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Towards an understanding of the
hydrological factors, constraints and
opportunities for irrigation in northern
Australia: A review
Cuan Petheram and Keith L. Bristow
CRC for Irrigation Futures Technical Report No. 06/08
CSIRO Land and Water Science Report No. 13/08
February 2008
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ISSN: 1834-6618
1
The Project Partners are: CSIRO, Land and Water Australia, National Program for Sustainable
Irrigation, CRC for Irrigation Futures, and the Governments of Australia, Queensland, northern
Territory and Western Australia.
Towards an understanding of the hydrological factors,
constraints and opportunities for irrigation in northern
Australia: A review
Cuan Petheram1 and Keith L. Bristow1,2
1
CSIRO Land and Water, PMB Aitkenvale, Townsville QLD 4814
2
CRC for Irrigation Futures, PMB Aitkenvale, Townsville QLD 4814
CRC for Irrigation Futures Technical Report No. 06/08
CSIRO Land and Water Science Report No. 13/08
February 2008
Acknowledgements
The authors gratefully acknowledge the contributions provided by Peter Jolly, Steven Tickell
(NT DNREA), Dr Anthony Smith (CSIRO Land and Water) and Bruce Pearce (Qld NRMW) in
the original discussions of the scope of this report. The authors would also like to thank
Malcolm Hodgen (CSIRO Land and Water) for GIS advice and Daryl Chin (NT Government),
Simon Rogers and Rosemary Lerch (WA Department of Water), and Edward Stephens (Qld
NRMW) for supplying data. This report would not have been possible without the tireless
efforts of the CSIRO librarians and their colleagues around Australia, thank you.
The streamflow section (Section 4.3) of this report draws heavily on a paper Dr Cuan
Petheram wrote in conjunction with Professor Emeritus Thomas McMahon and Dr Murray
Peel (University of Melbourne).
The guidance and assistance they provided with data
analysis and interpretation for the above mentioned manuscript is greatly appreciated.
Dr Jim Wallace, Mr Jeff Camkin, Dr Richard Cresswell, Dr Anthony Smith and Dr Peter
Hairsine (CSIRO Land and Water) are thanked for their comments on various drafts of this
report. Dr Jim Wallace provided valuable assistance with the section on evaporation. The
authors would also like to thank Professor Robert Henderson (James Cook University) for his
comments and discussion on geology and Dr Neil McKenzie and Dr Mike Webb (CSIRO
Land and Water) for discussion and comments on soils.
This work has been carried out as part of a suite of activities being undertaken by the
Northern Australia Irrigation Futures (NAIF) project. The NAIF project is funded by a number
of private and public investors including the National Program for Sustainable Irrigation, the
Cooperative Research Centre for Irrigation Futures, the Australian Government and the
Governments of the Northern Territory, Queensland and Western Australia, and their support
is gratefully acknowledged.
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Executive Summary
The last few years has seen renewed enthusiasm for northern Australia to strive for higher
agricultural performance from its ‘abundant’ water resources. This has been partly fuelled by
increased competition for water within the southern states, and by a perception that there are
large amounts of water and suitable land in the North. However, many Australians believe
that northern Australia holds iconic ecological and heritage status that should be carefully
managed. Visions of vast untouched landscapes, untamed rivers and a rich cultural heritage
are held close to heart.
To indigenous north Australians the surface and groundwater
systems of the North also have very strong cultural significance.
Reconciling these
contrasting visions is particularly challenging given the chequered history of cultivated
agriculture, and particularly irrigation, in the North and South of Australia.
Numerous studies and reports have examined reasons for European development either
failing or never starting in the north of Australia.
understand the northern environment.
A much cited factor is a failure to
In this report we seek to lay foundations for
understanding the hydrology of northern Australia, by providing a broad overview of the
surface and groundwater resources with respect to irrigation development. In doing so we
aim to: 1) provide a review of key literature on climate and hydrology in northern Australia,
relevant to irrigation; and 2) highlight key bio-physical issues, opportunities and constraints
for irrigation in northern Australia.
Northern Australia (or the North) is defined here as that area north of the Tropic of Capricorn
(23.5o S), encompassing approximately forty percent of Australia’s land mass. Northern
Australia has been classified into three broad climatic zones: wet-dry tropics (Köppen Aw),
semi-arid zone (Köppen BSh) and arid zone (Köppen BWh). A small area of Köppen Af and
Am exists along the north-east Queensland coast where the orographic uplift of moist
easterly winds creates a distinct wet tropical zone.
Because of its position and the
orientation of the Australian continent within the global circulatory system, northern Australia
is characterised by high year-round temperatures, a distinct seasonal rainfall pattern, some
of the greatest rainfall intensities in the world, large inter-annual variability in rainfall and
large evaporation rates.
The lack of rainfall during the dry (winter) months in northern Australia means that irrigation
is essential for cultivated agriculture or perennial horticulture during this period.
The strong seasonal component to rainfall and the high evaporation rates in northern
Australia mean that a greater volume of water is required to irrigate a given area of perennial
pasture in the North than in the South. For example, a perennial pasture grown in the three
major northern Australian drainage divisions (North-East Coast, Timor Sea and Gulf of
Hydrology of northern Australia: A review
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Carpentaria) would require between 20 and 80% more water than the same pasture grown in
the Murray Darling Basin.
Twelve major drainage basins characterise the Australian continent.
Half are partly or
entirely located within northern Australia, and approximately 60% of Australia’s runoff is
generated north of the Tropic of Capricorn. The majority of surface runoff occurs in the three
externally draining Divisions located predominantly in Köppen zone Aw where the majority of
rivers are ephemeral. In Köppen Aw, BSh and BWh, where rivers are well connected to
groundwater within carbonate karstic systems (e.g. Daly, Roper and Gregory Rivers) or
coarse, unconsolidated sedimentary deposits (e.g. Jardine and Wenlock Rivers), perennial
flow may occur.
While these perennial river systems are of strong interest to irrigators
because of their large dry season flows, they also hold particular ecological and cultural
importance.
A key feature of streamflow in northern Australia is that it is strongly seasonal and has a
large inter-annual variability when compared with rivers of similar climate elsewhere in the
world. This highly seasonal streamflow means that permanent settlements and irrigation
during the dry season requires surface water storage structures, unless suitable groundwater
resources are available. Large variation in flow from season to season and from year to year
requires that sizeable storage structures be built to accommodate volume fluctuations and
meet demand.
In this report we also describe some simple calculations that were used to estimate a
theoretical upper limit of the amount of water that could be used for human purposes (ie the
total potentially exploitable water) and, if this volume of water were all used for irrigation, the
maximum amount of land that could theoretically be irrigated in northern and southern
Australia. These calculations were based on historical estimates of rainfall, evaporation and
runoff only, and ignored other values and potential user of this water (e.g. urban, industry,
environment, tourism) as well as other factors that may constrain implementation of
sustainable irrigation such as the availability of suitable land and soil, economics, crop type,
etc. These calculations show that some 40% of Australia’s total potentially exploitable water
is located in northern Australia. If all of this potentially exploitable water was used for
irrigation, 20 to 25% of Australia’s irrigation by area could theoretically be located in northern
Australia. In reality, however, the maximum area under irrigation will be significantly less than
this when environmental, social, cultural and other values are considered in the water
allocation planning process. While these estimates suggest that from a purely water volume
point of view there is potential for additional irrigation in northern Australia, efforts towards
achieving and maintaining sustainable irrigation in southern Australia will continue to be
central to Australia’s long term irrigation future.
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There are few documented studies on groundwater recharge, discharge or lateral
groundwater flow in northern Australia. Hence this review provides limited discussion of the
potential to use groundwater for irrigation in the North.
In summary, considerable
groundwater resources occur in some Quaternary unconsolidated sediments (e.g. the Lower
Burdekin) and large sedimentary basins in the North.
Artesian Basin, is already considered fully committed.
The largest of these, the Great
Because extensive geological
exploration has taken place in northern Australia over the last 40 years, it is unlikely that any
large sedimentary basins with significant groundwater resources remain undiscovered.
Sustainable irrigation with groundwater in semi-arid (Köppen BSh) and arid (Köppen BWh)
zones will require a recharge area that is several orders of magnitude greater than the
irrigated area. If groundwater is developed in these arid zones, it may be very challenging to
maintain existing ecological values.
Policies designed to retain unique aspects of tropical environments and allow for
development will present major new challenges to sustainable irrigation in the North.
Aspects of irrigation planning and design that need particular attention include:
•
Drainage management in regions of large seasonal watertable fluctuations and large
quantities of surface runoff
•
Social, economic and biophysical costs and benefits of irrigation mosaics
•
Aquifer Enhanced Recharge within an irrigation context in a highly seasonal tropical
environment.
•
Management of irrigation tail waters in highly ephemeral systems
•
Water harvesting in the wet-dry tropics of northern Australia.
The range of concerns about irrigated agriculture in northern Australia highlights the need for
irrigation design to be carried out as part of a much broader development planning process,
which considers the physical and economic aspects of resources and society, as well as the
cultural, social and ethical values held by the people. The relatively small number of players
in northern Australia (compared to the situation in the South), however, provides a unique
opportunity for collaboration, and to plan proactively and not be reactive to problems and
failures.
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Acronyms
BFI
Base Flow Index
BHWSS
Burdekin Haughton Water Supply Scheme
BoM
Bureau of Meteorology
ENSO
El Nino Southern Oscillation
ET
Evapotranspiration
FAO
Food and Agriculture Organisation (United Nations)
FDC
Flow Duration Curve
GAB
Great Artesian Basin
GBR
Great Barrier Reef
GCM
Global Circulation Model
GDE
Groundwater Dependent Ecosystem
GL
Gigalitres
ITCZ
Intertropical Convergence Zone
KDDA
Katherine Douglas Daly Area
LB
Lower Burdekin
NAIF
North Australian Irrigation Futures
ORIA
Ord River Irrigation Area
RoW
Rest of the World
SOI
Southern Oscillation Index
SSP
Sea Surface Pressure
SST
Sea Surface Temperature
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Table of Contents
Acknowledgements ............................................................................................................... 4
Executive Summary............................................................................................................... 5
Acronyms ............................................................................................................................... 8
1 Introduction................................................................................................................... 10
1.1
2
2.1
2.2
2.3
2.4
2.5
3
Major surface water drainage divisions ................................................................................. 13
Distribution of major groundwater basins .............................................................................. 16
Coastal plains of northern Australia ...................................................................................... 23
Soil resources of northern Australia ...................................................................................... 25
Flora and fauna of northern Australia.................................................................................... 29
Climate of northern Australia ...................................................................................... 31
3.1
3.2
3.3
3.4
4
Report outline ........................................................................................................................ 11
North Australian landscape......................................................................................... 12
Rainfall................................................................................................................................... 32
Evaporation ........................................................................................................................... 43
The interaction between the amount, timing and intensity of rainfall and evaporation ......... 48
Representative irrigation demand for each of the major drainage divisions ......................... 49
Hydrology of northern Australia ................................................................................. 53
4.1
4.2
4.3
4.4
Excess water (the non-evaporated component of rainfall).................................................... 54
Potential recharge ................................................................................................................. 56
Streamflow and runoff ........................................................................................................... 67
Quantity of exploitable surface water by drainage division ................................................... 81
5 Concluding remarks..................................................................................................... 87
6 References .................................................................................................................... 91
Appendix 1 ......................................................................................................................... 104
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1 Introduction
There is growing interest in developing the water resources of northern Australia for
irrigation. This trend is partly fuelled by widespread perceptions of abundant water resources
in northern Australia, perceptions of declining rainfall trends across most of Australia, and
recognition that some water resources in the southern states are over allocated and over
used. Recent statements like ’rivers in northern Australia ”contain” 70% of Australia’s water
resources‘ (Anon. 2004a; Gehrke et al. 2004; Anon 2005; Gehrke 2005) serve to re-enforce
these perceptions without acknowledging the more seasonal nature of streamflow and large
evaporation in northern Australia. Among the main proponents for developing the water
resources of northern Australia are large and small irrigators. However, there is recognition
that mistakes have been made in the past with respect to irrigation development and water
allocation practices, which have compromised ecological and cultural values and future
industries (Woinarski and Dawson 1997).
In addition to its ecological and bio-physical attributes, northern Australia’s rivers hold iconic
status among many contemporary Australians. To indigenous Australians, the surface and
groundwater systems of the North also have a strong cultural significance (Jackson 2004).
With rapidly changing attitudes to development and sustainability, irrigation developers face
unprecedented scrutiny of their proposals from local communities, tourism and other
businesses, conservation interest groups, governments and the broader Australian
community.
Attitudinal changes are being reflected in recent policy initiatives that require new proposals
for development, including irrigation, to have a sound economic basis and be justifiable in
terms of impact on social, cultural and environmental values.
The key purpose of this report is to review the literature on the hydrology of northern
Australia and to highlight key issues, constraints and opportunities for irrigation in the North.
Particular emphasis has been placed on illustrating the differences between water systems in
northern Australia and temperate southern Australia—the latter being more familiar to most
Australians. This report has been written for a wide range of readers using general scientific
language, and seeks to provide a sufficient base level of knowledge that will assist all
stakeholders engage in an informed debate on the future of irrigation in northern Australia.
Time and financial constraints often mean that policy makers are placed in positions where
they are required to make decisions and develop policy based upon information that is
readily at hand. It is intended that this report will provide policy makers with a document that
contains a broad base of information and references on issues related to hydrology and
irrigation across northern Australia.
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Although the focus of this report is on bio-physical factors and more specifically hydrology,
the authors are cognisant of economic, environmental, social and cultural factors that may
individually or collectively form boundaries and guide or influence decisions regarding the
water resources of northern Australia.
1.1 Report outline
Section 2 provides an outline of the north Australian landscape, including the major drainage
divisions of Australia and the spatial distribution of the major groundwater basins in northern
Australia.
Key features of the coastal plains, soils and natural environment of northern
Australia also are discussed to provide a broader context for the other material outlined in
this report. Section 3 examines precipitation and evaporation in the north of Australia and
the irrigation demands in each major drainage division are evaluated.
recharge, surface runoff and streamflow are examined in Section 4.
Groundwater
Finally concluding
remarks discuss the key implications for irrigation in the North and key knowledge gaps and
challenges are identified.
Appendix 1 summarises the key messages contained in this
report.
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2 North Australian landscape
The North Australian Irrigation Futures (NAIF) project has defined northern Australia as that
area of Australia north of the tropic of Capricorn. Adopting this definition, this hydrological
review covers much of Queensland, almost the entire Northern Territory and the northern
portion of Western Australia (Figure 1). This area encompasses about three million square
kilometres, or 40% of Australia’s landmass.
This section outlines some of the general
features of the north Australian landscape.
Figure 1 States and territories of Australia. Not shown is the Australian Capital Territory
which lies in New South Wales. Large towns are shown by pink circles. Black dashed
horizontal line indicates Tropic of Capricorn. White lines indicate boundaries of major
drainage divisions. Numbers indicate major irrigation regions in the North (left to right): 1Carnarvon; 2- Ord River Irrigation Area; 3- Katherine Douglas Daly Area; 4-Darwin rural; 5Ti-tree; 6-Atherton; 7- Lower Burdekin; 8-Mackay/Withsundays; 9-Emerald.
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2.1
Major surface water drainage divisions
Perhaps the most strikingly visible characteristics of northern Australia and Australia in
general are the extensive plateaus and flatness of the landscape (Figure 2). This has been
largely attributed to the age of the landscape relative to that of other continents. Australia
has no high mountain ranges, active volcanoes or glaciers. Instead it is characterised by
extensive low plateaus situated behind narrow coastal plains. The plateaus are often sharply
truncated by scarp retreat and by the incision of narrow gorges, and slope gently inwards to
collectively create a vast depressed arid interior of extensive inward and often uncoordinated
drainage systems, which rarely flow (see Section 4.3).
The stability of the continent through geological time has been attributed to its central
location within a tectonic plate that includes Papua New Guinea, which is located along a
convergent boundary (Johnson 2004). Tectonic activity tends to occur at plate boundaries
(Marshak 2005) and as a result there has been minimal mountain building activity in
Australia. Australia’s largest mountain range, the Great Dividing Range, attains a maximum
height of 2228 m in the South and 1622 m in the North (Mt Bartle Frere), which is very low by
world standards. In contrast, Papua New Guinea is mountainous with many peaks over
4000 m in elevation. Despite its relatively low elevation the Great Dividing Range is a key
geomorphologic feature stretching almost along the entire length of the east coast of
Australia and forms the boundary of the Interior Lowlands and the Eastern Uplands. North of
the Tropic of Capricorn other regions of relatively high elevation are (from west to east in
Figure 2): Hamersley Ranges (Number 1), Kimberley Plateau (2), MacDonnell Ranges (3),
Arnhem Land (4) and the Barkley Tablelands (5).
Of these topographic high points, only the Great Dividing Range poses an obstruction to
large atmospheric circulatory systems (see Section 3). The relative lack of recent tectonic
activity has meant that more recent depositional processes are thought to have been much
more effective in regional differentiation than in other continents, where structure may have
greater importance (Jennings and Mabbutt 1986).
Twelve major drainage divisions characterise the Australian continent (Figure 2). Half of
these are partly or entirely located within northern Australia. Narrow zones of co-ordinated
external drainage occur in the north, west and east. In northern Australia, water resources in
the externally draining Timor Sea (VIII), Gulf of Carpentaria (IX) and the North East (I)
drainage divisions are attracting considerable interest.
The interior of the continent is
drained by two major co-ordinated drainage systems. Of these, only the Murray Darling
Basin (MDB) (within southern Australia in this report) has sufficient flow to maintain a mouth
to the sea. The other internally draining system is the Lake Eyre division, which intermittently
discharges to Lake Eyre (see McMahon et al. 2005).
Hydrology of northern Australia: A review
Elsewhere in northern Australia,
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drainage is either un-coordinated (e.g., the Western Plateau, which is also known as the
riverless region) or non-existent except during rare periods of heavy rainfall.
Figure 2 Topographic map of Australia (brown – high; blue – low; AUSLIG 9” digital
elevation model). Major drainage divisions of Australia shown by solid black lines. Two
transects across northern Australia show topography (black), annual precipitation (blue) and
pan evaporation (red). Transect A-A’ corresponds with Broome to Ingham and; B-B’
corresponds with Darwin to Mallacoota. Tropic of Capricorn is shown by black dashed line.
Numbers correspond with major topographic features of northern Australia: 1- Hamersley
Ranges; 2-Kimberley Plateau; 3-MacDonnell Ranges; 4-Arnhem land; 5-Barkley Tablelands;
6-Great Dividing Range. Roman Numerals correspond with major drainage divisions: INorth-East Coast; II-South-East Coast; III-Tasmania; IV-Murray Darling Basin; V-South
Australian Gulf; VI- South-West Coast; VII-Indian Ocean; VIII-Timor Sea; IX-Gulf of
Carpentaria; X-Lake Eyre; XI-Bulloo-Bancannia; XII-Western Plateau.
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Box 2.1 Geological age descriptions
Geological timescale and major events in Australia’s geologic history. Boundaries represent a major
change in sedimentation patterns. Age is in millions of years. Adapted from Anon. (1990), Twidale
and Campbell (1995) and discussion with Professor Robert Henderson (JCU).
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2.2
Distribution of major groundwater basins
At the continental scale, the Australian landmass can be conceptualised as being comprised
of a series of igneous blocks (rises in the Craton2) and sedimentary basins (zones of
subsistence within the Cratons hosting younger sedimentary material) over which lies a
surface veneer of unconsolidated regolith and alluvial sediment formed in younger time (e.g.
Quaternary) (Figure 3).
With respect to groundwater, sedimentary basins are of
considerably more interest than continental blocks because they usually make the best
groundwater reservoirs in terms of extraction and storage (see Box 2.2), particularly where
the sedimentary material is coarse (i.e. sand or gravel) and where diagenetic processes have
not had a major influence.
Figure 3 Sedimentary and igneous rock provinces. These roughly correspond with major
continental scale blocks and basins. Hatched polygons are predominantly regions of
igneous rock/blocks. Numbers correspond with selected major sedimentary basins and
blocks: 1-Carnarvon Basin; 2-Perth Basin; 3-Yilgarn Block; 4-Canning Basin; 5-Eucla Basin;
6-Amadeus Basin; 7-Wiso Basin; 8-Daly Basin; 9-Money Shoals; 10–Arafura Basin; 11Georgina Basin; 12-Great Artesian Basin; 13-Carpentaria Basin; 14-Laura Basin; 15-Murray
Basin. Source: NLWRA 2000.
At the continental scale, rocks and sedimentary material have been categorised here into
four broad groups based upon their permeability characteristics: 1) crystalline rocks and
Palaeozoic and older sedimentary basins; 2) Early to Middle Palaeozoic carbonate rocks; 3)
Cainozoic to Mesozoic sedimentary rocks and geological basins; and 4) surficialunconsolidated, non-lithified and predominantly Quaternary sediments. These groups are
briefly discussed in turn and examples for northern Australia provided. Sedimentary basins
illustrated in Figure 4 are underlined in the following discussion.
2
Ancient crustal material comprising ancient volcanogenic, clastic and chemical sediments, which are
intruded by igneous rock and upon which basins of variously deformed and metamorphosed volcanic
and sedimentary rocks are superimposed Allen (1997).
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Box 2.2 Rock types
There are two main rock types: 1) sedimentary rocks; and 2) crystalline rocks. Also
included in this discussion are unconsolidated sediments. Particular emphasis is made to
their hydrologic properties because these broad groupings also form the main types of
groundwater flow systems.
Unconsolidated Sediments
Unconsolidated sediments are ‘loose’ grains or aggregates derived from weathering of
igneous or sedimentary rocks, which have been transported and then deposited once the
transportation medium no longer has sufficient energy to entrain the particles. Typically
unconsolidated sediments have high porosity. However, porosity decreases with tighter
packing (e.g. due to increasing pressure with depth of burial) and in poorly sorted
material. The ability of fluid to flow through porous media is partly a function of the size
and the interconnectedness of the pores, referred to as its permeability. The permeability
of unconsolidated sediments increases with sorting and grain size. As a result aquifers
comprised of unconsolidated sands and gravels are often very productive from a
groundwater extraction point of view. Very fine materials (e.g., clay) often have large
porosity; however, the pores are very small and not always interconnected, which reduces
the permeability. Water in these materials does not easily flow.
Schematic illustrating how porosity changes with grain size, sorting, compression and
cementation in sedimentary material and an illustration of the porosity in crystalline rock.
Shapes are not to scale.
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Sedimentary Rocks
Sedimentary rocks can be considered clastic or non-clastic. Siliclastic rocks are
formed from unconsolidated sediments that have been compacted and lithified due
to increased heat and pressure (usually due to depth of burial) and chemical
changes. Ultimately this results in a consolidated rock, which has reduced porosity
due to a more tightly packed fabric and also to mineral cements that precipitate in
the pore spaces (either a silicate mineral like quartz or a non-silicate mineral like
calcite).
The collective term for the processes that result in changes to the porosity,
mineralogy and chemical composition of unconsolidated sediments upon
deposition is digenesis. Typically this occurs at temperatures and pressures
higher than ambient surface conditions but lower than 300 degrees. Temperatures
higher than 300 degrees usually result in metamorphism.
Non-clastic rocks are formed mainly by the precipitation of minerals from water
through various chemical and biochemical processes. They include evaporates,
chert and phosphorites. However, by far the most abundant non-clastic rocks are
the carbonate rocks, which are particularly prevalent in parts of northern Australia.
Carbonate rocks are formed by the precipitation of calcium carbonate minerals to
form limestones and dolomite (typically through biological agencies). They may
also have a clastic component if they are comprised of rock that is the product of
the re-working of past carbonate deposits. Like clastic rocks, carbonate rocks are
also subject to diagenetic processes but are more susceptible to dissolution,
recyrstallization and replacement processes. Changes to the porosity of carbonate
rocks is complex but porosity is generally reduced by compaction and cementation
and enhanced by dissolution. Generally, carbonate rocks have negligible primary
permeability but may have considerable secondary permeability due to dissolution
along fracture or bedding planes.
Crystalline Rocks
Crystalline rocks are comprised of igneous and metamorphoic rocks. Igneous
rocks are those that have crystallised from a silicate melt. They may be extrusive
(i.e. crystallise on the earths surface - basalts) or intrusive (i.e. the silicate melt
intrudes into and cools within the rocks that form the earths crust – granite
plutons). Metamorphic rocks are rocks that have undergone secondary change by
intense heat and pressure (typically greater than that required to undergo digenetic
change) and chemically active fluids. They are extensively recrystallised. These
two rock types usually have a very low primary porosity (< 2%) and the pores are
very small and not inter-connected. Where there is an absence of weathering or
fracturing they are considered essentially impermeable. However, where there is
fracturing, these rocks may exhibit secondary permeability and the quantity of
water preferentially flowing along the lines of weakness may be orders of
magnitude greater than that which flows through the rock matrix.
Sedimentary rocks (typically very old) may also exhibit secondary permeability due
to fracturing. Fractured rock systems rarely yield large quantities of water but they
are often an invaluable potable water resource for remote communities in northern
Australia.
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16
Figure 4 Hydrogeological map of Australia based upon broad scale geology. Numbers
correspond with major sedimentary basins and blocks: 1-Carnarvon Basin; 2-Perth Basin; 3Yilgarn Block; 4-Canning Basin; 5-Eucla Basin; 6-Amadeus Basin; 7-Wiso Basin; 8-Daly
Basin; 9-Money Shoals; 10–Arafura Basin; 11-Georgina Basin; 12-Great Artesian Basin; 13Carpentaria Basin; 14-Laura Basin; 15-Murray Basin; 16-Lower Burdekin
Group 1 – Palaeozoic and older sedimentary basins and groundwater systems
In most pre-Mesozoic siliclastic sedimentary rocks permeability has been strongly influenced
by diagenetic overprint, to the extent that the primary porosity of these rocks is often minimal.
For this reason, sedimentary basins older than Palaeozoic have been grouped with
crystalline rocks (see Box 2.2). Aquifers may form in these rocks where joints and fractures
are present (secondary porosity), although specific yields are usually small and water quality
may be variable.
The Atherton Tablelands is an example of fractured igneous rock in
northern Australia (Cook et al. 2001) and the Bowen Basin is an example of a large preMesozoic sedimentary basin with negligible primary porosity.
Most major ore bodies are situated in very old igneous and metamorphic rocks where hot
fluids from great depths transport and then precipitate minerals (Figure 5). This is facilitated
by reworking of the crust by folding and mountain building activity; the older the rock the
greater the chance it has been exposed to these activities. The crust of Australia has not
Hydrology of northern Australia: A review
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experienced significant reworking since the Permian (Taylor 1958), which is why most
mineral deposits of economic importance occur in rocks older than Permian. Though mining
is a relatively small user of water (2% Australia wide; Anon. 2004b), it can impose critical
stresses on local groundwater resources because the sustainable yield of rock aquifers is
often small.
Figure 5 Mine locations in Australia. Shaded regions are igneous and metamorphic blocks
(Source: NLWRA 2000). Unshaded regions are basins.
Group 2 - Early to Middle Palaeozoic carbonate rocks
It is thought that following the extensive Precambrian ice age, large areas of warm shallow
water were rapidly produced with the waning of glaciers (Johnstone 2004) and a seaway
across the Northern Territory was formed (Anon. 1990). Warm water favours carbonate
precipitation because increasing temperature causes a decrease in the solubility of carbon
dioxide, thus raising the pH (Boggs 2001). This led to the formation of the Early to Middle
Palaeozoic carbonate rocks, which are of primary interest to irrigation investors and
developers in the Territory due to their favourable water storage and baseflow
characteristics. Unlike the extensive limestone formations in the Eucla Basin (Parkinson
1988)—deposited in Aragonite rich seas during the Tertiary (Boggs 2001)—the carbonates of
the Northern Territory were deposited under conditions favourable to the formation of
dolomitic limestones. These rocks are characterised by dissolution cavities near the water-
Hydrology of northern Australia: A review
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table and primary porosity due to dolomitic recrystalisation3 (Anon. 1987).
Dissolution
features can act as preferential flow paths and, if intercepted by an extraction well, can yield
large quantities of water, in excess of one-hundred litres per second.
Early to Middle Palaeozoic carbonate rocks of northern Australia include the Oolloo
Dolostone and Tindal Limestone, which are separated by the dolomitic siltstone and
mudstone of the Jinduckin Formation). These sediments were deposited in shallow water
environments within the Daly Basin (Tickell 2002) and there are extensive deposits in the
Wiso and Georgina Basins.
Groundwater extraction in the Darwin rural region, which
contains the Northern Territory’s largest irrigation area, is predominantly from dolomitic
limestone aquifers (e.g., Palmerston dolomite).
Group 3 - Cainozoic to Mesozoic sedimentary rocks and geological basins
The Mesozoic was a significant Era in the formation of a number of important sedimentary
basins that contain contemporary groundwater systems (Taylor 1958); for example,
deposition commenced in the Great Artesian Basin (GAB). By the start of the Cretaceous
the continent was already quite flat and shallow seas covered large parts of the Australian
landmass during the Cretaceous eustatic transgression (Ollier 1986). Due to Australia’s high
latitude at that time, the water temperature was cool and there was very little carbonate
deposition. In the GAB, the typically ‘muddy’ Cretaceous sediments are thought to be of
terreginous4 rather than marine origin and act as a thick confining layer above the more
porous sandstone aquifer. The Jurassic sandstone formations outcrop in higher regions in
the north east (NRM 2005) where there was further uplift and erosion of overlying layers.
Underlying the western part of Cape York is the Carpentaria Basin, one of three interconnected sedimentary sub-basins that comprise the GAB. The Carpentaria Basin and the
inter-connected Laura Basin contain consolidated Mesozoic sediments, including the
Dalrymple Sandstone, Gilbert River Formation and Helby Beds (Horn et al. 1995). Recharge
in the sourthern and northern sub-basins occurs along the elevated eastern divide.
Groundwater in the Carpentaria Basin is relatively undeveloped because the fluoride
concentration is too high for domestic use (Anon. 1987) and good quality groundwater can
be readily obtained from the shallower and younger Karumba Basin aquifers (Horn et al.
1995a). The Karumba Basin was largely filled with sediment of fluvial origin between the late
Cretaceous and Pliocene (Horn et al. 1995). The aquifers of the Karumba Basin, principally
the Bulimba Formation, are considered to be the most significant groundwater resource in
Cape York, although the aquifer is heterogeneous and siting successful production bores is
3
Dolomitic rocks are calcium carbonate rocks where 50% of the cation sites are filled by Magnesium
and 50% by Calcium (Boggs 2001). Upon calcite recrystallising (changing form but not chemical
composition) to dolorite there is an increase in porosity of 13% (assuming no subsequent compaction
or cementation) (Tucker 2001).
4
Originally of volcanogenic material from the Whitsunday Volcanic Province (Bryan et al. 2000;
Johnstone 2004).
Hydrology of northern Australia: A review
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difficult (Horn et al. 1995). Knowledge of the groundwater flow and recharge characteristics
of the region is relatively poor (Horn 2000).
During the Cretaceous eustatic transgression, seawater also encroached over the northern
part of the Northern Territory, albeit relatively briefly (< 5 my, Anon. 1990) forming poorly
consolidated Cretaceous sandstones (Money Shoal Basin) in Arnhem Land and on Bathurst
and Melville Islands.
Some groundwater from these formations discharges to perennial
streams (Jolly and Chin 1991).
Box 2.3 Groundwater timelags
Studies in dryland systems in southern Australia have indicated that the time it takes for a local
scale groundwater system (≈ 10’s km in length) to respond to change in recharge or discharge
(i.e. acquire a new hydrologic equilibrium) may be in the order of 10s of years, an intermediate
system (many 10’s to a couple of 100 km in length) many decades to over a hundred years, and
regional scale systems (many 100’s and even 1000’s km in length) may take many hundreds of
years (Coram et al. 2000; Dawes et al. 2001). Groundwater age in the GAB has been measured
to an age of up to 2 million years (Airey et al. 1983).
The figure to the right illustrates
generic time response of Local,
Intermediate and Regional
groundwater systems. Reproduced
with permission from Dawes et al.
(2001).
1.0
Response to Change
The time lags associated with lateral
groundwater flow and the paucity of
data on groundwater recharge and
flow present numerous challenges to
the sustainable management of
groundwater resources and
associated ecosystems. Two high
profile examples of the challenges in
managing the time lags associated
with groundwater processes are
surface water - groundwater
interactions and secondary salinity.
Local
0.8
Intermediate
0.6
0.4
0.2
Regional
0.0
0
50
100
150
200
Time (Years)
Group 4 – Surficial-unconsolidated, non-lithified and predominantly Quaternary
sediments
Unconsolidated Quaternary sedimentary deposits occur in association with current and prior
drainage systems and coastal sand dunes. Aquifers in these deposits have a variety of yield
and storage properties and contain groundwater of variable quality. Because of the relatively
small sizes of these features they may have local importance but are difficult to map at the
continental scale. A notable region of Quaternary deposition in the north of Australia is the
area east of the Great Dividing Range along the north Queensland coast, where most of
northern Australia’s cultivated agriculture and population are situated.
For example, the
Lower Burdekin is one of Australia’s most intensive groundwater use areas. It contains more
than 2000 extraction bores (Arunakumaren et al. 2001) and the sustainable groundwater
yield is estimated to be 350000 million litres per year (NLWRA 2000).
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2.3
Coastal plains of northern Australia
The coastal plains of north Queensland support the largest areas of dryland and irrigated
agriculture in northern Australia but they are atypical of many of the coastal plains west of the
tip of Cape York.
Box 2.4 Evolution of coastal plains
Over the past 1.8 million years sea level has risen and fallen at least seventeen times
(Fink and Kukla 1977); the last peak in sea level rise in northern Australia was
between 7 and 9000 years ago (Wolanski and Chappell 1996). Since then, sea level
has fallen slightly but has been stable for about the last 6000 years, defining the
contemporary shoreline and familiar shape of the continent. Higher sea level caused
the sea to encroach inland, drowning existing river valleys along the periphery of the
continent and creating inlets and estuaries5 (Warner 1988). These newly created
estuaries then started to slowly infill with sediment delivered by their rivers. Provided
there is a continuous supply of sediment and sea level remains stable, all estuaries
will eventually evolve into deltas (Heap et al 2004). With each subsequent fall in sea
level, the river systems ‘rejuvenate’ as the coastline regresses (Twidale and Campbell
1995) and some or all of the sedimentary deposits that accumulated during the
previous sea level rise are eroded—though new deposition may occur in some places.
In many cases, the type of coastal plain sediments and their lithology are largely
reflective of the present depositional environment (Harris and Heap 2003) because
sea level has remained largely unchanged for thousands of years, although relict
sediments from previous sea level high stands may sometimes be present. A number
of researchers (e.g. Galloway 1975; Dalrymple et al. 1992; Harris and Heap 2003)
have shown that fluvial, tidal and wave energy processes largely control estuarine
evolution. Along sections of coast without a river outlet, the siliclastic coastal
depositional environment depends on the ratio of mean wave power to mean tidal
power (Harris et al. 2002).
In north and north-western Australia, tidal processes tend to dominate estuary5 evolution
(Harris et al. 2002) and fine texture sedimentary deposits (i.e. silt and clay, often referred to
as marine muds) are typically deposited adjacent to the main channel during high tide events
(Coleman and Wright 1978) (Figure 6 illustrates wave height and tidal range around
Australia). Because the tidal length of these channels can be very large, the resulting subcoastal plains can be relatively extensive. For example, the tidal length of the Daly River is
greater than one hundred kilometres (Chappell 1993; van Diemen Gulf, Woodroffe 1993). In
a study of the sub-coastal plains of the Adelaide River (NT), CSIRO survey teams concluded
that the heavy estuarine clays were unsuited for mechanical cultivation (Chapman and
Basinski 1985). In addition to the trafficability difficulties that the estuarine clays presented,
strong tidal activity in the adjacent river and seasonal and prolonged flooding from rainfall
were additional hazards to mechanised farming.
Along the North Queensland coast, wave energy is slightly greater than elsewhere in
northern Australia (Harris et al. 2002) but small relative to southern Australia due to the
5
Geologically an estuary is defined as “the seaward portion of a drowned valley system which
receives sediment from both fluvial (i.e. river) and marine sources and which contains facies
influenced by tide, wave and fluvial processes” (Dalrymple et al. 1992).
Hydrology of northern Australia: A review
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presence of tropical reefs (Jennings and Bird 1967). Tidal ranges in northern Australia are
also larger (Figure 6). The most distinctive aspect of north Queensland is large surface
water discharges off the precipitous coastal escarpments (Transect A-A’ Figure 2), which
ensure appropriate hydraulic conditions for the delivery of fine and coarse sediment to the
coastal flood plain. As rivers enter the flat coastal areas there is a marked reduction in fluvial
energy and sedimentation results. The large discharge volumes and sediment transport
capacity of rivers like the Burdekin, Herbert (Jennings and Bird 1967) and Fitzroy, combined
with the narrow coastal margin, has resulted in prograding alluvial deltas of relatively coarse
sedimentary material. Coarse sedimentary deposits result in highly permeable groundwater
systems and sediments in the Lower Burdekin contain one of Australia’s most transmissive
and intensively used aquifer systems (Anon. 1976).
In an analysis of 280 rivers discharging to the ocean, Milliman and Syvitski (1992) report that
sediment loads are a log-linear function of basin area and maximum elevation. Their model
provides an explanation for why prograding alluvial deltas are less prevalent along the southeastern margin of the Great Dividing Range. Easterly draining catchments in the southeast
have a smaller sediment supplies because they are smaller and have lower rainfall erosivity.
Figure 6 Tidal range and wave height map of Australia. Numbers signify mean tidal range.
Horizontal dashed line indicates Tropic of Capricorn. Adapted from Davies (1986).
Hydrology of northern Australia: A review
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2.4
Soil resources of northern Australia
Box 2.5 Soil formation
What is soil?
“Soils are independent natural bodies consisting of weathered mineral and organic matter often
occurring in genetically related horizons formed in response to subarial processes” Hubble et
al. 1983.
Key soil formation processes
Soils do not occur within a landscape by chance, but rather form complex patterns as a result
of the inter-play of five key factors: parent material, climate, organisms, topography and time
(Fitzpatrick 1986).
Soil type is only weakly correlated with rock type, where the same rock can give rise to many
different soil types. Correlation is strongest between rock type and soil texture e.g. sandstones
which are high in quartz give rise to sandy soils. Where soils are mature (i.e. have been in
place for long period of time – factor number five) it is found that climate, especially
temperature and rainfall, have perhaps the most marked effect on soil formation. These
parameters effect soil formation directly and indirectly (e.g. climate is a key factor in
determining vegetation type which in turn can influence soil formation processes). The main
effect of temperature on soils is to increase the speed of soil formation, where the speed of
chemical reactions increase by a factor of two to three for every 10oC rise in temperature
(Fitzpatrick 1986). Rainfall controls the amount of moisture in the soil, which is the primary
mechanism by which ions and small particles are transported within a soil matrix. Attributing
the characteristics of a soil to a particular climate is challenging because often a soil is the
integrated result of many past climates, as is the case in many parts of Australia.
Organisms include plants, vertebrates, microorganisms and mesofauna (e.g. earthworms,
termites, ants). These organisms influence soil formation in many ways, but of particular note
plants contribute organic material to the soil surface, while microorganisms (e.g. bacteria, fungi,
algae) perform services like decomposing the organic matter and fix atmospheric nitrogen,
which can then be used by plants. Mesofauna assist with the decomposition of organic matter
and transportation of material.
Topography acts as a control on erosion and deposition (i.e. soil thickness) as well as the
spatial distribution of moisture in the landscape. Soils typically take thousands even millions of
years to form. Of these five factors only time can be considered independent of the others.
Generally accepted standards for describing soil profile morphology in Australia are given in a
number of publications e.g. Isbell 1996; McKenzie et al. 2004.
Australia’s northerly latitude during the Pleistocene ensured that only a very small portion of
the main continent was affected by glaciations during this Epoch (Anon. 1990). As a result
many Australian soils are relatively old, some having been associated with landscapes that
have been weathered for millions of years (McKenzie et al. 2004). Old, deeply weathered
soils tend to be depleted of nutrients, a much cited characteristic of Australian soils. While
soil nutrient deficiencies are not unique to the Australian situation, Australia does have an
extraordinary large area of relatively poor soil and a relatively small area of good quality soil
(Leeper 1970). These good quality soils are generally geologically ‘young’ being derived
either from fairly ‘recent’ volcanic activity (e.g. Cainozoic basalts) associated with the uplift of
the Great Dividing Range (during the Tertiary) or from unconsolidated sediments on alluvial
floodplains. However, there are exceptions, for example Vertosols, which have a very high
clay content, are able to retain essential nutrients despite prolonged periods of weathering
Hydrology of northern Australia: A review
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and leaching, while some young soils like Holocene sand dunes, which have a low content of
organic matter and a high proportion of weather resistant quartz have very low fertility.
Until superphosphate fertilizer was developed in 1890s, the use of high yielding pastures and
crops in Australia was very limited (Anon. 1982).
The application of large amounts of
superphosphate, subterranean clover (for nitrogen), trace elements and more recently large
inputs of nitrogen and potassium in southern Australia during the twentieth and twenty-first
century allowed many of the nutrient deficiencies in southern soils to be partly overcome.
Because there is no substitute for phosphorous in agriculture (USGS 2007) and phosphorous
is fundamental to plant growth, any future global shortages of phosphorous6 has the potential
to constrain some agricultural activities (Abelson 1999).
Despite the distinct climatic difference between northern and southern Australia, making
north-south regional scale distinctions in soil type is a difficult task (Figure 7).
This is
because many of Australia’s soils have been exposed to a variety of climates over Geologic
time due to climatic fluctuations and the gradual northward drift of the Australian continent
since the Tertiary. Perhaps the main difference between the soils of northern and southern
Australia is that the physical structure and chemical composition of southern soils have been
extensively modified through cultivation and the application of fertilisers (Leeper 1970).
A strong regional scale distinction in northern Australia’s soils lies east and west of the Great
Dividing Range. Along the Great Dividing Range soils derived from the Cainozoic basalts
generally provide fertile soils.
East of the Divide, fluctuating sea levels (i.e. alternating
baselevels) during the Pleistocene caused the short, steep streams to rejuvenate, stripping
old soils and depositing ‘fresh’ sediments, from which new soils formed. West of the Great
Dividing Range there are extensive areas of deeply weathered mantle that have been
preserved since the Tertiary period. Many of these deeply weathered profiles have almost
been completely leached of essential plant nutrients. In many regions of northern Australia
the deficiency of key nutrients in the soil means that intensive cultivated agriculture will
require fertiliser additives.
There is little information on the soil resources of northern Australia relative to southern
Australia. Few regions have been mapped at a scale of 1:50 000 or finer (Figure 8) and
limited soil related literature explicitly discusses the soil resources of the region, soil function
and response.
6
Phosphorous forms on geological timescales, either by guano deposits, or sedimentary processes
thought to be associated with ocean up-welling along specific continental margins (Boggs 2001).
Hydrology of northern Australia: A review
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Australian
Soil
Classification
-15°
Order
Calcarosols
Chromosols
Dermosols
-25°
Ferrosols
Hydrosols
Kandosols
Kurosols
Organosols
Podosols
-35°
Rudosols
Sodosols
Tenosols
Vertosols
Lakes
110°
120°
130°
140°
150°
160°
Figure 7 Generalised distribution of soil orders belonging to the Australian Soil Classification, modified from Isbell et al. (1997).
Hydrology of northern Australia: A review
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Completed
and published
1:25 000
1:50 000
1:100 000
Completed,
not published
No data
To be surveyed
Survey in progress
Broad scale
1:100 000 to
1:250 000
Figure 8 Soil map availability (not to scale).
Soil/landform
1:100 000
Hydrology of northern Australia: A review
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Detailed mapping
< 1:50 000
2.5
Flora and fauna of northern Australia
During the late Cretaceous, sea floor spreading saw Australia separate from Antarctica and
start to drift north (Anon. 1990). This separation led to Australia’s fauna and flora evolving in
relative isolation (Ollier 1986). As a result many plant and animal species are endemic to
Australia. The Australian flora is considered to have developed a range of adaptations to the
low nutrient status of the Australian soils (and poor water storage capacities) and low and
variable rainfall e.g. cluster roots, mycorrizas, sclerophylly, tubers and lignotubes (Eamus et
al. 2006). In the wet-dry tropical regions of northern Australia, deep rooted vegetation is
predominantly evergreen, which is different to the deep rooted vegetation found in other wetdry tropical regions of the world (predominantly deciduous).
Bowman and Prior (2005)
attribute this phenomenon to the variability in climate, soil infertility and deeply weathered
regolith, from which evergreens can exploit water during seasonal drought.
Many ecosystems across northern Australia range from being episodically dependent to fully
dependent upon groundwater.
These ecosystems are referred to as Groundwater
Dependent Ecosystems (GDE). In northern Australia, GDE include riparian and other deep
rooted vegetation dependent upon the presence of groundwater in the subsurface during the
dry season (e.g. see O’Grady 1999; Hutley et al. 2001); invertebrate groundwater ‘animals’
(collectively known as stygofauna; Figure 9) usually found in course alluvial or carbonate
deposits (Humphreys 2006) and; terrestrial and aquatic ecosystems, which depend upon the
surface water expression of groundwater (e.g. mound springs7 or groundwater baseflow).
The potential impact of future agricultural development on the latter of these GDE is
attracting considerable attention in the Daly River system, Northern Territory (e.g. Anon.
2003; Erskine et al. 2003; Blanch et al. 2005).
Freshwater flows from rivers also have a profound influence on coastal ecosystems
(Gillanders and Kingsford 2002), affecting circulation patterns and vertical stability of marine
waters, mixing and nutrient exchange processes, and the delivery of particulate organic and
inorganic compounds, which form part of the marine food chain (Drinkwater and Frank 1994).
In large river systems the effect of freshwater has been observed to extend many hundreds
of kilometres into the ocean beyond the river mouth (e.g. Moore et al. 1988). Ecologically,
freshwater flow has been strongly linked to the health and production of certain estuarine and
marine fish and shellfish (Robins et al. 2005) with most studies showing a positive
relationship between fish abundance and river discharge (Drinkwater and Frank 1994).
The commercial fisheries in the estuaries and near-shore waters of tropical Australia have a
combined value of approximately A$220 million (Robins et al. 2005).
Other important
commercial species include penaeid prawns, finfish, sharks and crabs. Barramundi, mud
7
Mound springs are groundwater discharge sites. Classically they form conical shaped mounds of
carbonate and clastic material and salts.
Hydrology of northern Australia: A review
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crabs and many other species have cultural importance to indigenous communities (Robbins
et al. 2005). The quality of water in mean and extreme flow events also has ecological
importance (Gillanders and Kingsford 2002). Along the north east coast of Queensland,
there is concern about the quality of terrestrial runoff and its impacts to the Great Barrier
Reef (GBR) and inshore reef systems (Brodie and Mitchell 2005). Sediments, nutrients and
pollutants entrained or dissolved in terrestrial runoff have been shown in many independent
field studies around the world to degrade coral reefs at local scales though relationships at
the regional scale are more difficult to establish because of confounding issues (Fabricius et
al. 2005). The reef systems of the GBR have been valued at over $6 billion per year to the
national economy (Access Economics 2005).
Figure 9 Nirripirti arachnoides (Dytiscidae) one of more than 100 species of blind diving
beetles known from calcrete aquifers of the Yilgarn region, WA, and Ngalia Basin, NT. Photo:
Chris Watts, South Australian Museum. Photography provided by William Humphrey,
Western Australian Museum.
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3 Climate of northern Australia
This section examines the climate of northern Australia, specifically precipitation and
evaporation. It does not explicitly examine or discuss the optimum climatic zone for crop
production with regards to parameters like temperature, frost or humidity.
Key factors controlling Australia’s climate
The primary characteristics of Australia’s climate are generally considered to be a
consequence of four major inter-related factors:
1. Its location within the subtropical pressure zone. Australia’s landmass is centrally
located within the dry descending air of the Hadley cell circulation. This results in
much of the continent being affected by large eastward travelling anti-cyclones.
These high pressure systems, which may extend up to 4 000 kilometres along their
west-east axes are responsible for the high temperatures and dryness that
characterise much of the continent.
Systems generating moisture occur either
between individual anti-cyclones or to the north or south of them (Warner 1986).
2. The size, shape and latitudinal range of the Australian continent. This has resulted in
a broad range of climates.
3. The subdued relief of the continent provides little obstruction to major atmospheric
circulatory systems. The exception is the Great Dividing Range along the east coast
of Australia.
4. Australia’s position with vast expanses of ocean to the east, west and south and the
long coastlines ensures that most of the continent is subject to oceanic influences
The net affect of these four factors is a large semi-arid/arid zone (Figure 10), moderate
seasonal variation and high inter-annual variability (Hobbs 1998) for which Australia is well
renowned. To the south of the arid centre (Köppen class B) the climate is Mediterranean
(Köppen class Cs); to the north the climate is tropical (Köppen class A) and is characterised
by highly seasonal, summer dominated rainfall and year round high temperatures and
evaporation rates.
Hydrology of northern Australia: A review
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Figure 10 Mean annual rainfall and months with no precipitation (Adapted from Warner
1986).
3.1
Rainfall
Rainfall generating mechanisms
Between the months of December and April a broad area of low atmospheric pressure
known as the Intertropical Convergence Zone (ITCZ), monsoon trough or thermal equator
moves south of the equator and intermittently crosses the northern shores of Australia.
When the trough comes close to or crosses over land it brings humid conditions with
showers and thunderstorms to northern Australia. The shallow and unstable air associated
with the north-westerly monsoonal flow does not penetrate deep inland and generally favours
the development of thunderstorms. This results in heavily localised rainfall and an inwardly
declining rainfall gradient (Figure 10). Throughout the course of a wet season the location of
the monsoon trough varies, and those periods where it temporarily retreats north of the
Australian coastline are referred to ‘inactive’ periods. Following a prolonged ‘build-up’ period,
typically a northern wet season is comprised of two or thee active/inactive cycles, each full
Hydrology of northern Australia: A review
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cycle lasting between four to eight weeks (Anon. 1998), with inactive periods usually being of
longer duration than active periods. The position of the trough is highly variable from one
wet season to another (Bonell et al. 1983), and in those years where the monsoonal trough
does not extend over northern Australia, ‘well organised rainfall’ (i.e. widespread, as opposed
to localised and spatially variable convective rainfall) does not occur.
The monsoonal rains associated with the ITCZ are also supplemented by heavy often
widespread rainfall from tropical cyclones originating from the seas to the north-west and
north-east of the continent. On average tropical cyclones produce 30% of the rain during
January - March period and up to 50% in drier regions like Port Headland and Broome. On
the north-west coast of Australia tropical cyclones have an average frequency of occurrence
of 2 per year, the Gulf of Carpentaria 1 per year and the north-east coast 1 to 2 (Anon.
1986). While many tropical lows along the north-east coast and in the Gulf of Carpentaria do
not fully develop into tropical cyclones they nevertheless can be significant rain producing
systems.
During June-September the ITCZ moves north of the equator and the high pressure cells
move northward (Figure 11). Cold fronts between subtropical high cells frequently move
across central Australia and may bring rain to these parts of the country.
North of 30
degrees and south of 14 degrees (i.e. where the Great Dividing range lies adjacent to the
Queensland coast), orographic uplift of the south-east trade winds results in year round
rainfall and high rainfall totals along the Queensland coast (illustrated in Figure 11 and Figure
12), particularly between Cardwell and Cooktown where the ranges are very steep and fringe
the coast (Sumner and Bonell 1986). On the western side of the escarpment there is a very
steep declining rainfall gradient. Having lost most of their moisture the trade winds then
sweep across the rest of the continent resulting in mainly mild, dry south-easterlies over
northern Australia (Figure 11). Other elevated areas in northern Australia e.g. Pilbara Range
on the west coast, or the inland McDonald Ranges, have a less dramatic effect on rainfall
because the prevailing air has a very low moisture content.
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Figure 11 Winter median rainfall (June – August). Source: Adapted from Anon. 1998 and
Warner 1986.
Figure 12 Summer median rainfall (December – February). Red dashed line illustrates
ITCZ. Source: Adapted from Anon. 1998 and Warner 1986.
Hydrology of northern Australia: A review
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Seasonality and intensity
Rainfall across northern Australia is highly season, although the magnitude of the seasonality
varies spatially. Darwin and Weipa, situated at the most northern parts of northern Australia,
are regularly affected by the monsoonal trough (Figure 13). During the dry season little to no
rainfall occurs because the predominantly south-easterly winds are dry having lost most of
their moisture along the north-east Queensland coast. Inland regions like Mount Elizabeth,
Katherine and Tennant Creek exhibit similar seasonal rainfall distributions but of lower
magnitude, a function of their distance inland and their latitude. Cairns is less affected by the
monsoonal trough than more northerly centres like Weipa, but orographic uplift off the northeast Queensland coast results in high wet season rainfall totals (Figure 12). Orographic
uplift of moist south-east trade winds during the dry season results in rainfall throughout the
year (Figure 11). Gove (centre top - Figure 13) appears to be in a slight rain shadow when
the prevailing winds are from the west. This may explain why summer rainfall totals are
lower than centres of equivalent latitude e.g. Darwin and Weipa. However, small dry season
rainfalls are observed, probably due to the dry south-easterly winds picking up moisture as
they move over the Gulf of Carpentaria. Rockhampton and Port Headland, situated just
above the Tropic of Capricorn, are not directly affected by the monsoon trough.
Rockhampton receives year round rainfall from moist south-easterly trade winds and
localised convection during summer and the occasional cyclonic depression. Port Headland
receives little rainfall, with most rainfall the result of cyclonic lows.
A unique characteristic of rainfall in northern Australia is its intensity at the daily time scale.
Bonell et al. (1983) provide an extreme example where at Bellenden Kerr on the North
Queensland coast, 1330 mm was recorded in a 30 hour period.
Not only is northern
Australia observed to have considerably higher daily rainfall intensities than southern
Australia (Figure 14), but its intensities are considered very high globally. For example,
Jackson (1986) found that for the whole of northern Australia, except the north-east coast,
rainfall is more concentrated with fewer rain days and higher mean daily intensities than one
would predict from its monthly totals when compared to other tropical regions around the
world. This is thought to be due to the importance of tropical cyclones to monthly rainfall for
much of northern Australia. While northern Australia may have some of the highest daily
rainfall intensities it does not necessarily imply that it also has the highest sub-daily
intensities (e.g. hourly). In other tropical regions of the world where other rain producing
mechanisms may be more prevalent they may produce high sub-daily rainfall intensities (e.g.
due to localised convective thunderstorms). The very high daily rainfall intensities observed
across much of northern Australia has numerous implications for the hydrology of the North
(discussed in Section 4), as well as soil erosion (Figure 15), agriculture, mining and
engineering works.
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Figure 13 Mean annual rainfall map of Australia accompanied by charts of median monthly rainfall (bar chart – each bar represents 50 mm) and pan
evaporation (black dashed line) for selected centres. Major Drainage Division boundaries and numbers are illustrated by thin black line and Roman
Numerals respectively. Source: Climate charts were generated from SILO data, rainfall map was sourced from NLWRA 2000 data library.
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Figure 14 Rainfall intensity. Mean rainfall per wet day. Adapted from Prescott in Leeper
(1970).
Figure 15 Hillslope erosion map of Australia. Modelled values. Erosion values are shown
in tonnes/ha/yr. Black dashed line indicates Tropic of Capricorn. Thin dark lines illustrate
major drainage divisions. Source: NLWRA 2000 data library.
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Variability and long term trends in rainfall
Australia’s rainfall variability is one of the defining
traits of the Australian climate and has been the
source of discussion in academic writings (Leeper
1970; McBride and Nicholls 1983, Peel et al.
2002a)
and
popular
literature
(e.g.
Banjo
Patterson). The spatial variability of annual rainfall
is shown in Figure 16.
Here the measure of
variation used is the difference between the 90th
and 10th percentiles divided by the median rainfall.
Box 3.2 The variability or dispersion of data is a
very important characteristic of data. The simplest
measure of variability is the Range. Perhaps the
most commonly used measure is the Coefficient of
Variation (Cv), the ratio of the standard deviation to
the mean.
In northern Australia it has been observed that for
a given mean annual rainfall total, the inter-annual
variability of rainfall is higher than that observed at
stations from the Rest of the World (RoW) for the
same climate type (Petheram et al. 2008). Rainfall
stations along eastern and northern Australia have
been observed to have a strong correlation (0.5 –
0.6) with the Southern Oscillation Index8 (SOI)
during spring (McBride and Nicholls 1983), where
rainfall stations with a consistent relationship with
El Nino – Southern Oscillation (ENSO) have a
higher inter-annual variability of rainfall than those
with a poor relationship. (Nicholls 1988; Peel et al.
2002b).
ENSO is a phenomenon that is
Box 3.1 Climate variability, trends
and climate change
The three terms: climate variability, trends
and climate change are inter-related but
subtly different.
Climate
variability
is
a
natural
phenomenon that occurs at a variety of
timescales, ranging from daily (e.g.
diurnal variation) through to intra-annual
(e.g. seasonal variation), and to many
millennia (e.g. Milankovitch cycles caused
by variations in the earths orbit).
A climatic trend is a long-term shift or
change in climate after variability
mechanisms operating at shorter time
scales have been accounted. They may
be due to processes (that have always
been present) that operate over very long
time scales or because of a ‘changing’
climate. Climate change is complicated
because the different mechanisms and
processes that induce climatic variability,
trends and change are not independent of
one another. Thus their net effect is not
simply the sum of their individual
contributions. Rather they exhibit nonlinear behaviour, where variation induced
by one process may simultaneously
cause and be a consequence of variation
induced by another process.
Geological studies indicate that the
earth’s climate is in a constant state of
change.
Whether or not the earth’s
climate is changing depends upon the
scale of interest. Concern over climate
change stems around concern that the
medium
scale
climatic
processes
controlling long-term (i.e. 100 year) trends
have changed in character or are
changing. Whether these changes are
due to non-linearities inherent in the
climatic system or due to external forcing
(e.g. human activities) is still under
debate, though the latter is now accepted
as exacerbating the former (IPCC 2007).
considered to be the primarily source of global
climate variability over the 2-7 year timescale. It is caused by a complex and unstable
interaction between the ocean and atmosphere over the tropical Pacific (Box 3.3). While
ESNO has now been recognised as affecting global climate conditions, it has its largest
influence on weather patterns and climate variability over the tropical pacific and the
8
The Southern Oscillation Index (SOI) provides a measure of the state of the ENSO cycle. It is
calculated from the monthly or seasonal fluctuations in air pressure difference between Tahiti and
Darwin. Sustained negative (positive) values of the SOI often indicate El Nino (La Nina) episodes.
Hydrology of northern Australia: A review
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continental landmasses on either side, i.e. coastal regions of South American, South East
Asia and north and eastern Australia.
Figure 16 Variation in rainfall and rainfall charts for key centres from 1900-2005. Source:
Bureau of Meteorology9. Measure of variation used was the difference between the 90th and
10th percentiles divided by the median rainfall. On the charts of rainfall, each horizontal bar is
indicative of 500 mm/yr and each vertical bar is representative of the rainfall in 1 year.
9
http://www.bom.gov.au/climate/averages/climatology/variability/IDCJCM0009_rainfall_variability.shtml
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Box 3.3 El Nino - Southern Oscillation (ENSO)
The El Nino – Southern Oscillation (ENSO) phenomenon is now considered to be the primarily source
of global climate variability over the 2-7 year timescale and is caused by a complex and unstable
interaction between the ocean and atmosphere over the tropical Pacific (IPCC 2001). ENSO
influences climatic variation by irregularly oscillating between two modes, La Nina (Spanish for little
girl) and El Nino (Spanish for little boy) – a term that has come to be synonymous for drought, in the
western Pacific and eastern and northern Australia. A description of the underlying processes is
provided below.
In ‘neutral’ years, an up-welling of cold, nutrient rich water off the Peruvian coast flows west along the
equator, increasing in temperature with distance and exposure to the tropical sun (see diagram below)
This results in a Sea Surface Temperature (SST) gradient across the Pacific with a temperature
difference (3 and 8 degrees) between the eastern and western Pacific Ocean. Strongly coupled to
these ocean circulatory features is the Walker circulation, an asymmetric atmospheric circulation
centred over the tropical Pacific which is driven to a large extent by differences in SST. The Walker
circulation strengthens the easterly tropical surface winds of the symmetrical Hadley circulation (which
are largely driven by solar radiation and the rotation of the earth). These westward wind stresses
simultaneously reinforce the SST difference across the Pacific by pushing warm surface waters west,
varying the sea level (Neelin et al 1998) and depth of the thermocline (see diagram below) resulting in
cold water being exposed near the surface in the east. Along the equator, the easterly induced ocean
surface currents drift to the north and south (referred to as Ekman drift) due to the rotation of the earth
(i.e. Coriolis effect), which in the eastern and central Pacific further drives the narrow band of cold
water up-welling, referred to as the ‘equatorial cold tongue’.
Diagram of ESNO. Red and blue indicate warm and cold sea temperatures respectively. The atmospheric convective loop is
part of the Walker circulation. Clouds form by convective processes over those parts of the Pacific with the warmest sea
surface temperatures. Right - Diagram of the El Nino effect. Red and blue indicate warm and cold sea surface temperatures
respectively. Positive temperature anomalies in the eastern Pacific cause the Walker circulation to be displaced to the east.
Source: Pacific ENSO Applications Centre
During El Nino (La Nina) episodes (defined as when SST anomalies of +0.5C (-0.5C) occur across the
central tropical Pacific Ocean for a period of greater than 5 months) the usually large SST difference
across the tropical Pacific is reduced (increased) by the occurrence of positive (negative) temperature
anomalies in the eastern equatorial Pacific. This causes and is a consequence of weaker (stronger)
easterly trade winds, which result in a fall (rise) and rise (fall) in sea level in the western and eastern
Pacific respectively. Weakened (strengthened) easterly currents along the equator result in a deeper
(shallower) thermocline in the eastern Pacific, further strengthening the positive (negative)
temperature anomaly in the eastern equatorial Pacific.
As a result of these anomalous ocean-atmosphere dynamics the Walker circulation is displaced to the
east (west), resulting in precipitation (no rain) in usually dry regions of the Pacific and dry (wet)
conditions prevailing over eastern and northern Australia and South East Asia. It is because of these
simultaneous anomalies in ocean temperature and Sea Surface Pressure (SSP) that the ENSO
phenomenon gets its name (i.e. where El Nino refers to the warming of the eastern Pacific and
Southern Oscillation is reference to the state of the Walker circulation).
While major strides forward have been made in understanding the general dynamics of ENSO, there
is still considerable uncertainty surrounding the precise generation of an El Nino episode (Tsonis et al.
2003). Discussions have centred upon the relative roles of the above mentioned factors (i.e. wind,
SST, up-welling currents), their connections and the mechanisms by which the warm anomaly in the
eastern Pacific is propagated and sustained. This is currently an active area of research.
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Trends
Long term trends in rainfall may be due to climatic processes that have always been present
but operating over much longer time scales than our current observational record, may be
due to anthropogenic forcing, or a combination of both. Figure 17 illustrates the change in
rainfall between 1900 and 2006 and 1950 and 2006 and rainfall trends for the last 100 years
for selected centres (expressed as the cumulative sum of the difference from the mean).
Most notable is the decline in rainfall in the major population centres along the east coast of
Australia and south-western Western Australia and in Australia’s ‘food bowl’, the MDB. In
contrast, in the north west of Western Australia apparently large increases in annual rainfall
have been observed, although this region has few rainfall stations with long-term climate
data.
At a number of centres across northern Australia (e.g. Fitzroy Crossing, Darwin,
Weipa, Tennant Creek in Figure 17), annual rainfall totals over the last couple of decades
have been considerably higher than the long term average. Figure 17 illustrates that the
magnitude and pattern of the change in rainfall is sensitive to the chosen time period.
Because of the complexity and non-linearity of the processes controlling the earth’s climate,
it is not possible to simply extrapolate current rainfall trends into the future. For example,
using an ensemble of climate models, Milly et al. (2005) qualitatively reproduced observed
trends in global runoff. However, initial observed increases in runoff in the twentieth century
in eastern equatorial South America, southern Africa and the western central plains of North
America were projected to reverse and decrease in the twenty first century.
Understanding the significance of current rainfall trends and predicting future medium to long
term rainfall patterns requires an understanding of the longer time scale processes and how
they might change with external forcing (e.g. global warming). This is done using two types
of approach, 1) analysis of historical observations (measurements as well as proximal
indicators10 of past climates) - using past climates as analogues of present and future
climates; and 2) using numerical modelling techniques to simulate the earths climate
systems under current conditions and external forcing (e.g. global warming). Climate change
and its potential impacts is a very active area research. The interested reader is directed to
IPCC (2007) and CSIRO (2007) for more detailed and specific information.
10
In the absence of longer unambiguous instrumental records, workers have made use of what is
referred to as proximal indicators also referred to as proximal evidence. These are in effect surrogates
for the parameter of interest. Proximal indicators have been extensively used to reconstruct large
scale temperature changes (e.g. Jones et al. 2001) and hence investigate mean global climate
change. However, there are no proximal parameters that relate directly to climate variability (Jones et
al. 2001) and instead they are often related to another parameter/s that can be related to climate
variability e.g. inorganic laminae deposits (Rodbel et al 1999), coral oxygen isotopes (Cole et al. 1993,
Tudhope et al. 2001).
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Figure 17 Change in rainfall from 1900 – 2006 and 1950 – 2006 (mm/10 years). Charts are
the mass residual curve of rainfall between 1900 and 2004 (normalised for mean annual
rainfall). Source: Adapted from Bureau of Meteorology11 and SILO dataset.
11
http://www.bom.gov.au/cgi-bin/silo/reg/cli_chg/trendmaps.cgi
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3.2
Evaporation
Box 3.4 Evaporation and transpiration
Evaporation is “the rate of liquid water transformation to vapour from open water, bare
soil, or vegetation with soil beneath” Shuttleworth (1993)
Transpiration is “that part of the total evaporation which enters the atmosphere from
the soil through the plants” Shuttleworth (1993)
There are two major ways in which evaporation (Ea) affects a regions potential for irrigation.
The first is via the catchment wide losses of evaporation that determine runoff (R) and
drainage (D) or the ‘excess water’ (R + D), which forms the basis of the potentially
exploitable resource.
(1)
R + D = P – Ea
The second way in which evaporation affects irrigation potential is via its influence on the
crop water requirement (ET), where
ET = Kc Erc
(2)
Kc is a crop specific coefficient that varies during the season and Erc is the ‘reference crop
evapotranspiration’.
The crop water requirement, ET, is often referred to as
evapotranspiration, the latter term being widely used to explicitly indicate the inclusion of
evaporation from the soil surface and transpiration through the plant leaves. Reference crop
evapotranspiration, Erc is a measure of the evaporative demand of the atmosphere and
following FAO guidelines (Allen et al. 1998), it is calculated using formulae that are based on
the Penman-Monteith equation (Allen et al. 1998). The primary factors that determine Erc in
this equation are radiation, air humidity and wind speed.
To obtain the actual water requirement of a fully irrigated crop, Erc is multiplied by a crop
specific factor, Kc, which may vary during the growing season. Tabulated values of Kc for
most common crops are given by Allen et al. 1998 and an example for sugar cane is shown
below (Figure 18).
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1.4
1.2
Crop factor
1
0.8
0.6
0.4
0.2
0
0
50
100
150
200
250
300
350
Day of season
Figure 18 An example of how the crop factor for sugar cane varies with different stages of
growth (Allen et al. 1998).
Figure 18 and equation (2) show that the total seasonal water requirement for sugar cane is
primarily determined by the evaporative demand of the atmosphere (via Erc) and so a given
crop grown in a location with a high evaporative demand will require more water than if it is
grown in a location with a lower evaporative demand. Table 1 illustrates water requirements
for sugarcane grown at the Lower Burdekin (LB), North Queensland.
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Table 1 Water-use by sugarcane grown in the Lower Burdekin. Assuming a June planting (dry
season) and April-May harvest (post wet season). Crop factor were derived from Figure 18. Where
the crop factor varies over the course of a month an average value was used (i.e. number in brackets)
to calculate water use. Climate data for Ayr was sourced from the Silo Point Patched Dataset12 and
Erc was calculated using the Penman-Monteith equation as recommended by FAO56 (Allen et al.
1998).
Month
Day of season
June
July
August
September
October
November
December
January
February
March
April
Total
0-30
31-62
63-94
95-125
126-157
158-188
189-220
221-252
253-281
282-313
314-344
Crop factor
0.4
0.4-0.8 (0.6)
0.8-1.25 (1)
1.25
1.25
1.25
1.25
1.25
1-1.25 (1.1)
0.8-1 (0.9)
0.75
Lower Burdekin (Ayr)
Erc
(mm)
85
95
110
130
160
165
165
155
130
135
115
Crop water use
(ML/ha)
0.34
0.57
1.10
1.63
2.00
2.06
2.06
1.94
1.43
1.22
0.86
15.21
In Australia, catchment wide (or areal) evaporation (Ea) is usually calculated using Morton’s
(1983) ‘complementary’ evaporation formulae and this forms the basis of the Australian
Bureau of Meteorology (BoM) maps of both potential and actual evaporation across the
Australian continent. Potential evaporation (PE) is determined by atmospheric conditions
and actual evaporation (Ea) is equal to or less than this according to the availability of
moisture in the soil (or ground water). Morton (1983) assumed that two forms of potential
evaporation existed; that which applied at a point (PEp) and that which applied to a large area
(PEa). He then calculated actual (areal) evaporation (Ea) as
Ea = 2 PEa - PEp
(3)
The BoM and the University of Melbourne used Morton’s complementary method to generate
the evaporation maps that appear on the BoM’s website13. There are complex theoretical
differences between Morton’s (1983) estimates of Ea and those used in the calculation of
crop water requirements (e.g. Erc) that are beyond the scope of this report. However,
Morton’s ‘point potential’ (PEp) estimate is closest to the ‘potential’ evapotranspiration
recommended by FAO (Allen et al. 1998), so this is used in the reminder of this section to
indicate how potential evaporation varies across the Australian continent.
Potential evaporation (PEp) is generally high over the Australian continent, significantly
exceeding rainfall in all but the wettest areas (Figure 19). It exceeds 2500 mm/year across
12
13
http://www.nrw.qld.gov.au/silo/ppd/index.html
http://www.bom.gov.au/climate/averages/climatology/evapotrans/IDCJCM0008_evapotranspiration.shtml
Morton’s complementary method was used to estimate evaporation because of a paucity of historical wind speed data across
Australia.
Hydrology of northern Australia: A review
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most of the tropical north and is extreme (approaching 10mm/day) during the wet season
(i.e. southern summer). Potential evaporation decreases with decreasing solar radiation as
occurs with increasing southern latitude and the onset of winter. It also decreases with
proximity to coasts, because of increasing cloudiness, rain and humidity (i.e. relative humidity
is lower).
Figure 19 Morton’s point potential evapotranspiration. Source: Bureau of Meteorology14.
Across most of Australia actual evapotranspiration (Ea) is the largest component of the
terrestrial water balance.
Australia wide, approximately 88% of rainfall is evaporated or
transpired (NLWRA 2000). Across northern Australia approximately 80% of rainfall is lost as
evapotranspiration compared with approximately 95% in southern Australia. At the regional
and continental scale, long term annual Ea has been shown to be largely dependent upon the
long term annual rainfall (e.g. Schreiber 1904, Budyko 1974). In northern Australia, Williams
et al. (1996) showed that tree height and density were also directly related to the long term
annual rainfall. Observations such as these can be used to explain why the periphery of the
relatively ‘well watered’ north, south and east of the continent have high annual actual
evapotranspiration totals relative to other parts of the continent of similar latitude (Figure 20)
and the central arid region of northern Australia has the lowest actual evapotranspiration
despite having a very high potential evaporation (Figure 20 and Figure 19).
14
http://www.bom.gov.au/climate/averages/climatology/evapotrans/IDCJCM0008_evapotranspiration.shtml
Hydrology of northern Australia: A review
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Figure 20 Annual actual evapotranspiration maps. Source: Bureau of Meteorology15.
In the most northerly parts of Australia (e.g. Darwin and Cape York regions) annual potential
evapotranspiration is around twice the annual actual evaporation near the coast, but the
difference increases rapidly with distance inland. Figure 19 illustrates that as you move
inland potential evaporation (PEp) increases due to a decrease in the relative humidity and
an increase in radiation increases, yet actual evapotranspiration (Figure 20) decreases
because less moisture is available and the vegetation becomes sparser. In the arid zones of
northern Australia where long term rainfall totals are very low the ratio of actual
evapotranspiration to potential evaporation is only about 0.1.
Seasonal variation
Northern Australia has a high seasonality of actual evapotranspiration, although unlike
southern Australia PEp remains relatively high throughout the year. The seasonality in actual
evapotranspiration is mainly associated with variations in the leaf area index of the
understorey in native vegetation. For example, during the wet season, Hutley et al. (2001)
found ‘stand’ evapotranspiration rates to be 2-18 times higher than during the dry season
and most of this difference (i.e. 80%) was attributed to transpiration from the annual grasses
and herbaceous plants in the understorey. If Hutley et al’s results are typical of other native
vegetation types then most of the actual evapotranspiration observed in northern Australia
can be attributed to transpiration by the understorey during the wet season.
15
http://www.bom.gov.au/climate/averages/climatology/evapotrans/IDCJCM0008_evapotranspiration.shtml
Hydrology of northern Australia: A review
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3.3
The interaction between the amount, timing and intensity of rainfall and
evaporation
Mean annual rainfall minus potential evaporation is referred to as the mean annual rainfall
deficit (Figure 21), a general measure of moisture availability. This metric is sometimes used
as a first cut assessment of the mean annual irrigation demand. To refine this estimate the
waterbalance needs to be calculated at shorter time intervals so that the intra-annual
variability in precipitation and evaporation can be accounted.
In the following section
(Section 3.4) this was done on a monthly time step to attain a ‘representative’ irrigation
demand for each of the major drainage divisions.
Figure 21 Rainfall deficit i.e. mean annual rainfall minus mean annual potential evaporation.
Source: derived from data supplied by the NLWRA 2000.
Climate characterisation
The amount and temporal distribution of precipitation and evaporation does much to
characterise climates, particularly in northern Australia where temperatures are uniformly
high. To characterise the climates of northern Australia, here we adopt the commonly used
Köppen-Geiger classification (Köppen, 1936). This classification divides the world into 30
climate types, based on the version used here (Peel et al. 2007), of which 7 are represented
in northern Australia. These are illustrated spatially in Figure 22. The three dominant climate
types are tropical Aw (‘wet-dry tropics’), semi-arid BSh and arid BWh, which stretch across
northern Australia in latitudinal bands from north to south. These latitudinal bands broadly
mirror the latitudinal rainfall deficit bands seen in Figure 21. Small regions of tropical Af and
Am are found along the north-east Queensland coast (‘Wet Tropics’) and patches of
temperate Cwa are found on the Atherton Tablelands and Mackay Highlands.
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Mackay the seasonal distribution of precipitation becomes more uniform, with Cwa giving
away to Cfa climate type.
In the Northern Territory and north-western Australia, the semi-arid (Köppen BSh)
landscapes may occur in areas with annual rainfalls totals of up to 800 mm. In southern
Australia, where potential evaporation rates are lower, the boundary between the semi-arid
and temperate zones coincides more closely with the 400 mm rainfall isohyets, while at subtropical latitudes (i.e. south-east Queensland) the semi-arid zone occurs up to annual
rainfalls of about 600 mm. Hence, mean annual rainfall values should be examined within
the context of the evaporative demand.
Figure 22 Köppen climate zones of northern Australia (Peel et al. 2007). Three major zones
are Aw (wet-dry tropics), BSh (semi-arid zone) and BWh (arid zone). Diagram sourced from
Petheram et al. 2008. Stream gauging stations from Petheram et al. (2008) shown by pink
circles.
3.4
Representative irrigation demand for each of the major drainage divisions
Here we estimated the annual irrigation demand for each major drainage division using a
monthly timestep.
The irrigation demand of a drainage division was calculated by the
weighted sum irrigation demand of one or more ‘representative’ centres (Figure 23a) so that
each of the components were taken to sum to 1. Representative centres were selected
using expert knowledge, based upon: 1) where irrigation is currently located within that
division (Figure 23b); 2) where the largest quantities of surface runoff occur (Figure 23c); and
3) in a location that is representative of the mean annual rainfall deficit in that division (Figure
23d).
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Figure 23 A) Location of representative centres; B) Irrigation water use; C) mean annual
runoff; D) mean annual rainfall deficit. Original datasets sourced from the NLWRA (2000).
Irrigation demand is equal to the crop water requirement (Equation 2) minus the effective
rainfall (PE). PE is equal to that rainfall that is not ‘lost’ to runoff (R) or deep drainage.
Because 1) there is no information on recharge at the drainage division scale; and 2)
recharge is usually a minor component of the water balance; this term has been neglected
and PE was simply calculated by multiplying the rainfall by the complement of the runoff
coefficient (RC) for the drainage division as shown in Equation 4:
PE = P × (1 − RC )
(4)
PE was evaluated for each major drainage division on a monthly basis. Monthly runoff
coefficients were calculated using monthly rainfall and runoff grids of Australia (sourced from
the National Land and Water Resources Audit data library16). Crop water requirement (ET)
data were sourced from the Silo Point Patched Dataset17, which calculated Reference
Evapotranspiration (ETrc) using the Penman-Monteith equation as recommended by FAO56
(Allen et al. 1998) and a uniform crop factor (KC) of 0.8 was used for each month (Equation
2). A crop factor of 0.8 is considered representative of a perennial pasture grown in the
MDB. Allen et al. 1998 recommend using a value of between 0.75-0.85 for grazing pasture
and turf grass.
16
17
http://ww.nlwra.gov.au/Data_library/index.aspx
http://www.nrw.qld.gov.au/silo/ppd/index.html
Hydrology of northern Australia: A review
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Where rainfall exceeded the crop water requirement in a particular month, the irrigation
demand was assigned a value of zero and it was assumed that no moisture is carried over to
the following month. The representative irrigation demand for each major Drainage Division
is summarised in Table 2.
Table 2 Rainfall equivalent depth of water required by 1 ha of crop for each drainage
division. Northern Drainage Divisions are shown in bold.
Drainage
division
No.
Drainage division
Annual
runoff
coefficient1
Representative
centre
Weighting
I
NE Coast
0.2
II
SE Coast
0.18
III
Tasmania
0.5
IV
Murray Darling
Basin
South Australian
Gulf
SW Coast
Indian Ocean
0.05
Ayr
Laura
Emerald
Taree
Bairnsdale
Hamilton
Burnie
Oatlands
Echuca
Narrabri
Wallaroo
0.5
0.25
0.25
0.33
0.33
0.33
0.5
0.5
0.66
0.33
1.0
V
VI
VII
0.03
Irrigation
demand by
drainage
division
(mm/year)
696
282
356
614
752
0.05
0.03
Collie
1.0
579
Carnarvon
0.75
1206
Pardoo
0.25
VIII
Timor Sea
0.16
Kununurra
0.5
1002
Katherine
0.5
IX
Gulf of
0.2
Richmond
0.5
1055
Carpentaria
Highbury Station
0.5
X
Lake Eyre
0.03
Longreach
1.0
1130
XI
Bulloo-Bancannia
0.02
Tibooburra
1.0
1183
XII
Western Plateau
0.00
Minnipa Ag. Centre
0.5
989
Ernebella
0.5
1. Annual runoff coefficients are presented here. However, monthly values of RC were used to
calculate PE on a monthly time step.
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Summary
Australia’s climate is the principal factor that influenced the evolution of Australian dryland
farming systems (Hook and Williams 1998), together with changing markets and prices. In
southern Australia relatively low rainfall totals and high inter-annual variability mean that
successful production of many higher value crops is dependent upon irrigation.
In northern Australia high evaporation rates, highly seasonal and variable rainfall severely
limits the range of dryland cropping. Even where there is sufficient moisture during the wet
season to overcome evaporative demands, the high rainfall intensities can severely constrain
agricultural operations, particularly where heavy machinery can cause direct erosion to the
seed bed (Delane 1987) or soil structure. In many parts of northern Australia, these factors
necessitate the need for irrigation for the production of crops, pastures and/or horticulture.
However, based upon the calculations in Table 2 irrigation demand for a perennial pasture (a
function of the quantity and timing of rainfall and evaporation) is higher in the northern
drainage divisions (e.g. Divisions I, VIII and IX) than the southern drainage divisions. For
example, 1 ha of perennial pasture grown in northern Australia (i.e. Divisions I, VIII and IX)
will require between 20 and 80% more supplementary water than 1 ha of perennial pasture
grown in the MDB.
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4 Hydrology of northern Australia
This section discusses the terrestrial water balance of northern Australia with particular
reference to those processes and components that are relevant to irrigation at the regional
scale (Box 4.1).
Box 4.1 - Regional scale water balance
A water balance follows the mass balance concept, where over a certain time period the sum of the
inputs, minus the outputs must be equivalent to the change in storage. What input and output
components are assessed depends upon the spatial and temporal scale of investigation and
objectives of the study. For example, in a crop water balance study at the paddock scale input
parameters may include rainfall and irrigation. Output parameters may include: evapotranspiration,
tail water drainage and deep drainage. The residual is equivalent to the change in water held in the
unsaturated zone. Over the course of a week the change in soil moisture may be a significant term
and hence should be measured. However, over the course of a year, the change in soil water
storage is often negligible compared with the input and output parameters.
This study is concerned with hydrology at the scale of northern Australia. Consequently we examine
those components of the water balance that have relevance at the catchment scale. Catchment
scale forms a useful integrator of many processes operating within the catchment.
At the catchment scale, under ‘natural’ conditions the input parameter is precipitation (in some cases
where groundwater divides do not align with catchment divides groundwater may also flow into the
catchment). Output parameters are streamflow, evapotranspiration and lateral groundwater flow.
The storage parameters are water stored in the groundwater and the unsaturated zone. Over the
course of a year the latter parameter is considered negligible. In situations where the subsurface
material has a very low permeability lateral groundwater flow is sometimes considered to be
negligible.
The two other terms shown below are recharge and surface runoff. If only a surface water balance
were being calculated recharge may form an additional output parameter. However, caution should
be exercised before ignoring the groundwater component of a waterbalance.
Schematic representation of catchment scale water balance
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The excess water component of rainfall (i.e. the non-evaporative component) is discussed
and data from northern Australia are examined. Next recharge, usually the smaller of the
two excess water components of rainfall, is examined. Because runoff (the larger of the two
excess water terms) dominates streamflow in many northern Australian catchments, it is
discussed within the context of streamflow, which follows a discussion on surface water –
groundwater interactions.
4.1
Excess water (the non-evaporated component of rainfall)
The non-evaporated component of the terrestrial water balance, often referred to as the
excess water component, is that water that is discharged from the catchment as streamflow
or groundwater (over a long time period).
In many circumstances lateral groundwater
discharge from a catchment is small relative to that flux of water that leaves the catchment as
streamflow. Because of difficulties in estimating lateral groundwater discharge it is often
ignored, unless the underlying groundwater system is highly transmissive.
Proportioning of evaporated and excess water in northern Australian catchments
Changes in transpiration in the form of clearing (Bosch and Hewlett 1982), grazing (Hanson
et al. 1970) or burning (Townsend and Douglas 2000) may alter water yield and/or quality. A
number of workers (e.g. Budyko (1974), Holmes and Sinclair (1986); Zhang et al. (2001))
have developed relationships between long term actual annual evapotranspiration, broad
vegetation type and long term annual rainfall. These simple water balance models can be
used to describe the effect of vegetation change on mean annual evapotranspiration. Here
we utilise the complement to the relationship developed by Zhang et al. (2001) to place
excess water observations from northern Australia into a broader context (see Box 4.2).
In Figure 24 we plot the long term annual average streamflow (assumed to be a function of
the long term runoff and recharge) against the long term annual catchment average rainfall
for the 99 catchments in northern Australia used by Petheram et al. (2008) and shown by
their streamflow gauging stations in Figure 22.
These catchments were predominantly
uncleared.
The actual evapotranspiration curves developed by Zhang et al. (2001) did not include any
study locations from northern Australia. Furthermore only about 1/5th of the studies were
from regions between the Tropic of Capricorn and the Tropic of Cancer, and 1/3rd of these
were from Yemen, largely a semi-arid environment (Zhang et al. 1999). The significance of
this is that the savanna regions of northern Australia are not represented by the ‘fitted’
relationships.
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Box 4.2 – Rational function approach to evapotranspiration (Zhang et al. 2001)
Rational function approach to evapotranspiration (Reproduced with permission from Zhang
et al. 2001).
Based upon the regional scale dependencies of evapotraspiration on rainfall and
vegetation, Zhang et al. (1999; 2001) developed a simple two-parameter water balance
model that related the mean annual evapotranspiration to rainfall, potential
evapotranspiration and plant available water capacity. The two parameters used in the
above model are the plant available water coefficient (w) and the potential
evapotranspiration (E0). The plant available water coefficient represents the ability of
plants to store water in the root zone for transpiration. Fitting the model to 240 catchments
world wide, the best fit for forested catchments yielded E0 of 1410mm for a w value of 2,
and for herbaceous plants E0 was 1100mm for a w value of 0.5. This simple water
balance model has since been applied used in southern Australia to describe the effect of
regional scale vegetation change on mean annual evapotranspiration and hence
determine catchment yield.
The underlying premise of this relationships is that at low mean annual rainfalls, actual
evapotranspiration is moisture limited and there is little difference between the long term
water use between trees and grass (i.e. nearly all water is used by both trees and
grasses). The difference in actual evapotranspiration between trees and grasses
increases with increasing mean annual rainfall as trees with their greater rooting depth are
able to utilise a greater proportion of the moisture. However, at long term annual rainfall
totals greater than about 1500 mm, the difference in actual evapotranspiration between
forest and grass expressed as a percentage of the mean annual rainfall starts to decline
(Peel et al. 2001) as radiation becomes increasingly limiting.
For the north Australian data, below an annual rainfall of 800 mm the data appear to broadly
follow the excess water curve for trees, although they exhibit a large degree of scatter above
and below the line. Above 2000 mm rainfall, however, the data lie along the excess water
curve for grasses, with the catchment with the largest annual rainfall (Russell River) almost
lying on the one-to-one relationship. Reasons for this are not fully understood but may be
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related to under estimates of the catchment average, long term annual rainfall (catchment
average rainfall data were sourced from the SILO dataset). All of the catchments with a
mean annual rainfall greater than 2000 mm are located in the wet tropics (Köppen Af and
Am) region of North Queensland. It is likely that rainfall stations in this region do not account
for cloud interception18; and methods for interpolating data between rainfall stations do not
properly account for the high orographic induced precipitation on the steep coastal
escarpments.
Figure 24 Excess water curves for trees and grass (derived from Zhang et al. (1999)) and
long term average streamflow and rainfall data for 99 northern Australian catchments
(predominantly uncleared).
4.2
Potential recharge
Recharge can be defined as being water that actually replenishes the underlying
groundwater system (Zhang and Walker 1998). Water that infiltrates the soil and passes
below the root zone of the vegetation is commonly referred to as potential recharge or deep
drainage (Bond 1998) and may or may not be equivalent to recharge. When a soil is wetted,
water flows downwards under the influence of gravity. However, after field capacity19 has
been attained water may flow laterally (Eamus et al. 2006) or upwards (i.e. capillary rise) in
response to moisture gradients induced by evapotranspiration (i.e. evaporation and or use of
soil moisture by plants) or increased drainage may be induced.
18
With respect to their study site on Mount Bellenden Ker (North Queensland), McJannet et al. (2007)
comment that “failure to account for the process of cloud interception would lead to an underestimate
of water inputs of up to 66% on a monthly basis and 30% over the longer term”.
19
The maximum amount of water that a soil can hold before it drains downward due to gravity.
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Very few studies have inferred deep drainage or recharge in northern Australia. However,
studies by Cook et al. (1998) and Wilson et al. (2006) suggest that in the wet and wet-dry
tropics of northern Australia recharge rates may be high, even under native vegetation (i.e.
up to 200 mm/yr). In these regions recharge is highly seasonal due to high year round
evaporation rates and the high seasonality of rainfall (as observed by Jolly and Chin 1991,
Cook et al. 1998 and many others). The large magnitude and high seasonality of recharge
can result in large intra-annual groundwater table fluctuations (see for example Figure 25) ,
with watertables declining over the subsequent dry winter months due to evapotranspiration
(i.e. direct evaporation and water use by phreatic20 vegetation) and lateral groundwater flow
into surface water bodies (i.e. creeks, wetlands and the ocean).
In the Howard River
catchment in the Northern Territory (mean annual rainfall of 1585 mm), Cook et al. (1998)
observed annual groundwater level variations of approximately 7 m, while Jolly and Chin
(1991) have observed groundwater levels to rise by as much as 10 m seasonally elsewhere
in the wet-dry tropics of the Northern Territory. Fluctuations of this magnitude in localised
surficial aquifers may result in groundwater levels approaching the ground surface, as
observed by Cook et al. (1998). In a region of central Kansas (USA) with relatively shallow
watertables (i.e. < 5 m), Sophocleous (1992) found watertable depth in the Spring months to
be one of the most influential variables affecting recharge.
Figure 25 Groundwater level measurements (blue dots) and inferred groundwater level
(black dotted line) in the Tindal Limestone Aquifer in the Daly geologic basin (RN029429).
The ground surface is shown by the horizontal green line. The red line indicates rainfall
mass residual curve (Katherine Council). Data provided by Steven Tickell (NT Government).
20
Vegetation that sources some or all of its water from groundwater
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Recharge mechanisms and controlling factors
The highly seasonal input of rainfall and recharge in the wet and wet-dry tropics of northern
Australia may result in a preference for different recharge mechanisms to those that may
occur in temperate climates. For example, preferential flow processes may occur during
episodes of saturation excess where water may by-pass the soil matrix through macro-pores.
By-pass flow is thought to only operate under conditions where the matrix is saturated
(Eamus et al. 2006).
In an uncleared portion of the Daly catchment (Northern Territory) Wilson et al. (2006)
inferred that 70% of recharge to the underlying Oolloo Dolostone aquifer occurred via
preferential/bypass flow, mostly likely through small depressions rather than ‘sink holes’,
which can be common in some carbonate settings (carbonate rocks, including limestone,
with extensive dissolution features are commonly referred to as karstic).
However, the
authors reasoned that because: 1) recharge was inferred (through measurement and
numerical modelling) to increase by a factor of two to four times under cleared land (i.e. from
50-200 mm to 300-540 mm); and 2) that the quantity of preferential/bypass flow should
remain the same under both vegetation types, the portion of recharge occurring via bypass
flow and matrix flow may be reversed in areas of cleared land. Nevertheless this study and
similar observations in the Tindal Limestone by Tickell (2002) indicates that bypass flow may
be a major recharge mechanism at least in some carbonate Karstic regions in northern
Australia and because of this the change in recharge may be less for different vegetation
types than that in southern Australia (as highlighted by Petheram et al. 2002 – see Figure
26). The results of Wilson et al. (2006) are consistent with those of Williams et al. (1997),
who, using the soil water balance model PERFECT (Littleboy et al. 1989), inferred an
increase in deep drainage of between 2-3 times when woodlands were replaced with pasture
in the Burdekin Catchment (i.e. wet-dry tropics).
No other studies have inferred deep
drainage/recharge through measurement under different vegetation types in northern
Australia.
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300
Annuals - Long-term
Perennials - Long-term
Grassland
Trees - Long-term
250
Annuals - Single year
Recharge (mm/year)
Trees - Single year
Excess water curve
200
Trees
150
100
50
0
0
200
400
600
800
1000
1200
Rainfall (mm/year)
Figure 26 Potential annual recharge under annual, perennial and native vegetation for
southern Australia (reproduced from Petheram et al. 2002). Dotted lines indicate excess
water curves of Zhang et al. (2001) which form hypothetical long term upper bounds to
recharge. Circles indicate measurements of preferential flow as observed/stated by the
original author.
Potential recharge in semi-arid/arid environments
In semi-arid and arid zones of northern Australia (i.e. < 800 mm/yr in Northern Territory),
which encompass nearly all of the internally draining drainage divisions, long term average
recharge rates are typically low, i.e. < 1mm/year (e.g. Harrington et al. 2002). Recharge
rates appear to be greatest along (ephemeral) river channels and landscape depressions,
where runoff is sufficiently concentrated that it can overcome evaporative demands (Jolly
and Chin 1991; Harrington et al. 2002). These irregular/intermittent recharge events may
result in a rise in groundwater levels of between 0.01 to 1 m (Jolly and Chin 1991).
Recharge zones may also be strongly correlated with large scale topography. This is in part
because of the correlation between rainfall amount and topography and that topography
driven zones of saturation enable surface runoff to be concentrated in quantities sufficient to
overcome evaporative demands and soil deficits, and may then recharge through ephemeral
stream channels (Harington et al. 2002). A well documented example is the Liverpool Plains
region of northern NSW, where surface runoff from the hills neighbouring the plains,
recharges the underlying aquifer system through alluvial fans at the foot of the ranges
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(Stauffacher et al. 1997).
In the intermediate and regional scale Mesozoic-Tertiary
Carpentaria and Laura Basins of Cape York (Horn et al. 1995) and the more southerly
Jurassic-Cretaceous Great Artesian Basins (Anon. 1987) of central northern Australia,
recharge zones occur on the upper slopes of the GDR where the water bearing aquifers
outcrop and are ‘exposed’.
During geological Periods of wetter climates and/or in basins where there was subsequent
deposition and diagenesis of sediments of very low permeability, recharge is thought to have
occurred in places in northern and central Australia where recharge is negligible today. This
can result in isolated volumes of ancient groundwater, commonly referred to as ‘fossil’ water.
Sustainable yield of groundwater
Sustainable yield remains an enigma to many in its definition and concept as well as in its
application. At the turn of the twentieth century a concept analogous to ‘sustainable yield’
was that of ‘safe yield’. Lee (1915) defined ‘safe yield’ as “the limit to the quantity of water
which can be withdrawn regularly and permanently without dangerous depletion of the
storage reserve”.
Meinzer (1920) defined ‘safe yield’ as “…the practicable rate of
withdrawing water from it (the aquifer) primarily for human use”.
Over the last hundred years the term, definition and concept of sustainable yield has
changed considerably, as documented by Kalf and Woolley (2005) and summarised here.
Later concepts of the term attempted to factor in economics (e.g. Meinzer 1923), water
quality (e.g. Stuart 1945), legality (Anon. 1961) and various permutations of these. Others
tried to make the definition more concise (Todd 1959), while others focused on trying to
remove some of the ambiguity that plagues the concept and the language used to describe it
(e.g. ASCE 1961).
The term sustainable yield came into use in the 1980s (Kalf and Woolley 2005). Bredehoeft
et al. (1982) and Bredehoeft (1997) proposed that sustainable groundwater extraction should
be based upon groundwater discharge and not natural groundwater recharge. This remains
an important point today. However, many water agencies around the world (and in Australia)
are still struggling to evaluate environmental groundwater requirements.
Many define
sustainable yield in terms of the average recharge rate of an aquifer in order to balance longterm groundwater withdrawal and recharge for human use. However, these definitions fail to
account for the environmental goods and services provided by naturally occurring
groundwater discharge.
With increasing awareness of environmental water requirements and changing community
values, ultimately the concept of ‘sustainable yield’ is moving towards capturing the essence
of sustainability i.e. “development that meets the needs of the present without compromising
the ability of future generations to meet their own needs” (Brundtland Report 1987). This
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requires a holistic approach, which includes an evaluation of bio-physical parameters (e.g.
groundwater discharge, induced recharge and groundwater storage) as well as
environmental water requirements where water can only be extracted to meet ones needs if
it does not compromise the needs of future generations. Nevertheless the term remains
contentious and steeped in ambiguity.
An example of a large sedimentary basin with a low ‘sustainable yield’ relative to its surface
area is the GAB.
The GAB is one of the worlds largest aquifer systems storing
approximately 8 700 000 GL (NLWRA 2000). It predominantly recharges in the north east
where the primary aquifer, the Jurassic sandstones, outcrops. Approximately 600 GL/yr of
groundwater is extracted from the GAB, yet the system is considered to be fully developed
(NLWRA 2000).
Distributed across the entire basin the ‘sustainable yield’ volume (see
Figure 27) equates to less than 0.1 mm/yr (Warner 1986). This highlights that the stored
volume of water in an aquifer is relatively meaningless quantity, within the context of
sustainable development.
Figure 27 illustrates the sustainable yield of the major
groundwater provinces in GL / year.
It should be noted that different definitions of
sustainable yield have been used by the States and in some cases environmental provisions
have yet to be properly incorporated into these volumes (NLWRA 200021). In the North East
Queensland province large volumes of groundwater are found in local/intermediate scale
unconsolidated systems.
Figure 27
21
22
‘Sustainable’ yield of groundwater provinces.
Source:
(NLWRA 200022).
http://www.anra.gov.au/topics/water/pubs/national/water_app3.html
http://audit.ea.gov.au/anra/water/water_frame.cfm?region_type=AUS&region_code=AUS&info=availability
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Connectivity of surface water – groundwater systems
It is increasingly recognised that groundwater and surface water systems are interconnected
(Figure 28) and interchangeable where development of either resource affects the quantity
and quality of the other (e.g. Winter et al. 1998; Alley et al. 1999; Fullagar 2004).
Figure 28 Gaining, loosing streams. Source: Adapted from Alley et al. (1999).
Double accounting of water invariably results in economic losses to downstream users and
reduces the flows available to in-stream and estuarine ecological systems at some future
time.
The concept of connectivity between surface and groundwater systems is not new and has
been discussed in the academic literature for many decades (e.g. Glover and Balmer 1954;
Jenkins 1968) and prominent water resource publications. For example, the Anon. (1976)
review of Australia’s water resources states “In the past, groundwater and surface water
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have tended to be viewed as separate resources, as a result, no doubt, of inherent
differences in their modes of occurrence, assessment and development. Yet they are often
hydraulically connected, and in any event, are complementary components of a single large
system, the hydrologic cycle.
Thus in assessing the water resources of a region,
independent measurements of groundwater and surface water yield are not necessarily
additive”.
In some parts of Australia, warnings of the connectivity of surface and groundwater systems
(e.g. Anon.1976) have gone unheeded for many decades and as a result in the more
populous regions the same portion of water has in effect been allocated twice, once as
groundwater and again as surface water. This is commonly referred to as double accounting
or double allocation of water. In most states of Australia, groundwater and surface water
allocations had been made independently of one another, often by different departments or
agencies (Fullagar 2004).
As a result, in some parts of Australia where surface water
allocation have been capped, landholders could still obtain a groundwater licence in the
same surface water catchment (Evans 2005).
It has taken increased competition and scarcity of water in the southern States to highlight
the importance of understanding the temporal and spatial connectivity between surface and
groundwater systems to water managers, policy makers and water allocation planners.
Unfortunately because of the time lags associated with lateral groundwater flow trying to
retrospectively address these problems in a fair and equitable manner is both technically and
socially challenging. In doing so not only do catchment managers need to consider current
surface and groundwater use, they also need be mindful of impacts of the activation of
groundwater ‘sleeper’ licences (Fullagar 2004).
Due to variations in watertable elevation and hydrogeological heterogeneity, the connectivity
of particular reaches of a river to the underlying groundwater system may vary from being
well connected through to being poorly connected (e.g. Figure 29). Largely because of the
uncertainties associated with groundwater recharge, discharge and flow there is a growing
cautionary consensus that in the first instance and at the catchment scale it should be
assumed that 1 megalitre of groundwater should equal 1 megalitre of surface water, unless
proven otherwise (e.g. Fullagar 2004).
However, there may be mitigating factors.
For
example where vegetation usage of groundwater is high (e.g. in some parts of the arid and
tropical zones) this may considerably reduce the impact of groundwater pumping on stream
flow (Evans 2005), causing adverse impacts on any GDE.
The connectivity of surface and groundwater systems is not just confined to water quantity
issues but also quality/chemistry.
Groundwater chemistry and surface water chemistry
cannot be dealt with separately (e.g. see Winter et al. 1998).
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The issue of cross-
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contamination of surface and groundwater bodies is particularly prevalent to the irrigation
industry.
Figure 29 Connectivity of surface and groundwater systems at the reach and catchment
scale.
In the extreme situation in Figure 30 the impacts on downstream users would be very similar
to that if water were extracted directly from the river. However, where production bores are
situated further from the river, particularly in large intermediate and regional groundwater
systems, the effects may go un-noticed for many decades because of the time lags
associated with groundwater flow.
Even once the issue of double allocation has been
addressed, the impact may continue for many decades until the groundwater system has
been sufficiently replenished.
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Figure 30 Installing a groundwater production bore adjacent to a river (Photograph provided
by Dr Rick Evans, SKM). It should be noted that sometimes this practice is also motivated
by water quality issues, where the sediments in the streambank act as a natural filter.
The time lag between groundwater pumping and having an effect on a nearby surface water
body may vary from days to decades depending upon the nature of the groundwater flow
system and the distance of pumping from the river (Figure 31).
For example, in a
homogenous, unconfined aquifer, where groundwater flow occurs through the aquifer matrix,
the time delay is proportion to the square of the flow length (Evans 2005). In groundwater
systems where preferential flow occurs (e.g. paleochannels, jointing) the time lag may be
less. What is particularly important to note is that regardless of the distance of the bore from
the stream, for a given extraction the eventual impact on the quantity of water in the stream
is always the same.
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Figure 31 Diagram illustrating time lags associated with groundwater extraction and lateral
groundwater flow.
With many of northern Australia’s water resources largely unallocated for human use
(NLWRA 2000), the ‘North’ has a unique opportunity to learn from the successes and
mistakes made in the south of Australia and overseas. One clear lesson is that proper
management of the water resources of northern Australia requires a good understanding of
the spatial and temporal links between surface and groundwater systems.
In northern
Australia these linkages can be difficult to estimate owing to a combination of low
topographic relief (i.e. groundwater gradients may not necessarily be reflective of the current
topographic surface), complex hydrogeology, potentially strong transpiration component and
the scale of the North.
Where groundwater discharge does occur it can be a very important source of water to local
communities (e.g. Ngukurr community, NT; Jolly et. al. 2004) and ecosystems (e.g. O’Grady
et al. 2006) during the winter dry season. However, mapping subsurface hydrogeological
features controlling zones of localised discharge is extremely difficult. Not only are these
connections difficult to identify, but they are also difficult to quantify.
Consequently
information on the spatial, temporal and degree of connectivity of these surface water
expressions to the underlying groundwater systems is very sparse.
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4.3
Streamflow and runoff
Stream flow is comprised of surface runoff and groundwater discharge into the stream.
Because streamflow in many north Australian rivers is dominated by surface runoff, rather
than discuss the runoff term separately it is discussed here within the context of streamflow.
Relatively few studies have documented the streamflow and runoff characteristics of
catchments in northern Australia.
Most studies have focused primarily on fluvial
sedimentation and erosion (e.g. Prosser et al. 2002; Fielding et al. 2004), particularly with
respect to the geomorphologic regimes of northern estuaries (e.g. Coleman and Wright 1978;
Chappell 1993; Wolanski and Chappell 1996), sediment and nutrient delivery to the GBR
(e.g. Moss et al. 1992; Neil et al. 2002; Brodie and Mitchell 2005; McKergow et al. 2005) or
ecological assessment (e.g. Pusey and Arthington 1996; Erskine et al. 2003).
The
applicability of results from continental scale analysis of Australian and global streamflow
datasets to northern Australia is limited by a lack of flow data for this region (e.g. McMahon
1982; McMahon et al. 1987, Haines et al. 1988; Finlayson and McMahon, 1992; McMahon et
al. 1992; Dettinger and Diaz 2000; Peel et al. 2001, 2002a, 2004a).
For this reason Petheram et al. (2008) assembled a database of 99 rivers from across
northern Australia to assess their general flow characteristics with respect to development.
The discussion presented here draws heavily on the findings of their work. To put their
results into a broader context Petheram et al. (2008) compared their data with data from the
RoW23 for the same Köppen classes and data from southern Australia (i.e. represented by
Köppen classes Csa, Csb, Cfb, Cfc, BSh and BSk). To provide an appropriate basis for
comparison and wider interpretation of the results of their analyses, they stratified the results
by Köppen climate type. The 7 Köppen classes of northern Australia are shown along with
the 99 catchments of Petheram et al. (2008) in Figure 22 (the catchments are shown by their
stream gauging stations).
This section discusses key flow characteristics of rivers in northern Australia: duration of
flow; seasonality, runoff coefficients, inter-annual variability of flow, annual flow series, and
flow characteristics of relevance to water harvesting.
Duration of flow
Few rivers across northern Australia exhibit perennial behaviour (Figure 32).
Notable
exceptions are the rivers in the wet tropics region (Figure 32) of Drainage Division I, where
some rivers flow all year round (e.g. North and South Johnston, Herbert). In the wet-dry
tropics of northern Australia (Drainage Divisions VII, VIII, IX), many winter months without
rainfall result in most water courses being ephemeral in nature (Figure 32). In the database
assembled by Petheram et al. (2008), of the 99 rivers examined as part of their study, 80%
23
In this report, RoW incorporates rivers from all continents excluding those located in Australia.
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were ephemeral (or intermittent) in nature on the monthly time step; with more than 50%
having no flow for greater than 35% of the time. Petheram et al. (2008) also found that the
proportion of months with no flow (PMF=0) for northern Australia (median value of 35%) was
greater than the PMF=0 for southern Australia (median value of 0%) and for the three major
Köppen classes represented in northern Australia (Aw, BSh, BWh) from the RoW dataset
(median value of 0%).
In the wet-dry tropics, for rivers to exhibit perennial behaviour, groundwater inflow must be
greater than evaporative demand to sustain year round flow. In northern Australia, west of
the Gulf of Carpentaria this primarily occurs in those rivers in carbonate karstic
environments, namely the Katherine-Daly River (NT) and the Gregory and Lawn Hill Rivers
(Qld), although small base flows also occur in drainage lines in Cretaceous sandstones e.g.
Arnhem Land (Jolly and Chin 1991). Along the western side of Cape York the Pascoe,
Wenlock and Jardine Rivers, which are located within siliclastic sedimentary systems, are the
only rivers to exceed 1 ML/day during the dry season (Horn 1995). The Jardine, which
drains extensive Quaternary sand deposits, is of particular significance as it has the highest
baseflows of any river in Queensland (Horn 1995). Smaller perennial rivers include the
Archer, Holroyd and the upper reaches of the Palmer River. While there are numerous
estuaries along the coastal margins of northern Australia with permanent water, these are
largely brackish (following the ecological definition of an estuary) and in most parts of
northern Australia subject to large tidal inundation (see Section 2.3).
A number of streams and rivers in the north of Australia have reaches with permanent pools
of water or seeps during the dry season. These pools are usually disconnected and the
reaches do not flow. In most cases the existence of these waterholes appears to be due to
surface flows from the previous wet season, with little evidence of groundwater contribution
(Bunn et al. 2006). In the Lake Eyre Basin, Costelloe et al. (2007) found the persistence of
waterholes in to be primarily dependent upon the depth when flow ceased.
In the arid zone, streams and rivers are generally classified as being intermittent, that is they
may only flow every few years, largely a function of the high inter-annual variability in rainfall.
Nearly all rivers in the arid zone are losing, regardless of time of year. For example, in a
study of the Lake Eyre basin, Costelloe et al. (2006) found average transmission losses of
approximately 80% in mid-catchment reaches of the Cooper Creek and Diamantina River
during low to medium sized floods. However, it should be noted that infiltration of river water
through the banks and beds of surface waterbodies in arid and semi-arid zones (i.e. initial
losses) can be a very important form of recharge to underlying groundwater systems (Jolly
and Chin 1991). These localised zones of recharge are the few locations where water is
able to concentrate sufficiently to over-come evaporative demands. Hence the harvesting of
river water in these regions may have implications to underlying groundwater systems.
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Figure 32 Perennial (blue) and ephemeral (yellow) streamflow around Australia. Major
drainage division are illustrated by thin black line. Source: Geosciences Australia. Inset:
flow duration curves for 5 rivers representative of the major Köppen climate zones across
northern Australia; 1 – Köppen Am/Af; 2 – Köppen Aw in carbonate karstic setting; 3 –
Köppen Aw non-carbonate karstic setting; 4 – Köppen BSh; 5 – Köppen BWh.
Seasonality
When the rivers of northern Australia do flow, most of their flow occurs during a relatively
short period of time (Figure 33). In more than 90% of the stations examined by Petheram et
al. (2008) the mean flow during the peak 3 month period was greater than 59% of the mean
annual flow and in half of the stations examined the mean flow during the peak 3 month
period was greater than 80% of the mean annual flow. The three major Köppen classes
represented across northern Australia demonstrated little difference in the percentage of
mean annual flow that occurred during the peak 3 month period (Peak3m). The median
Peak3m for northern Australia (median value of 81%) was observed to be greater than the
median Peak3m for southern Australia (median value of 46%) and for the RoW (median value
of 59%) for the same Köppen class (Petheram et al. 2008).
The Am and Af Köppen zones of northern Australia (the Wet Tropics), that exist because of
orographic induced precipitation during the dry season, have a weaker seasonal streamflow
signal (median Peak3m of 55%, n = 3) than the three major Köppen classes represented
across the North.
However, it should be noted that their seasonal streamflow signal is
Hydrology of northern Australia: A review
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slightly stronger than that from Am and Af Köppen zones from RoW (median Peak3m of 42%;
n = 88).
Seventy-five percent of those gauging stations north of the Tropic of Capricorn (84) recorded
their highest flows during the January to March period. Anomalies are likely to be due to
sampling variability. The only region of consistent spatial difference was the Wet Tropics
region along the North Queensland coast, which appears to have highest flow during the
February to April period. These observations agree with Haines et al. (1988) who identified
14 annual flow regimes around the globe, two of which, the ‘extreme late Summer and ‘early
Autumn’ were identified as occurring in northern Australia. The latter of these two regimes
was confined to the ‘Wet Tropics’ region along the North Queensland coast.
Unlike the MDB, which with its large latitudinal extent encompasses a number of broad
climatic zones, the externally draining coastal divisions of northern Australia have relatively
short flow lengths with most rivers falling in a single climatic zone. These divisions are
largely located within a zone of tropical convergence, which typically have large seasonal
streamflow gradients (Dettinger and Diaz, 2000).
The size, shape, longitudinal range of Australia and the position of the GDR (i.e. along the
eastern seaboard), are likely to be secondary factors influencing the seasonality of
streamflow across most of the North and may explain why seasonal streamflow in the three
major Köppen classes of northern Australia are higher than similar Köppen classes from the
RoW (many stations of which are also located in zones of tropical convergence). During the
dry season, anti-cyclonic circulation centred over central Australia ensures that most of the
North is subject to dry, stable air. These anti-clockwise winds are readily heated as they
move across the large expanse of the hot, dry, flat continental interior, and play an important
role in evaporation and the removal of excess water (Gentilli 1986). The proximity of the
GDR to the north-east coast of Australia ensures that precipitation from tropical easterlies is
limited to the coastal escarpments and that easterly moving air is much drier on the inland
side of the GDR. As a consequence of these factors there is an absence of large permanent
water bodies and other long term natural storage (e.g. snow, glaciers) within the northern
interior of the continent, further compounding the strong seasonal climatic signal. The lack of
natural storage capacity within the northern Australian landscape results in minimal lag
between the peak precipitation month and the peak streamflow month (Peak1m). Dettinger
and Diaz (2000), observed a lag of less than 1 month for northern Australia (the minimum
threshold reported).
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Figure 33 Monthly specific discharge (monthly volume / catchment area) for selected rivers in northern Australia (left-right Jan-Dec). Pink arrows
indicate approximate gauge location. Background colour indicates surface elevation (high – brown; low – blue). White line indicates southern limit of
summer dominant flow regions proposed by Haines et al. (1988). Dashed black line indicates Tropic of Capricorn. Black lines indicate major
Drainage Division boundaries. White dotted line indicates the Wet tropics region. Streamflow gauges 8140010 and 912105 are located within
carbonate rock formations. Data averaged over a minimum of 10 years
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The highly seasonal nature of streamflow in northern Australia means that permanent
settlements and/or irrigation during the dry season will require storage structures if suitable
groundwater resources are absent.
Where adaptive water management practices are
employed and water allocations are based upon recent and predicted rainfall and streamflow
trends, the highly seasonal nature of streamflow in northern Australia may enable post wet
season water allocations to be granted with a relatively high degree of certainty.
Runoff coefficients
Australia is the world’s driest inhabited continent with the lowest annual runoff volumes, on
average 11% of rainfall (Table 3; Figure 34). The greatest annual volumes of runoff per
square kilometre occur along the narrow and relatively small humid zone of the wet tropics
region, North Queensland, where orographic uplift due to steep coastal escarpments
generates high rainfall totals and along the west coast of Tasmania (Division III).
The
internally draining, Drainage Division X and the zone of un-co-ordinated drainage, Drainage
Division XII, experience little to no runoff, except in those rare years where cyclonic
depressions extend considerable distances inland.
Figure 34 Mean annual runoff map of Australia (Source: NLWRA 200024) and the average
runoff coefficient (runoff / precipitation) for each drainage division (Roman numerals). Runoff
coefficients derived from the NLWRA 2000 rainfall and runoff data.
24
http://audit.ea.gov.au/anra/water/water_frame.cfm?region_type=AUS&region_code=AUS&info=availability
Hydrology of northern Australia: A review
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Runoff coefficients (i.e. the ratio of runoff to rainfall) across northern Australia vary from
almost 0% during the dry season over much of the arid and semi-arid in-land regions to over
80% in northern humid regions. In the tropics of North Queensland where the soil maintains
near saturated conditions during December to mid-June, Bonell et al. (1983) observed that
surface runoff occurs almost instantaneously during this period. The 99 streamflow gauging
stations used by Petheram et al. (2008) had a long term median runoff coefficient of 0.13.
Runoff events in the wet-dry tropics and arid regions of northern Australia appear to be highly
dependent upon the timing and intensity of rainfall.
For example in the Station Creek
2
catchment, (size 148 km ; Köppen Aw) approximately 30 km inland and west of Townsville,
on two consecutive (rain) years runoff coefficients were 0.4% and 19.5% for annual rainfalls
of 333 mm/year and 363 mm/year respectively (Post et al. 2006).
Inter-annual variability
Inter-annual variability of runoff in Australia has been a subject of great interest to scientists
and engineers as it has considerable ecological importance (e.g. Pusey and Arthington 1996)
and great significance to design of water storage (e.g. McMahon 1975) and conveyance
structures and the location of urban centres. Petheram et al. (2008) observed the variability
in runoff from rivers in northern Australia to be between 2 and 3 times that of rivers located in
similar Köppen climates from the rest of the world and between 10 and 30% greater than
southern Australia for the same mean annual runoff (Figure 35).
This finding is similar to the results from a series of studies over the last 30 years (e.g.
McMahon 1978, McMahon 1982, Finlayson and McMahon 1988, McMahon et al. 1987,
McMahon and Finlayson 1988, McMahon et al. 1992) that found there to be significant
differences between the variability in runoff between southern Australia and southern Africa
and the Rest of the World. More recent work by Peel et al. (2001, 2004) using databases of
global precipitation and runoff found that the variability of runoff in temperate Australia to be
significantly higher than for other continents in the Köppen climatic zones Csb, Cfa, Cfb.
Peel et al. (2001) suggested that not all of the observed variability in runoff in (southern)
Australia and Southern Africa (ASF) could be attributed to variability in precipitation. They
proposed that another key factor in the variability of runoff in ASF may be the prevalence of
evergreen vegetation in temperate zones of the southern hemisphere and deciduous
vegetation in temperate zones of the northern hemisphere.
Mechanisms for why the
variability in runoff may be greater under evergreen than deciduous vegetation are unclear,
but lysimetery (Penman 1967) and catchment water balance studies (Bosch and Hewlett
1982) suggest that actual evapotranspiration is greater from evergreen vegetation than
deciduous vegetation.
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10
North Australia
RoW
Southern Australia
-0.319
y = 3.40x
Annual Cv
-0.224
y = 2.68x
Mean annual runoff (mm)
1
0.1
0
1
10
100
1,000
10,000
-0.140
y = 0.718x
0.1
0
Figure 35 Coefficient of variation of annual streamflows versus mean annual runoff
comparing North Australian rivers (red symbol and line) with those for equivalent climate
zones from the Rest of the World (blue symbol and line) and with southern Australian river
(brown symbol and line) (without climate class differentiation) (The equations in the figure are
not weighted for record length.). Reproduced from Petheram et al. (2008).
Evergreen vegetation is also prevalent in northern Australia, which is apparently anomalous
to other wet-dry tropical regions of the world (Bowman and Prior 2005). The prevalence of
evergreen vegetation in northern Australia has been attributed to a lack of key nutrients
(namely phosphorous) in ‘ancient’ Australian soils and the high seasonality of rainfall,
resulting in vegetation that does not re-grow leaves annually (Bowman and Prior 2005).
The difference in Cv between northern Australia (e.g. Wet Tropics) and southern Australia
(e.g. Tasmania) at higher runoff is most likely due to different Köppen climates (i.e. different
rainfall patterns and evaporative demand). In regions with a high evaporative demand (e.g.
northern Australia) runoff generation is more dependent upon the timing and intensity of
rainfall than in regions with a low evaporative demand. Another factor that may potentially
contribute to the higher Cv in runoff in northern Australia than southern Australia in high
rainfall zones may be the prevalence of Aboriginal and natural burning in the North (Williams
1991): Annual grasses and herbaceous vegetation in the understorey of tropical savanna
ecosystems cover approximately two million km2 across northern Australia (Beringer et al.
2003).
The understorey of these savanna ecosystems, which have been measured to
contribute approximately 80% of wet season vapour flux (Hutley et. al. 2001) are particularly
prone to fire, it has been estimated that between 1990 and 1999 on average approximately
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300 000 km2 of northern savannas were burnt each year (Russell-Smith et al. 2003). A
reduction in transpiration from tropical savannas immediately following fire has been
observed in northern Australia (Beringer et al. 2003) and based upon observations of
increased water yield due to a reduction in transpiration elsewhere (Brown et al. 2005) it
seems reasonable that in northern Australia this too may result in an increase in water yield.
However, studies to date are inconclusive.
For example, in a ‘paired’ catchment study
spanning 3 years in Kakadu National Park, Townsend and Douglas (2000) could not identify
a discernable difference in water yield between an early burnt (area 18 km2), late burnt (area
6.7 km2) and unburnt catchment (area 6.6 km2).
Implications of high inter-annual variability for irrigation in the North
High inter-annual variability of streamflow is of great significance to the design of water
storage structures. If the discharge of rivers was constant or varied only within narrow limits,
there would be no need to construct large dams to regulate flow over the season or years.
Water storages may still be required to provide for variations in demand but would not have
to be as large as they would where variation in flow is high. The greater the variation in flow
from season to season and from year to year, the greater the amount of storage required to
reduce the fluctuations and to assure that the demand can be met at most times. For all
other factors held equal, the required storage capacity is approximately proportional to the
square of the variability in runoff (McMahon 1975).
Annual sequences of flow, persistence and drought
‘Lag-1 serial autocorrelation coefficient’ (ρ) is a measure of how consecutive flows are
correlated and is in effect a measure of persistence. In a positive autocorrelation series,
positive departures from the mean tend to be followed by positive departures from the mean
and negative departures from the mean tend to be followed by negative departures from the
mean. A negative autocorrelation series tends to have positive departures from the mean
followed by negative departures from the mean and vice versa.
Series with a zero
autocorrelation exhibit no correlation between consecutive values. Positive autocorrelation
series are also referred to as persistence and in hydrology is indicative of carry-over storage
in the landscape (e.g. snow melt). In hydrology there is no physical explanation for series
with a negative correlation between values and it may reflect short record lengths. In a
global study of 1221 rivers McMahon et al. (2007b) observed an inverse relationship
between lag 1 serial auto correlation and streamflow record length.
In their study of 99 rivers across northern Australia, Petheram et al. (2008) calculated that
72% of rivers exhibited a negative autocorrelation coefficient, most likely a consequence of
the short length of streamflow records. Of the 99 rivers, only 8 had ρ values that were
statistically different from zero at the 5% level (the expected value would be 5).
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Petheram et al. (2008) also examined three metrics of drought in northern Australia: drought
length (number of consecutive years of streamflow below the median), drought magnitude
(the largest negative deviation from the median in each ‘run’ of dry years) and drought
severity (the product of the drought length and the drought magnitude).
The details of their findings are presented elsewhere (Petheram et al. 2008). In summary,
drought length for northern Australia was found to be consistent with that of a first order
linear autoregressive model ((AR(1)) and hence they concluded that drought length in
northern Australia was not unusual compared with other similar parts of the world in any way.
Drought magnitude, was found to be greater than other parts of the world for the same
climate types. Petheram et al. (2008) observed that drought magnitude in northern Australia
was related to the coefficient of variability of runoff, a result consistent with Peel et al. (2005)
in their global analysis of runs of years of annual precipitation and runoff equal to or below
the median (i.e. drought). Because northern Australia has a considerably higher coefficient
of variability of runoff than other parts of the world for the same climate type, one may infer
that drought magnitude would also be greater.
Streamflow drought severity was found to be greater in northern Australia than similar
climatic regions of the world due to the high drought magnitude (see Peel et al. 2005 for
comparison by Köppen class) and ‘normal’25 drought length.
The implications of these
findings are that agricultural enterprises seeking to source water from rivers in northern
Australia should especially establish contingency measures for the relatively likely event of
severe drought.
Potential for water harvesting
There is growing interest in the concept of irrigation mosaics for northern Australia i.e.
patches of irrigation distributed across the landscape (Paydar et al. 2007; Cook et al. 2007).
Where there is no suitable groundwater, water for this form of irrigation development would
most likely be ‘harvested’ from rivers during suitable flow events and stored in on-farm
storages. Petheram et al. (2008) used Base Flow Index (BFI), Flow Duration Curves (FDC)
and Spells Analysis to gain insight into low flow characteristics of north Australian rivers so
as to assess the potential to harvest water for on-farm storage. FDC have already been
briefly discussed therefore this section provides an outline of BFI and Spells Analysis only.
Continuous daily stream flow data were used and aspects of these analyses are discussed
below.
25
Consistent with that of a first order linear autoregressive model (AR(1)
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Base Flow Index
BFI is a dimensionless index, defined as the volume of slow flow divided by the volume of
total flow (Nathan and McMahon 1992). It can be used to provide a measure of shape of the
hydrograph and the opportunity to harvest river flow.
The task of separating baseflow (sub-surface flow) from river discharge data has many
practical difficulties (Appleby 1970; Kirchner 2003) and a variety of methods exist (Nathan
and McMahon 1990; Chapman 1999; Eckhardt 2005). This study utilised the Lyne and
Hollick digital filter (Grayson et al. 1996). While the quick and slow flow responses resulting
from the application of this method have little physical reality (slow flow may be comprised of
both interflow and baseflow), the filter has been widely applied and there is a considerable
body of data available for comparative purposes (e.g. Lacey 1996).
An example of quick and slow flow separation is illustrated in Figure 36 for a river in the wetdry tropics draining carbonate rocks (A) and a river in the Arid zone (B). Chart A is indicative
of a river with a high natural storage and Chart B is indicative of a river with limited natural
storage.
Figure 36 Quick (blue line) and slow (pink line) flow response using the Lyne and Hollick
digital filter for a river in the wet-dry tropics (A) and a river in the arid zone (B) over a 3 month
period.
Petheram et al. (2008) separated quick and slow flow using the Lyne and Hollick digital filter
(Grayson et al. 1996) at 99 gauging stations across the north of Australia, the results are
illustrated in Figure 37. Based upon the data in this figure it can be seen that most rivers
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across northern Australia have a low BFI (i.e. < 0.4), which is indicative of short duration,
high intensity rainfall events, a lack of topographical relief (Lacey 1996) and rivers with
limited natural storage (e.g. wetlands) and a poor connection to the underlying groundwater
system26 (e.g. Chart B in Figure 36). Here stream flow is largely a function of surface runoff
and hence exhibits seasonal characteristics similar to that of the climate. Also plotted on this
figure are 330 catchments from southern Australia (data provided by Dr Francis Chiew,
CSIRO Land and Water).
1.00
SA (Koppen Cfa Cfb
Cfc Csa Csb)
Koppen Af & Am
0.80
Koppen Aw
Koppen BWh
Koppen BSh
0.60
BFI
Koppen Cwa
Koppen Cfa
0.40
0.20
0.00
1
10
100
Mean annual discharge (10^6m^3)
1000
10000
Figure 37 Baseflow Index plotted against mean annual discharge and stratified by Köppen
class. BFI data for southern Australia (SA) were provided by Dr Francis Chiew (pers. comm.
using data from Peel et al. 2000). This figure was reproduced from Petheram et al. (2008).
Spells Analysis
Spells Analysis (Nathan and McMahon 1990) is used to characterise the nature of periods of
flow above and/or below a certain threshold (i.e. ‘a spell’) and Spells Analysis indicators are
commonly used to describe the environmental flow regimes/requirement of rivers and to
compare natural and regulated flow regimes (e.g. SKM 2005). Common indices include:
mean Spell Number (average number of low/high flow spells), mean Spells Duration (mean
length of low/high flow spell); and Spells Interval (measure of time between high and low flow
spells).
In the absence of consensus on appropriate ecological flow indicators (Arthington et al.
2006) and a paucity of information on meaningful ecological threshold values for northern
Australia, Petheram et al. (2008) chose a threshold value of zero and adopted the intent of
26
It should be noted that many rivers across northern Australia have wetlands in their lower reaches.
However, gauging stations are often located above these to avoid the affects of tides, which are large
across much of the North.
Hydrology of northern Australia: A review
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current Northern Territory approach that at least 80% of the flow in any part of a river should
be allocated to the environment. An additional restriction on water extraction was adopted in
that water can only be extracted from the falling limb of a hydrograph and hence ‘a Spell’ is
registered after each peak in the hydrograph and lasts until the next increase in flow or until
flow ceases.
Variations to this restriction have been recommended by studies on
environmental flow requirements in northern Australia (e.g. Erskine et al. 2003) and
elsewhere.
Petheram et al. (2008) found that Mean Spell duration varies little between Köppen classes.
However, mean Spell frequency and mean annual extraction decreases with increasing
aridity. Median mean annual extraction (Em) under the previously mentioned restrictions for
the Aw, BSh and BWh Köppen classes were found to be 28, 4 and 1 mm/year depth
equivalent respectively. Based upon these values and the following assumptions;
•
irrigation water requirements (Irrreq) of 10 (Aw), 15 (BSh) and 20 ML / ha (BWh) per
year;
•
an average dam depth (Dd) of 4 m;
•
net evaporative loss (Evapdepth) of 0.7 (Aw), 1.3 (BSh) and 1.8 m / yr (BWh) and
ignoring conveyance losses;
Equations 5 and 6 can be used to approximate the catchment area (CA) (ha) required to
generate sufficient runoff to irrigate 1 ha of land using on-farm dams.
CA =
Irrreq × 1 + Evapvol
(5)
E m × K1
where K1 is a unit conversion factor, which in this case is equal to 10 000 (to convert units to
ha) and Evapvol (ML) is the volume of water lost to evaporation and is equal to;
Evapvol =
Irrreq × 1
Dd − Evap depth
× Evap depth
(6)
Contributing areas of approximately 45, 560 and 3640 ha are required to generate sufficient
runoff to irrigate 1 ha of land in Köppen zones Aw, BSh and BWh respectively. Based upon
the above assumptions the required storage areas (Sarea) (ha) are equivalent to 30% (Aw),
56% (BSh) and 91% (BWh) of the irrigated area (Equation 7).
S area =
Irrreq + Evapvol
Dd × K 2
(7)
Where K2 is a unit conversion factor, which in this case is equal to 10 (to convert units to ha).
The quantity of water that can be extracted from the falling limb of a hydrograph expressed
as a proportion of the volume under the hydrograph (Vr) has a strong linear relationship with
the logarithm of BFI and this is illustrated in Figure 38. Restricting water extraction to the
falling limb of a hydrograph has greatest implications to those catchments with a small BFI,
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where events of short duration and short, steep recession ‘limbs’ present little opportunity to
harvest flow.
With many catchments across northern Australia having low BFI values,
hypothetical regulated yields would be seriously constrained in many regions under rules that
restricted water extraction to the falling limb of a hydrograph.
1
0.8
Vr
0.6
0.4
0.2
0
0.001
0.01
0.1
1
BFI
Figure 38 Proportion of the total volume that can be extracted from the falling limb of a
hydrograph (Vr) plotted against BFI for north Australian data. This figure was reproduced
from Petheram et al. (2008).
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4.4
Quantity of exploitable surface water by drainage division
Approximately 60% of Australia’s runoff is generated in northern Australia, the vast majority
in Drainage Divisions I, VIII and IX (Figure 39). It is not possible to use all of this water. With
the exception of the steeply draining catchments east of the Great Dividing Range (Drainage
Division I), northern Australia has subdued relief with relatively few opportunities for large
carry over storages27. Where it is possible to site large storages the proportion of streamflow
that can be allocated with a high degree of certainty for human uses is constrained by the
high inter-annual variability of runoff (see Section 4.3).
Figure 39 Mean annual runoff map of Australia (Source: NLWRA 200028) and the
percentage of Australia’s runoff that occurs in that particular drainage division (Roman
numerals). Data were sourced from the NLWRA 2000.
Here we employ simple water balance techniques to provide first cut estimates of the area of
land that could theoretically be irrigated in different parts of Australia (APot), based solely on
the 3 major components of the water balance, rainfall, evaporation and surface runoff. To
evaluate APot estimates of: 1) the irrigation demand (IrrD); and 2) the potential yield (YPot) are
required as shown in Equation 8:
27
Carry-over storages are storages that are sufficiently large to carry over water from one year to the
next and hence help mitigate the effects of drought.
28
http://audit.ea.gov.au/anra/water/water_frame.cfm?region_type=AUS&region_code=AUS&info=availability
Hydrology of northern Australia: A review
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APot =
YPot
IrrD
(8)
Estimates of irrigation demand for each major drainage division were sourced from Section
3.2 and these were used to estimate the amount of land that could potentially be irrigated
based solely upon the quantity of exploitable water (Note that this estimate does not account
for the actual availability of soil/land). These calculations of APot are summarised in Table 3
and illustrated in Figure 40.
Although physically possible, these theoretical ‘potential’ areas of irrigated land would not be
sustainable, because they do not account for environmental, cultural, or other human water
requirements, nor do they consider other factors upon which sustainable irrigation is
dependent (e.g. suitable soil, economics, crop physiology). While these estimates of YPot
and APot can be considered as providing an absolute upper bound to the amount of water and
land that could be irrigated, discussion centred on the relative values between regions is
likely to be more useful than discussion centred on the absolute values.
Spatial and temporal scale of analysis
In this report waterbalance calculations were undertaken for each of the 12 major drainage
divisions of Australia on a monthly basis. These spatial and temporal scales were selected
because of the availability of consistent datasets, convenience of reporting (i.e. it facilitates
north versus south comparisons), and because these scales were deemed commensurate
with the potential errors in the input data and the simplistic nature of the calculations.
Exploitable yield
In the absence of more recent data for all of Australia, the percentage of water that could
potentially be exploited (E%) from each of the major drainage divisions was made by using
estimates from a review of Australia’s water resources in 1975 by the Australian Water
Resources Council (Anon. 1976). In this report values of E% are provided for each major
drainage division. Australian and State government water authorities made these estimates
of E% at the point of lowest practical downstream development, for each river basin in
Australia and these were aggregated up to the major drainage division scale.
These
estimates attempted to take into account factors including: average annual flow, variability of
flow, water quality and the availability of suitable sites for storage. They did not take into
account economic, social, cultural or environmental considerations. From the authors review
of the literature these are the most readily available and recent estimates of E% available for
all of Australia. The potential yield was calculated as per Equation 9:
YPot = R × E %
(9)
Remarkably, in all drainage divisions except the Gulf of Carpentaria, the estimates of surface
runoff made by the 1975 review of Australia’s water resources (Anon. 1976), were within
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±10% or ± 1000 GL of those made by the 2000 National Land and Water Resources Audit
(NLWRA 2000). Hence it was deemed appropriate to use the E% made by the 1976 review
in-conjunction with the more recent NLWRA runoff estimates (although in reality it would
have made little difference which runoff values were used).
In the case of the Gulf of
Carpentaria drainage division, the NLWRA (2000) estimates of surface runoff are
approximately twice that of the 1976 review.
As a result, for this drainage division two
estimates of YPot and APot have been made. For the first estimate it is assumed that the small
E% is primarily due to unfavourable flow characteristics and high evaporation (i.e. storage is
not the limiting factor). Hence E% is simply applied to the larger NLWRA surface runoff value
for this division. For the second case it was assumed that the small E% is primarily because
of limited storage capacity in the Gulf of Carpentaria drainage division. In this case, despite
the upwardly revised estimate of surface runoff for this division by the NLWRA, the YPot is
equivalent to the Anon. (1976) surface runoff estimate (i.e. half that of the NLWRA). These
two estimates effectively form an upper and lower bound to the actual YPot value for the
division, although in all likelihood the actual YPot value will lie somewhere between these two
estimates.
Results and discussion
Although actual volumes of water and potential areas of irrigation are provided in Table 3, the
relative values between the divisions and northern and southern Australia are a more useful
discussion point. This is because the actual volumes and areas in Table 3 act as an upper
bound and are highly unlikely to be attainable. For example, in Table 3 the theoretical
potential area of irrigation in the MDB is 80% larger than the existing area (18 000 km2;
Bryan and Marvaneck 2004), yet many parts of the MDB are widely recognised as being over
allocated (under stress). Similarly, while Table 3 suggests that the water resources of the
Timor Sea drainage division could support an irrigated area of about 18 500 km2, these
calculations ignore transmission losses, assume optimal irrigation practice and do not
consider the water requirements of other users (e.g. urban, industry, mining, the environment
etc) or other factors that may constrain sustainable irrigation (e.g. suitable land and soil,
economics, etc).
In the following discussion where comparisons are made between northern and southern
Australia these are based upon northern Australia being comprised of a proportion of the
following divisions:
Northern Australia = 4/5 * D1 + 2/3 * D7 + D8 + D9 + 2/3 * D10 + 1/4 * D12
where D1, D7, D8, D9, D10 and D12 correspond with drainage divisions I, VII, VIII, IX, X, XII
respectively. Incidentally the results are very similar if northern drainage divisions I, VIII and
IX are compared to the southern drainage divisions II, II, IV and VI.
Hydrology of northern Australia: A review
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Approximately 60% of Australia’s runoff is generated in northern Australia. However, only
about 20-24% of this runoff can be feasibly exploited for human use. As a result 40% of
Australia’s total YPot is located in northern Australia. Expressing this volume of potentially
exploitable water as an area under irrigation, between 22 and 25% of Australia’s APot could
be located in northern Australia. The implicit assumption here is that it is economically viable
to transfer water from Tasmania to Victoria. If the Tasmanian drainage division is ignored on
the basis that there is insufficient land area (Column 11 in Table 3) and it is not viable to
transfer water across Bass Strait to Victoria, the APot in northern Australia is approximately
35% of Australia’s APot.
An alternative to in-river carry-over storages is on-farm dams, where one or several dams
may service one or more patches of irrigation. Large on-farm dams may require the use of
‘active’ or ‘passive’ water harvesting techniques29. In areas of northern Australia with low
relief, or for large irrigation developments, ‘active’ water harvesting (which involves extraction
of river water during high flow events and storing it for use during the dry ’winter’ months)
may be necessary. This is done either by pumping water using large capacity low head
pumping plants, or (where topography is favourable) using gravity diversions to divert flow
from the river into the storage. The suitability of the rivers in northern Australia for water
harvesting is broadly discussed in Section 4.3. However, no data are currently available at
the continental scale to evaluate the potential of this method in each drainage division.
29
Water harvesting is “the process of collecting natural precipitation from watersheds for beneficial
use” (Courrier 1973)
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Table 3 Potential exploitable surface water yield. Northern drainage divisions are shown in bold text.
Drainage
Division No.
Drainage Division
Name
Drainage
Mean annual
2
(‘000 km )
Potential
Annual
Runoff
average
yield (E%) as
(P)
(R)
runoff
a % of total
(GL)
(GL)
coefficient
surface
rainfall
Area
Mean annual
(Rc)
2
Source of data
3
4
Anon. 1976
NLWRA 2000
1
3
yield
6
NLWRA 2000
Columns 5/4
#
irrigation
Potential
Potential
irrigated area
demand
(YPot)
(APot)
(IrrD)
(mm/year)
7
8
9
10
Anon. 1976
Column 5 *
Section 3.4
4
APot expressed
irrigated area
as a ratio of
(% drainage
APot of MDB
5
division area)
2
(km )
runoff
5
Representative
(GL)
(%)
Column No.
Potential
2
7
#
Column 8/9
11
#
Column 10/3
12
#
Colum 10/10
#
I
North East Coast
451
367 454
73 411
0.2
34
25 000
700
35 900
7.8
1.11
II
South East Coast
274
238 035
42 390
0.18
43
18 500
280
64 700
24
2.01
III
Tasmania
68
91 156
45 582
0.5
71
32 500
360
91 000
134
2.82
IV
Murray-Darling
1 063
510 801
23 850
0.05
83
20 000
610
32 300
3.0
1.00
V
South Australian
82
27 658
952
0.03
31
500
750
400
0.5
0.01
Gulf
VI
South West Coast
314
137 960
6 785
0.05
37
2 500
580
4300
1.4
0.13
VII
Indian Ocean
519
152 025
4 609
0.03
13
500
1210
500
0.1
0.02
VIII
Timor Sea
547
505 233
83 320
0.16
22
18 500
1000
18 300
3.3
0.57
IX
Gulf of Carpentaria
638
489 787
95 615
0.2
21
20 000
1060
19 000
3.0
0.59
(9500)
(1.5)
(0.295)
(10 000)
X
Lake Eyre
1 170
264 250
8 638
0.03
4
500
1130
300
0.03
0.01
XI
Bulloo-Bancannia
101
26 671
546
0.02
0
0
1180
0
0
0.00
XII
Western Plateau
2 455
660 258
1 486
0.00
0
0
990
0
0
0.00
Total
7 680
3 471 289
390 000
0.11
35
138 000
N/a
266 700
3.5
8.27
(257 500)
(3.4)
(128 000)
Footnotes overleaf
Hydrology of northern Australia: A review
85-of-111
#
Footnotes to Table 3
1. Calculated using mean annual rainfall data from the NLWRA 2000 data library.
2. E% were obtained from a review of Australia’s water resources in 1975 (Anon. 1976). These
estimates take into account average annual flow, variability of flow, water quality and the
availability of suitable sites for storage, but do not take into account economic social, cultural
or environmental considerations.
3. YPot was calculated using the NLWRA 2000 mean annual runoff estimates and the E% given in
Column 7. This was deemed acceptable because in all Drainage Divisions, except Division IX
(Gulf of Carpentaria) the NLWRA 2000 runoff estimates were within 10% or 1000 GL of Anon.
(1976) runoff estimates. In Division IX estimates of mean annual runoff increased by 64%
between 1975 and 2001. For this Drainage Division two values have been given. The first is
the YPot using the NLWRA (2000) runoff data and the E% from Anon. (1976) exploitable yield
percentage. The second value (in brackets) is the Anon. (1976) YPot estimate for this Division
made in 1976 based on 1976 estimates of runoff (assumes this Division is storage limited).
These two values effectively provide a lower and upper bound.
4. APot does not factor in land availability or soil suitability, environmental, social or cultural flow
considerations.
5. An anomaly occurs in the Tasmanian drainage division where the area that could potentially
be irrigated (as calculated and summarised in Table 3) is in excess of the total land area of the
drainage division.
# Figures have been rounded for clarity of presentation.
Figure 40 Rainfall, runoff and useable yield volumes for each major drainage division and
area that could potentially be irrigated based upon water resources, climate and carry-over
storage only. Data sourced from Table 3.
Hydrology of northern Australia: A review
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5 Concluding remarks
In a review of past developments in northern Australia, Woinarski and Dawson (1997)
commented that there was “a pattern of general disregard for information and scant concern
for environmental consequences of success (or failure)” and that there was a perception that
the environment in the north of Australia was “so extensive and of so little value that little
safeguard needs to be built into development proposals”. Changing community values and
recent policy initiatives (e.g. COAG 1994; NWI 2004; NPWS 2007) have now shifted
community and government focus to water efficiency, full cost recovery, water trading,
separating water rights from land title, integrated water resource management and
acknowledgement of the environment as a legitimate user of water. Consequently there is
now a need for information to support the introduction of these policy reforms and enable
decisions regarding irrigation in northern Australia to be made with the best available
information.
Climate
In northern Australia high evaporation rates and high seasonality and intensity of rainfall limit
the range of dryland cropping more so than in the South. The almost total absence of rainfall
during the dry (winter) months in northern Australia means that irrigation is essential for
cultivated agriculture or perennial horticulture during this period.
Even where there is
sufficient moisture during the wet season to overcome evaporative demands, high rainfall
intensities can severely constrain agricultural operations through erosion, and heavy
machinery can cause direct damage to the seed bed or soil structure.
At many centres across northern Australia (e.g. Fitzroy Crossing, Darwin, Weipa, Katherine
and Tennant Creek) rainfall during the past few decades was considerably greater than the
long term average. Decisions about irrigation development in northern Australia that are
based on the recent rainfall record may lead to an overestimate of the long-term water
resource.
It is estimated that perennial pastures will require between 20 – 80% more irrigation water in
the drainage Divisions in northern Australia than in the MDB (ignoring conveyance and
seepage losses).
Surface water
Approximately 60% of Australia’s runoff is discharged from northern Australia. However,
most rivers in northern Australia have little to no flow during the dry season, which means
that storage of surface water (or the use of groundwater) is essential for irrigation. Even
before economic, environmental and cultural values are considered, the available data
suggest that only about 20-25% of the water discharging from rivers in northern Australia can
potentially be exploited because of the flow characteristics of rivers in the region and the
Hydrology of northern Australia: A review
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limited carry-over storage opportunities.
This volume of ‘exploitable’ water represents
approximately 40% of Australia’s total potential exploitable volume.
Higher evaporative
demands in northern Australia ensure that for a given crop (i.e. similar crop factor), crop
water use and irrigation demand is greater than in southern Australia. Ignoring other factors
that act as constraints to sustainable irrigation (e.g. availability of suitable land and soils,
market factors, and climatic variables that control crop physiology and human comfort), if the
volume of ‘exploitable’ water in the North is expressed as an irrigated area and taking into
account the higher irrigation demands in northern Australia, then approximately 20 and 25%
of Australia’s total potential irrigation could be located in northern Australia. From a water
resources perspective, there is potential for further irrigation development in the north of
Australia, but these results suggest that efforts towards achieving and maintaining
sustainable irrigation practices in the South are central to assuring Australia’s long term
irrigation future.
Groundwater
Considerable groundwater resources occur in some Quaternary unconsolidated sediments
and large sedimentary basins in the North. The largest of these is the GAB, which is already
fully committed (NLWRA 2000). Some of the best prospects for development of groundwater
resources in the North lie in the Dolomitic carbonates of the Daly and Georgina Basins.
However, these groundwater systems have strong connectivity with surface drainage
features to which they provide year round baseflow (e.g. Daly and Roper Rivers). While
these few perennial rivers may be of particular interest to irrigation investors and developers,
these rivers support ecological communities that are dependent upon the quantity, quality
and timing of these groundwater flows.
Because most rivers in northern Australia are
ephemeral, these perennial rivers have high ecological significance.
Any extraction of
groundwater from these systems will most likely result in a reduction in streamflow at some
point in time. The impacts of these reductions and whether those impacts are acceptable is
a key management question.
The management of local and some intermediate groundwater systems in the wet-dry tropics
of northern Australia appears to have several advantages over their management in
temperate Australia.
Firstly, the timing of the onset of the wet season in the North is
relatively predictable. Secondly, once the northern wet season has ended, groundwater
levels are known and there is little likelihood of further recharge until the following wet
season.
In local and some intermediate groundwater systems this would enable
groundwater allocation decisions for the dry season to be made with relative certainty
(assuming the discharge characteristics of the groundwater system are known).
In
unconfined surficial groundwater systems where storage capacity is consistently exceeded
each wet season (i.e. too much recharge), groundwater allocation decisions may vary little
Hydrology of northern Australia: A review
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between years, providing water users (environment and/or human) with a relatively high
degree of long-term certainty from one year to the next.
In the semi-arid and arid zones of northern Australia catchment average recharge rates are
very low (Section 4.1). ‘Sustainable’ irrigation in these areas would require recharge areas
several orders of magnitude greater than the irrigated area. This means that the large, often
topographically driven, recharge areas required to support aquifers that can supply
commercial quantities of water, are often of intermediate or regional scale and have very
long flow paths (i.e. recharge and discharge zones may be many hundreds or thousands of
kilometres apart).
Long time lags exacerbate the difficulties in ‘sustainably’ managing
aquifers receiving little recharge. Difficulties associated with geological heterogeneities and
subdued relief means that often there is a poor understanding of the connection between
recharge and discharge zones. Where groundwater systems discharge in semi-arid zones
they often provide an essential source of water to surrounding flora and flora. In these arid
zones it may be very difficult to maintain existing ecological values if groundwater resources
are developed.
Key knowledge gaps and challenges
The design of irrigation for tropical systems needs targeted research if irrigation is to be
developed and practiced in a sustainable manner in northern Australia. Important design
topics include:
•
Water harvesting.
The feasibility of water harvesting operations has not been
assessed at the regional scale across the North.
How environmental flow
requirements and the highly seasonal and variable flows in northern Australia may
limit water harvesting operations should be investigated.
•
Irrigation mosaics. Mosaic designs have received little attention in the literature. If
surface water is to be stored in northern Australia without large storages, on-farm
dams will be required. It is likely that multiple small storages will result in a greater
preference for mosaic style developments rather than water storage in large carry
over schemes.
However, there is currently little information on the hydrological,
environmental, social, economic or cultural benefits and costs of irrigation mosaics.
•
Drainage management in regions with large water table fluctuations. Experience in
northern irrigation schemes, e.g. the Ord (Smith et al. 2006) and Lower Burdekin
(Petheram et al. 2006), illustrate that failure to manage deep drainage accessions will
result in irrigation-induced salinity. Future irrigation development in the North should
require sub-surface drainage management (Petheram et al. 2008b). In the wet-dry
tropics of northern Australia groundwater levels may seasonally fluctuate by as much
as 10 m (Jolly and Chin 1991; Cook et al. 1998). These seasonal fluctuations are
considerably greater than those experienced in irrigation districts in southern
Hydrology of northern Australia: A review
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Australia. For sustainable irrigation in these systems, suitable subsurface drainage
technology/design will need to be developed and used.
Other hydrology related knowledge gaps include:
•
Paucity of data on streamflow and rainfall data (Figure 41)
•
Groundwater – surface water connectivity
•
Assessment of the hydrological needs of the environment
•
The impacts of climate and land use change (including burning) on tropical water
balances
•
Information on groundwater recharge and flow
•
Detailed water resource assessment.
•
Understanding soil /landscape properties, function and response across northern
Australia.
This information is fundamental to helping governments and communities decide if, where
and what type of irrigation they may want in northern Australia. The relatively small number
of players in northern Australia (compared to the situation in the South) provides a unique
opportunity for collaboration and to plan proactively and not be reactive to problems and
failures.
Figure 41 - Rainfall (red) and stream gauging (blue) stations. Rainfall stations illustrated
here have more than 50 years data that is greater than 80% complete (Source BoM).
Stream gauging stations were sourced from the NLWRA 2000. Note many gauging stations
shown here (particularly in northern Australia) are no longer currently operational, do not
have an established rating curve and/or have only a couple of years of continuous record.
Hydrology of northern Australia: A review
90-of-111
6 References
Abelson PH (1999) A potential phosphate crisis. Science 26 March 1999: Vol. 283. no. 5410, p. 2015.
Access Economics (2005) Measuring the economic and financial value of the Great Barrier Reef
Marine Park. Access Economics Pty Ltd. Great Barrier Reef Marine Park Authority Research
Publication
No.
84.
Retrieved
20
July,
2007,
from
http://www.gbrmpa.gov.au/corp_site/about_us/documents/economic_values_report.pdf
Airey PL, Bentley H, Calf GE, Davis SN, Elmore D, Gove H, Habermehl MA, Phillips F, Smith J,
Torgersen T (1983) Isotope hydrology of the Great Artesian Basin, Australia. In Papers of the
International Conference on Groundwater and Man, Sydney, 1983.
Australian Water
Resources Council, Conference Series 8(1), 1-12.
Allen AD (1997)
Groundwater the strategic resource.
A geological perspective of groundwater
occurrence and importance in Western Australia. Geological Survey of Western Australia.
Allen RG, Pereira LS, Raes D. Smith M (1998) Crop evapotranspiration - Guidelines for computing
crop water requirements - FAO Irrigation and drainage paper 56. FAO - Food and Agriculture
Organization of the United Nations, Rome, 1998.
Alley WM, Reily TE, Franke OL (1999) Sustainability of ground-water resources. US Geological
Survey Circular 1186.
Appleby VC (1970) Recession and baseflow problem. Water Resources Research 6(5), 1398-1403.
Anon (1961) Ground-water management. Manual of Engineering Practice 40. American Society of
Civil Engineering.
Anon (1976). Review of Australia’s water resources 1975.
Department of National Resources.
Australian Water Resources Council. Australian Government Publishing Service, Canberra
1975.
Anon (1982)
Agriculture.
Atlas of Australian Resources.
Volume 3.
Third Series.
Division of
National Mapping, Canberra 1982. Commonwealth of Australia.
Anon (1986) Climate. Atlas of Australian resources. Third Series. Volume 4. Division of National
Mapping, Canberra, 1986 Commonwealth of Australia
Anon (1990) Australia. Evolution of a continent. BMR Palaeographic group. Australian Government
Publishing Service.
Anon (1987) Hydrogeology of Australia. Bureau of Mineral Resources. BMR Bulletin 227.
Anon (1998) Climate of the northern Territory. Bureau of Meteorology. June 1998.
Anon (2003)
Draft conservation plan for the Daly Basin bioregion. Department of Infrastructure,
Planning and Environment. Northern Territory Government. August 2003.
Anon (2004a) Securing the North: Australia’s tropical rivers. Published by WWF Australia 2004.
Hydrology of northern Australia: A review
91-of-111
Anon. (2004b)
Water Account, Australia.
Australian Bureau of Statistics.
Commonwealth of
Australia.
Anon (2005) Australia’s tropical rivers. Program plan and prospectus 2005-2010. Land and Water
Australia.
Arthington AH, Bunn SE, Poff NL, Naiman RJ (2006) The challenge of providing environmental flow
rules to sustain river ecosystems. Ecological Applications 16, 1311-1318.
Arunakumaren NJ, Bajracharya K, McMahon GA (2001) The Burdekin Delta Groundwater Model.
Water Assessment & Planning Natural Resource Science. Natural Resources and Mines.
Beringer J, Hutley LB, Tapper NJ, Coutts A, Kerley A, O’Grady AP (2003) Fire impacts on surface
heat, moisture and carbon fluxes from a tropical savanna in northern Australia. International
Journal of Wildland Fire, 12, 333-340.
Blanch S, Rea N, Scott G (2005) Aquatic conservation values of the Daly River Catchment, Northern
Territory, Australia. WWF- Australia 2005. pp 30.
Boggs S Jr. (2001). Principles of sedimentology and stratigraphy. Third Edition. Prentice Hall. New
Jersey, USA.
Bond W (1998) Soil physical methods for estimating recharge. Part 3 of the Basics of recharge and
Discharge (Ed Zhang). CSIRO Publishing.
Bonell M, Gilmour DA, Cassells DS (1983) Runoff generation in tropical rainforests of northeast
Queensland, Australia, and the implications for land use management.
In ‘Hydrology of
Humid Tropical Regions’ (Ed Reiner Keller). IAHS Publication No. 140.
Bosch JM, Hewlett JD 1982. A review of catchment experiments to determine the effect of vegetation
changes on water yield and evapotransipration. Journal of Hydrology 55: 3-23.
Bowman DMJS, Prior LD (2005) Turner Review No. 10. Why do evergreen trees dominate the
Australian seasonal tropics? Australian Journal of Botany 53, 379-399.
Bredehoeft JD (1997) The water budget myth. Ground Water 35(6):929.
Bredehoeft JD, Papadopoulos SS, Cooper HH Hr (1982) The water budget myth. In: Scientific basis
of water resource management, studies in geophysics.
National Academy Press,
Washington, DC, pp 51-57.
Brodie JE, Mitchell AW (2005)
Nutrients in Australian tropical rivers: changes with agricultural
development and implications for receiving environments. Marine and Freshwater Research
56, 279-302.
Brown AE, Zhang L, McMahon TA, Western AW, Vertessy RA (2005) A review of paired catchment
studies fro determining changes in water yield resulting from alterations in vegetation. Journal
of Hydrology 310, 28-61.
Brundtland Report (1987) Our common future. World Commission on Environment and Development.
Hydrology of northern Australia: A review
92-of-111
Bryan SE, Ewart A, Stephens CJ, Parianos J, Downes PJ (2000) The Whitsunday volcanic Province,
Central Queensland, Australia: lithological and stratigraphic investigations of a silic-dominated
large igneous province. Journal of Vulcanology and Geothermal Research 99, 55-78.
Bryan BA, Marvanek S (2004) Quantifying and valuing land-use change for ICM evaluation in the
Murray-Darling Basin 1996/97 – 2000/01. CSIRO Land and Water Stage 2 Report for the
Murray-Darling Basin Commission.
Budyko MI (1974) Climate and life, Academic, New York, 1974.
Bunn S, Davies P, Donaldson J, Douglas M, Garnett S, Olley J (2006) On TRACK for Better River
and Coastal Management in northern Australia: Introducing a new Research Initiative. In
‘Proceedings of Australian national Committee on irrigation and drainage’.
National
Conference and Exhibition Darwin 15th to 18th October.
Chapman T (1999) A comparison of algorithms for stream flow recession and baseflow separation.
Hydrological Processes 13, 701-714.
Chapman A, Basinski J (1985). Coastal Plains research station. In ‘The northern Challenge. A
history of CSIRO crop research in northern Australia’ (Eds. J Basinski, I Wood, J Hacker).
Published by CSIRO Division of Tropical Crops and Pastures.
Chappell J (1993)
Contrasting Holocene sedimentary geologies of lower Daly river, northern
Australia, and lower Sepik-Ramu, Papua New Guinea. Sedimentary Geology 83, 339-358.
COAG (1994) Attachment A – Water resource policy. Council of Australia Governments. Retrieved
from http://www.coag.gov.au/meetings/250294/attachment_a.htm on June 2006.
Cole J, Fairbanks R, Shen G (1993). Recent variability in the southern oscillation: isotopic results from
a Tarawa atoll coral. Science, 260, 1790-1973.
Coleman JM, Wright (1978) Sedimentation in an arid macrotidal alluvial river system: Ord River,
Western Australia. Journal of Geology, 86, 621-642.
Cook, F.J., Xevi, E., Knight, J.H., Paydar, Z. and K.L. Bristow. 2008. Analysis of biophysical processes
with regard to advantages and disadvantages of irrigation mosaics. CSIRO Land and Water
Science Report No. ??/08, CRC for Irrigation Futures Technical Report No. 09/07 64 pp.
Cook P, Hatton TJ, Pidsley D, Herczeg AL, Held A, O’Grady A, Eamus D (1998) Water balance of a
tropical woodland ecosystem, northern Australia: a combination of micro-meteorological, soil
physical and groundwater chemical approaches. J. Hydrol., 210:161-177.
Cook P.G., Herczeg A.L. and McEwan K.L (2001) Groundwater recharge and stream baseflow,
Atherton Tablelands, Queensland. CSIRO Land and Water, Tech. Rep. 08/01.
Coram J, Dyson PR, Houlder PA, Evans WR (2000) Australian flow systems contributing to dryland
salinity. National Land and Water Resources Audit.
Costelloe JF, Shields A, Grayson RB, McMahon TA (2007) Determining loss characteristics of arid
zone river waterbodies. River Research and Applications 23, 715-731.
Hydrology of northern Australia: A review
93-of-111
CSIRO (2007) CSIRO, Australian Bureau of Meteorology. Climate change in Australia: technical
report 2007. CSIRO. 148 pp.
Dalrymple RW, Zaitlin BA, Boyd R (1992) Estuarine facies models: conceptual basis and stratigraphic
implications. Journal of sedimentary Petrology 62, 1130-1146.
Dawes WR, Gilfedder M, Walker GR, Evans WR (2001) Biophysical modelling of catchment scale
surface and groundwater response to land-use change. In ‘Proceedings of MODSIM 2001’.
(Eds F Ghassemi, P Whetton, R Little and M Littleboy). Pp. 535-540. (Modelling and
Simulation Society of Australia and New Zealand: Canberra).
Davidson BR (1965) The northern myth: a study of the physical and economic limits to agricultural
and pastoral development in tropical Australia. Melbourne: Melbourne University Press.
Davidson BR (1969) Australia wet or dry? The physical and economic limits to the expansion of
irrigation. Melbourne University Press.
Davies JL (1986) The Coast. In ‘The Natural Environment’ (ed. DN Jeans).Sydney University Press,
Sydney, 1986.
Delane RJ (1987)
Climate of the Ord River Irrigation Area Western Australia. Division of Plan
Research. Western Australia Department of Agriculture.
Dettinger MD, Diaz HF (2000)
Global characteristics of stream flow seasonality and variability.
Journal of Hydrometeorology, 1 289-310.
Drinkwater KF, Frank KT (1994) Effects of river regulation and diversion on marine fish and
invertebrates. Aquatic Conservation Freshwater and Marine Ecosystems 4, 135-151.
Eamus D, Hatton T, Cook P, Colvin C (2006) Ecohydrology. Vegetation function, water and resource
management. CSIRO Publishing, Collingwood, Australia.
Eckhardt K (2005) How to construct recursive digital filters for baseflow separation. Hydrological
Processes 19, 507-515.
Erskine WE, Begg GW, Jolly P, Georges A, O’Grady A, Eamus D, Rea N, Dostine P, Townsend S and
Padovan A (2003) Recommended environmental water requirements for the Daly River,
northern Territory, based on ecological, hydrological and biological principles. Supervising
Scientist Report 175. (National Riverhealth Program, Environmental Flow Initiative, Technical
Report 4), Supervising Scientist, Darwin NT.
Evans R (2005) Groundwater surface water interaction. Groundwater surface water interactions in the
tropics. northern Territory branch of IAH Australia workshop, Darwin 26-27 May 2005.
Fabricius KE (2005) Effects of terrestrial runoff on the ecology of corals and coral reefs: review and
synthesis. Marine Pollution Bulletin 50, 125-146.
Fielding CR, Trueman, J, Alexander J (2004). Sedimentology of the modern and Holocene Burdekin
River Delta of north Queensland, Australia – Controlled by river output, not by waves and
tides. In ‘ Deltas’ (eds J. Bhattacharya and L. Giosan), SEPM Special Publication.
Hydrology of northern Australia: A review
94-of-111
Fink J, Kukla GJ (1977) Pleistocene climates in Central Europe: at least 17 interglacials after the
Olduvai event. Quaternary Research 7, 363-371.
Finlayson BL, McMahon TA (1988) Australia v the World: A comparative analysis of Streamflow
Characteristics. In ‘Fluvial geomorphology of Australia (Ed. Robin F. Warner). Academic
Press.
Finlayson BL, McMahon TA (1992): Global Runoff. In: W.A. Nierenberg (Ed.), Encyclopaedia of Earth
Systems Science (Academic Press Inc., Florida, USA) 2: 409-422.
Fitzpatrick EA (1986).
An introduction to soil science.
Second edition. Long Man Scientific &
Technical; New York 255p.
Fullagar I (2004)
Rivers & Aqufiers: Towards conjunctive water management.
Workshop
Proceedings: Adelaide, 6-7 may 2004.
Galloway WE (1975) Process framework for describing the morphologic and stratigraphic evolution of
deltaic depositional systems.
In: Broussard ML, editor. Deltas, models for exploration.
Houston, TX: Houston Geological society, 1975. p. 87-98.
Gehrke P (2005) Sustainable futures for Australia’s tropical rivers: introduction to current research
and information needs. Marine and Freshwater Research, 56, v-vii.
Gehrke P, Bristow KL, Bunn S, Douglas M, Edgar B, Finlayson M, Hamilton S, Loneragan N, Lund M,
Pearson R, Prosser I, Robson C (2004). Sustainable Futures for Australia’s Tropical Rivers: A
strategy for developing research directions for Australia’s tropical river systems. CSIRO Land
and Water Technical Report No 17/04.
Gentilli J (1986) Climate. In ‘The Natural Environment. Australia – A geography Volume One’. (Ed. D
N Jeans). Sydney University Press.
Gillanders BM, Kingsford MJ (2002) Impact of changes in flow of freshwater on estuarine and open
coastal habitats and the associated organisms.
Oceanography and Marine Biology: an
Annual Review, 40, 233-309.
Glover RE, Balmer CG (1954)
Stream depletion resulting from pumping a well near a stream.
American Geophysical Union Transactions 35(3), 168-470.
Grayson RB, Argent RM, Nathan RJ, McMahon RA, Mein RG (1996) hydrological recipes. Estimation
Techniques in Australian Hydrology. Cooperative Research Centre for Catchment Hydrology.
Haines AT, Finlayson BL, McMahon TA (1988) A global classification of river regimes. Applied
Geography, 8, 255-272.
Hanson CL, Kuhlman AR, Erickson CJ, Lewis JK (1970) Grazing effects on runoff and vegetation on
western South Dakota rangeland. J. Range Management 23, 418-420.
Harrington GA, Cook PG, Herczeg AL (2002) Spatial and temporal variability of groundwater recharge
in central Australia: a tracer approach. Ground Water 40: 518-527.
Hydrology of northern Australia: A review
95-of-111
Harris PT, Heap AD (2003) Environmental management of clastic coastal depositional environments:
inferences from an Australian geomorphic database. Ocean & Coastal Management 46, 457478.
Harris PT, Heap AD, Bryce SM, Porter-Smith R, Ryan DA, Heggie DT (2002).
Classification of
Australian clastic coastal depositional environments based upon a quantitative analysis of
wave, tidal and river power. Journal of Sedimentary Research 72, 858-870.
Heap AD, Bryce S, Ryan DA (2004) Facies evolution of Holocene estuaries and deltas: a largesample statistical study from Australia. Sedimentary Geology 168, 1-17.
Hobbs JE (1998) Present climates of Australia and New Zealand. In: Climate of the Southern
Continents, Present, Past and Future. (Eds.), Hobbs, J. E., Lindesay, J. E. and Bridgman, H.
A., John Wiley & Sons, Chichester, 63-105.
Holmes JW, Sinclair JA, (1986) Water yield from some afforested catchments in Victoria. Hydrology
and Water Resources Symposium, Griffith University, Brisbane, 25-27 November 1986. The
Institution of Engineers, Australia.
Hook RA, Williams K (1998) Dryland farming regions and systems of Australia. In ‘Farming action
catchment reaction’ (Eds J Williams, RA Hook, HL Gascoigne)
(CSIRO Publishing:
Melbourne).
Horn AM (1995) Surface water resources of Cape York Peninsula. Cape York Peninsula Land Use
Strategy, Office of the Co-ordinator General and the Department of Primary industries,
Government of Queensland, Brisbane and the Department of Environment Sport and
Territories, Canberra.
Horn AM (2000) An Assessment of the water resources of Cape York Peninsula and its application in
natural resource management in the context of environmental change. PhD thesis. James
Cook University.
Horn AM, Derrington EA, Herbert GA, Lait RW, Hillier JR (1995) Groundwater resources of Cape York
Peninsula. Cap York Peninsula Land Use Strategy.
Office of the Co-ordinator General
Queensland, Brisbane. Department of Environment, Sport and Territories, Canberra.
Queensland
Department
of
Primary
Industries,
and
Australian
Geological
Survey
Organisation, Mareeba.
Hubble GD, Isbell RF, Northcote KH (1983) Features of Australian soils. In ‘Soils an Australian
viewpoint’. Division of Soils. p 17. (CSIRO: Melbourne/Academic Press: London.)
Humphreys WF (2006) Aquifers: the ultimate groundwater-dependent ecosystems. Australian Journal
of Botany, 54, 115-132.
Hutley LB, O’Grady AP, Eamus D (2001) Monsoonal influences on evapotraspiration of savanna
vegetation of northern Australia. Oecologia 126, 434-443
IPCC (2001) Climate change 2001: The scientific basis. Contribution of working group I to the Third
Assessment Report on the Intergovernmental Panel on Climate Change. Published for the
Intergovernmental Panel on Climate Change. Cambridge University Press.
Hydrology of northern Australia: A review
96-of-111
IPCC (2007). Intergovernmental Panel on Climate Change Fourth Assessment Report – Climate
Change 2007 Retrieved from http://www.ipcc.ch/#.
Isbell (2002) The Australian Soil Classification. Revised Edition. Australian soil and land survey
handbook Series Volume 4. CSIRO Publishing 152pp.
Jackson IJ (1986) Relationships between rain days, mean daily intensity and monthly rainfall in the
tropics. Journal of climatology 6:117-134.
Jackson IJ (1988) Daily rainfall over northern Australia: deviations from the world pattern. Journal of
Climatology 8:463-476.
Jackson S (2004) Preliminary report on Aboriginal perspectives on land-use and water management
in the Daly River region, northern Territory. A report to the northern Land Council.
Jenkins CT, 1968. Techniques for computing rate and volume of stream depletion by wells. Ground
Water 6(2), 37-46.
Jennings JN, Bird ECF (1967)
Regional geomophological characteristics of some Australian
Estuaries. Estuaries: Geomorphology. 121-128.
Jennings JN, Mabbutt JA (1986) Physiographic outlines and regions. In ‘The Natural Environment’
(ed DN Jeans). Sydney University Press, Sydney 1986.
Johnson DP (2004) The Geology of Australia. Cambridge University Press.
Jolly PB, Chin DN (1991) Long term rainfall – recharge relationships within the northern Territory,
Australia – the foundations for sustainable development. International Hydrology and Water
Resources Symposium. Perth 2-4 October 1991 p 824-829.
Jolly P, Knapton A, Tickell S (2004) Water availability from the aquifer in the Tindal Limestone South
of the Roper River.
Report No. 34/2004D
Department of Infrastructure, Planning and
Environment. northern Territory Government.
Jones P, Osborn T, Briffa K (2001) The evolution of climate over the last millennium. Science 292,
662-667.
Kalf F and Woolley D (2005)
Applicability and methodology of determining sustainable yield in
groundwater systems. Hydrogeology Journal 13, 295-312.
Kelly JH (1966) Struggle for the North. Australasian Book Society. Sydney 1966.
Kirchner JW (2003)
A double paradox in catchment hydrology and geochemistry.
Hydrological
Processes 17, 871-874.
Köppen W (1936), Das geographisca System der Klimate. In Köppen W & Geiger G (Eds.), Handbuch
der Klimatologie, 1. C. Gebr, Borntraeger, 1–44.
Lacey GC (1996) Relating baseflow to catchment properties. A scaling approach. Cooperative
Research Centre for Catchment Hydrology Report 96/8.
Lee CH (1915) The determination of safe yield of underground reservoirs of the closed basin type.
Trans Am Soc Civil Eng 78:148-151.
Hydrology of northern Australia: A review
97-of-111
Leeper GW (ed) (1970)
The Australian environment, CSIRO and Melbourne University Press,
th
Melbourne, 4 ed.
Littleboy M, Silburn D, Freebairn D, Woodruff D, Hammer GL (1989)
PERFECT: a computer
simulation model of productivity erosion runoff functions to evaluate conservation techniques.
QDPI, Brisbane.
Marshak S (2005) Earth. Portrait of a planet. Second Edition. WW Norton & Company. New York.
McBride JL, Nicholls N (1983) Seasonal relationships between Australian rainfall and the Southern
Oscillation. Monthly Weather Review 111, 1998-2004.
McJannet D, Wallace J, Reddell P (2007) Precipitation interception in Australian tropical rainforests:
II. Altitudinal gradients of cloud interception, stemflow, throughflow and interception.
Hydrological Processes, 21, 1703-1718.
McKenzie N, Jacquier D, Isbell R, Brown K (2004) Australian soils and landscapes. An illustrated
compendium. CSIRO Publishing.
McKergow LA, Prosser IP, Hughes AO, Brodie J (2005) Sources of sediment to the Great Barrier
Reef World Heritage Area. Marine Pollution Bulletin 51, 200-211.
McMahon TA (1975) Variability, persistence and yield of Australian Streams. Hydrology Symposium
1975, Inst. Engrs. Aust., pp.107-111.
McMahon TA (1978)
Hydrological characteristics of Australian streams, Monash University Civil
Engineering Research Report 3/1979).
McMahon TA (1982).
World hydrology: Does Australia fit? Hydrology and Water Resources
Symposium 1982, Institution of Engineers Australia, Nat. Conf. pubn. No. 82/3, 1-7.
McMahon TA, Finlayson BL (1988). Australia versus the World. A comparative analysis of streamflow
characteristics. In Fluvial Geomorphology of Australia, R.F. Warner (ed.) Academic Press, pg
17-40.
McMahon TA, Finlayson BL, Haines AT, Srikanthan R (1987): Runoff variability: a global perspective.
IASH-AISH 168: 3-11.
McMahon TA, Finlayson BL, Srikanthan R, Haines AT (1992): Global Runoff: Continental
Comparisons of Annual Flows and Peak Discharges. (Catena Verlag, Cremlingen-Destedt,
West Germany), 166 pp.
McMahon TA, Murphy R, Little P, Costelloe JF, Peel MC, Chiew FHS, Hayes S, Nathan R & Kandel
DD, (2005) Hydrology of Lake Eyre Basin. Report prepared for Department of Environment
and Heritage. Pages 139.
McMahon TA, Vogel RM, Peel MC, Pegram GGS (2007) Global hydrology – Part 1 Characteristics of
annual and monthly streamflows. Journal of Hydrology In review.
Meinzer OE (1920) Quantitative methods of estimating groundwater supplies. Bull Geol Soc Am 31.
Meinzer OE (1923) Outline of groundwater hydrology, with definitions, US Geol Surv Water Supply
Pap 494.
Hydrology of northern Australia: A review
98-of-111
Milliman JD, Syvitski PM (1992) Geomorphic/tectonic control of sediment discharge to the ocean: the
importance of small mountainous rivers. The Journal of Geology; 100, 525-544.
Milly PCD, Dunne KA, Vecchia AV (2005) Global pattern of trends in streamflow and water availability
in a changing climate. Nature 438, 347-350.
Moore WS, Sarmiento JL, Key RM (1988) Tracing the Amazon component of surface Atlantic Water
using 228Ra, salinity and silica. Journal of Geophysical Research, 91, 2574-2580.
Morton FI (1983) Operational estimates of areal evaporation and their significance to the science and
practice of hydrology. Journal of Hydrology 66, 1-76.
Moss AJ, Rayment GE, Reilly N, Best EK (1992) A preliminary assessment of sediment and nutrient
exports from Queensland coastal catchments, Queensland Department of Environment and
Heritage, Technical Report 5, Brisbane, Queensland government 27p.
Nathan RJ, McMahon TA (1990) Evaluation of automated techniques for base flow and recession
analyses. Water Resources Research Vol 26, No. 7 1465-1473.
Nathan RJ, McMahon TA (1992) Estimating low flow characteristics in ungauged catchments, Water
Resource Management 6, 85-100.
Neelin JD, Battisti DS, Hirst AC, Jin F-F (1998) ENSO theories. J. Geophy Res 103: 14261-14290
Neil DT, Orpin AR, Ridd PV, Yu BF (2002) Sediment yield and impacts from river catchments to the
Great Barrier Reef lagoon: Marine and Freshwater Research 53, 733-752.
Nicholls N (1988) El Niño-Southern Oscillation and rainfall variability, Journal of Climate. 1: 418-421.
NLWRA
(2000)
Australian
water
resources
assessment
2000.
Retrieved
from
http://audit.ea.gov.au/anra/water/docs/national/Water_Contents.html on 12 February 2006.
NLWRA (2002)
Australia’s natural resources 1997-2002 and beyond.
National Land & Water
Resources Audit. Commonwealth of Australia 2002.
NPWS (2007) National Plan for Water Security. Department of Environment, Water, Heritage and the
Arts. Retrieved 12 January 2007,from http://www.environment.gov.au/water/action/npws.html
NRM (2005) The Great Artesian Basin. Natural Resources and Mines. Retrieved April 4, 2006, from
www.nrm.qld.gov.au/water/gab
NWI (2004) National Water Initiative. Council of Australian Governments meeting 25th June 2004.
Retrieved June 2006 from http://www.coag.gov.au/meetings/250604/index.htm
O’Grady A, Eamus D, Cook P, Lamontage S (2006) Groundwater use by riparian vegetation in the
wet-dry tropics of northern Australia. Australian Journal of Botany, 52, 145-154.
O’Grady PA, Eamus D, Hutley LB (1999) Transpiration increases during the dry season: patterns of
tree water use in eucalypt open-forests of northern Australia. Tree Physiology 19, 591-597.
Ollier CD (1986) Early landform evolution. In ‘The Natural Environment’ (ed. DN Jeans).Sydney
University Press, Sydney, 1986.
Hydrology of northern Australia: A review
99-of-111
Parkinson G ed. (1988) Geology and minerals. Atlas of Australian resources: third series Volume 5
Geology and Minerals. 59p.
Paydar, Z., Cook, F.J., Xevei, E. and K.L. Bristow. 2007. Review of the current understanding of
irrigation mosaics. CSIRO Land and Water Science Report No. 40/07, CRC for Irrigation
Futures Technical Report No. 08/07. 38 pp
Peel MC, Finlayson BL, McMahon TA, (2007) Updated world map of the Köppen-Geiger climate
classification. Hydrological and Earth Systems Sciences Discussions 4: 439-473.
Peel MC, McMahon TA, Finlayson BL (2004a): Continental differences in the variability of annual
runoff – update and reassessment. Journal of Hydrology 295: 185-197.
Peel MC, McMahon TA, Finlayson BL, (2002b). Variability of annual precipitation and its relationship
to the El Niño-Southern Oscillation. Journal of Climate. 15(5): 545-551.
Peel MC, McMahon TA, Finlayson BL, Watson FGR (2001): Identification and explanation of
continental differences in the variability of annual runoff. Jour. Hydrology. 250: 224-240.
Peel MC, McMahon TA, Finlayson BF Watson FGR (2002a): Implications of the relationship between
catchment vegetation type and annual runoff variability. Hydrological Processes 16: 29953002.
Peel MC, McMahon TA, Pegram GGS (2005). Global analysis of runs of annual precipitation and
runoff equal to or below the median: Run magnitude and severity. International Journal of
Climatology, 25: 549-568.
Penman HL (1967)
Evaporation from forests: a comparison of theory and observation.
In
International Symposium on Forest Hydrology, Sopper WE, Lull HW (eds). Pergamon Press:
Oxford; 373-380.
Petheram C, McMahon TA, Peel MC (2008) Flow characteristics of rivers in northern Australia:
implications for development. (Submitted to Journal of Hydrology).
Petheram C, Walker G, Grayson R, Thierfelder T, Zhang L (2002) Towards a framework for predicting
impacts of land-use on recharge: 1. A review of recharge studies in Australia. Australian
Journal of Soil Research, 40, 397-417.
Post DA, Loong D, Dowe J, Hodgen M, Keen R, Hawdon A, Petheram C, Burrows D, Faithful J (2006)
Ecological Monitoring of the Townsville Field Training Area (TFTA) 2005/06. CSIRO Land and
Water and ACTFR Client Report to Department of Defence, November 2006
Prosser, I. P., Moran, C. J., Scott, A., Rustomji, P., Stevenson, J., Priestly, G., Roth, C. H. and Post,
D. (2002)
Regional patterns of erosion and sediment transport in the Burdekin River
catchment. Technical Report 5/02. CSIRO Land and Water , Canberra.
Pusey BJ, Arthington AH (1996) Streamflow variability within the Burdekin River basin, Queensland:
implications for in-stream flow assessments. 23rd Hydrology and Water Resources
Hydrology of northern Australia: A review
100-of-111
Robins JB, Halliday IA, Staunton-Smith J, Mayer DG, Sellin MJ (2005) Freshwater-flow requirements
of estuarine fisheries in tropical Australia: a review of the state of knowledge and application of
a suggested approach. Marine and Freshwater Research 56, 343-360.
Rodbell D, Seltzer G, Anderson d, Abbott M, Enfield D, Newman J (1999). An 15000 year record of El
Nino driven alleviation in southwestern Ecuador. Science 283, 516-520.
Russell-Smith J, Yates C, Edwards A, Allan GE, Cook GD, Cooke P, Craig R, Heath B, Smith R
(2003) Contemporary fire regimes of northern Australia, 1997-2001: change since Aboriginal
occupancy, challenges for sustainable management. International Journal of Wildland Fire,
12, 283-297.
Schreiber P (1904) Uberdie Beziehungen zwischen dem Niederschlag und der Wasserfhrung der
Flusse in Mittleeuropa, Meteorologische Zeitungen 21 (1904), pp. 441–452.
Shuttleworth WJ (1993), Evaporation Chapter 4. In: D.R. Maidment, Editor, Handbook of Hydrology,
McGraw-Hill Inc, New York (1993).
SKM (2005) Environmental flow recommendations for the Upper Avoca River System. Final Report.
Smith AJ, Pollock DW, Salama RB, Palmer D (2006) Groundwater Management Options to control
rising groundwater level and salinity in the Ord Stage 1 Irrigation Area. CSIRO Land and
Water Client Report. February 2006.
Sophocleous M (1992) Groundwater recharge estimation and regionalization: the Great Bend Prairie
of central Kansas and its recharge statistics. Journal of Hydrology 137, 113-140.
Stauffacher M, Walker G, Evans R (1997) Salt and water movement in the Liverpool Plains – What’s
going on? Occasional Paper 14/97. Land and Water Resources Research and Development
Corporation, Canberra.
Stuart (1945) Groundwater resources of the Lansing area. Mich Dept of Cons, Geolo Surv Div Rep
no. 13.
Sumner G, Bonell M (1986) Circulation and daily rainfall in the North Queensland wet seasons 19791982. Journal of Climatology 6: 531-549.
Taylor G (1958) Australia. Methuen & Co Ltd. London.
Tickell SJ (2002) Groundwater resources of the Oolloo dolostone.
Department of Infrastructure
Planning and Environment. Natural Resources Division. Report 17/2002.
Todd DK (1959) Groundwater hydrology. Wiley, New York.
Townsend SA, Douglas MM (2000) the effect of three fire regimes on stream water quality, water yield
and export coefficients in a tropical savanna (northern Australia). Journal of Hydrology, 229,
118-137.
Tucker ME (2001) Sedimentary Petrology. 3rd edn., Blackwell, Oxford, 2001, pp 262.
Tsonis A, Hunt A, Elsner J (2003).
On the relation between ENSO and global climate change.
Meteorological Atmospheric Physics 84, 229-242.
Hydrology of northern Australia: A review
101-of-111
Tudhope A, Chilcott C, McCulloch M, Cook E, Chappell J, Ellam R, Lea D, Lough J, Shimmield G
(2001) Variability in the El Nino-Southern Oscillation through a Glacial-Interglacial Cycle.
Twidale CR, Campbell EM (1995) Australian landforms: Structure, process and time. Gleneagles
publishing. 568p.
USGS (2007) Phosphate Rock US Geological Survey, Mineral Commodity Summary, January 2007.
Retrieved
from
http://minerals.usgs.gov/minerals/pubs/commodity/phosphate_rock/phospmcs07.pdf on the 4
January 2008.
Warner RF (1986) Hydrology. In ‘The Natural Environment’. Australia – a geography Volume one
(Ed. DN Jeans). Sydney University Press.
Williams J, Bui EN, Gardner EA, Littleboy M, Probert ME (1997) Tree clearing and dryland salinity
hazard in the Upper Burdekin catchment of North Queensland. Australian Journal of Soil
Research, 35, 785-801.
Williams MAJ (1991) Evolution of the landscape. In ‘Monsoonal Australia. Landscape, ecology and
man in the northern lowlands’ (Eds CD Haynes, MG Ridpath, MAJ Williams). A.A. Balkema
Publishers, Rotterdam, Netherlands.
Williams RJ, Duff GA, Bowman DMJS, Cook GD (1996) Variation in the composition and structure of
tropical savannas as a function of rainfall and soil texture along a large-scale climatic gradient
in the NT, Australia. Journal of Biogeography 23, 747-756.
Wilson D, Cook P, Hutley L, Tickell, S, Jolly P (2006) Water quality in the Dally River. A multidisciplinary management approach. northern Territory Department of Natural Resources the
Environment and the Arts, Technical Report No. 18/2006D.
Winter TC, Harvey JW, Franke OL, Alley WM (1998) Ground water and surface water. A single
resource. US Geological Survey Circular 1139.
Woinarski J, Dawson F (1997) Limitless lands and limited knowledge: coping with uncertainty and
ignorance in northern Australia. In Ecology, Uncertainty and Policy: managing ecosystems for
sustainability. (eds.) Norton, T.W., Handmer, J.W. & Dovers, S.R. Cambridge University
Press, Cambridge.
Wolanski E, Chappell J (1996) The response of tropical Australian estuaries to a sea level rise.
Journal of Marine Systems 7, 267-279.
Woodroffe CD (1993) Late Quaternary evolution of coastal and lowland riverine plains of Southeast
Asia and northern Australia: an overview. Sedimentary Geology, 83, 163-175.
Zhang L, Walker GR (Eds) (1998), Studies in Catchment Hydrology - The Basics of Recharge and
Discharge, CSIRO Publishing, 1998.
Zhang L, Dawes WR, Walker GR. (2001) Response of mean annual evapotranspiration to vegetation
changes at catchment scale. Water Resources Research 37: 701–708.
Hydrology of northern Australia: A review
102-of-111
Zhang, L., Dawes, W.R., Walker, G.R. (1999)
Predicting the effect of vegetation changes on
catchment average water balance. CRC for Catchment Hydrology Technical Report, 99/12.
1999.
Hydrology of northern Australia: A review
103-of-111
Appendix 1
Summary of key points
This report reviews the hydrologic aspect of northern Australia’s landscapes. This review has
an emphasis on assessing the suitability of these landscapes for development of irrigated
agriculture. Key hydrological factors, constraints and opportunities for irrigation in northern
Australia are highlighted and it is anticipated that this report will provide a broad knowledge
base that will enable all stakeholders the opportunity to partake in debate over the future of
irrigation in northern Australia.
Particular emphasis has been placed on illustrating the
differences between water systems in northern Australia and those found in temperate
southern Australia; to which most Australians are familiar. This report has been written for
readers with a general scientific background. The key points from the report are summarised
below.
Geology and geomorphology
•
In this report, northern Australia is defined as that area north of the Tropic of
Capricorn (23.5o S) encompassing approximately forty percent of Australia’s land
mass.
•
Because Australia has experienced recent glaciations or volcanism, many of the
landscapes are very old and, with age, have become flat.
The antiquity of the
landscapes is often cited as the reason for the relative infertility of many of Australia’s
soils.
•
Twelve major drainage basins characterise the Australian continent. Half are partly
or entirely located within northern Australia.
The major rivers of the three major
drainage divisions of northern Australia (Timor Sea, Gulf of Carpentaria and North
East) are externally draining and have short flow lengths relative to rivers in the
internally draining divisions and the Murray Darling Basin. The three major drainage
divisions of northern Australia are largely located in the wet-dry tropic climate zone.
•
The Great Dividing Range which runs along the east coast of Australia imparts an
east-west distinction on Australia’s soil and water resources.
Soils
•
Australia has a highly complex soil pattern.
•
Despite the contemporary climatic difference between northern and southern
Australia, identifying north-south, regional-scale soil distinctions is difficult because
many of Australia’s soils have been exposed to a variety of climates over geologic
time.
One of the main differences between the soils of northern Australia and
Hydrology of northern Australia: A review
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southern Australia is the modified physical structure and chemical composition of
southern soils caused by cultivation and the application of fertilisers.
•
A strong regional -scale distinction in northern Australia’s soils occurs east and west
of the Great Dividing Range.
basalts are generally fertile.
Along the Divide soils derived from the Cainozoic
Fluctuating sea level and marine transgression and
regression during the Pleistocene caused the short, steep streams east of the Divide
to rejuvenate, stripping the old soils and depositing ‘fresh’ sediments from which new
soils formed. West of the Great Dividing Range there are extensive areas of deeply
weathered mantle that have been preserved for millions of years. Many of these
deeply weathered profiles have been almost completely leached of essential plant
nutrients.
•
As in southern Australia, many regions of northern Australia are deficient in key
nutrients in the soil and intensive cultivated agriculture will require fertilizer additives.
•
In north and north-western Australia, tidal processes dominate estuary evolution and
fine textured sediments (i.e. silt and clay, often referred to as marine muds or
estuarine clays) are typically deposited adjacent to the main channel during high tide
events. The estuarine clay soils of these sub-coastal plains, which are reportedly
superficially similar to soils in the (labour-intensive) rice growing regions of south-east
Asia (Woodroffe 1993), were found to be unsuited for mechanical cultivation in the
Adelaide River region. In addition to the trafficability difficulties that the estuarine
clays present, strong tidal activity in the adjacent river channel and seasonal and
prolonged flooding from rainfall present additional hazards to mechanised farming.
Flora and Fauna
•
Australia separated from Antarctica during the early Tertiary (approximately 100
million years ago). As a consequence of its isolation, Australia has unique flora and
fauna with a very high degree of endemism30 and intrinsic value. The deep-rooted
vegetation in northern Australia is predominantly evergreen and differs in this respect
from other wet-dry tropical regions of the world.
Australian vegetation has also
developed a number of other unique adaptations.
Climate
•
Rainfall across the north of Australia is considerably more seasonal than that of
southern Australia.
This is primarily due to the position and orientation of the
Australian continent within the global circulatory system.
•
Evaporation is very high in northern Australia exceeding 3000 mm per year in many
places.
30
Endemism is the ecological state of being unique to a place. Endemic species are not naturally
found elsewhere (Wikipedia 2007)
Hydrology of northern Australia: A review
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•
Northern Australia can be considered to have three broad climatic zones: wet-dry
tropics (Köppen Aw), semi-arid zone (Köppen BSh) and arid zone (Köppen BWh).
Along the north-east Queensland coast, orographic uplift of moist easterly winds
creates a distinct wet tropical zone (Köppen Af and Am).
•
The semi-arid zone in the Northern Territory extends into regions with average annual
rainfall of up to 800 mm. In southern Australia, the semi arid zone corresponds with
areas that receive less than 400 mm per year, largely because evaporation rates are
less.
•
Stand evapotranspiration rates in northern Australia are 2-18 times greater during the
Wet season than during the Dry season. Most of this seasonal difference is caused
by transpiration from annual grasses and herbaceous plants in the understorey.
•
In the wet-dry tropics most rain falls from December to March, with some northern
centres recording over 90% of their annual rainfall during this period.
•
On average, tropical cyclones produce 30% of the rain during January to March in
most centres in northern Australia, and up to 50% in more arid locations like Port
Headland and Broome.
•
Northern Australia’s predominantly convective rainfall-generating weather systems do
not penetrate far inland. As a result, rainfall is greatest near the coast and decreases
rapidly with distance inland.
•
Northern Australia has some of the largest daily rainfall intensities in the world. Large
rainfall intensities can severely constrain agricultural operations, particularly where
the disturbance of soil vegetative cover by heavy machinery disrupts the seed bed or
soil structure.
•
The importance of cyclonic depressions as rain-generating weather systems and the
influence of the El Nino – Southern Oscillation across the east and north of Australia
is reflected by large inter-annual variability in rainfall.
•
At many centres across northern Australia (e.g. Fitzroy Crossing, Darwin, Weipa,
Katherine and Tennant Creek) rainfall during the past few decades was considerably
greater than the long term average.
Decisions about irrigation development in
northern Australia that are based on the recent rainfall record may lead to an
overestimate of the long-term water resource.
•
The irrigation water demand for a perennial pasture is larger in northern Australia
than in southern Australia. For example, a perennial pasture grown in the three major
northern Australian drainage divisions (North-East Coast, Timor Sea and Gulf of
Carpentaria) would require between 20 and 80% more water than the same pasture
grown in the Murray Darling Basin.
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Groundwater systems of northern Australia
•
In the wet and wet-dry tropics of northern Australia, large potential evaporation and
seasonal rainfall result in distinctly seasonal aquifer recharge. Seasonal groundwater
levels may vary by as much as 10 m. Large seasonal deep drainage and large
seasonal fluctuations of groundwater tables are likely to have considerable
implications for the design of irrigation sub-surface drainage.
•
The management of local and some intermediate groundwater systems in the wet-dry
tropics of northern Australia appears to have several advantages over their
management in temperate Australia. Firstly, the onset of the wet season in the north
can usually be predicted several months in advance with relative certainty. Secondly,
once the northern wet season has ended, groundwater levels are known and there is
little likelihood of further recharge until the following wet season.
•
Few recharge studies have been conducted in northern Australia.
Initial studies
suggest that the relative difference in recharge under different vegetation types may
be less than that in southern Australia. More work is required.
•
In semi-arid and arid zones, topography is likely to be an important control on
groundwater recharge. Topographically driven zones of saturation may occur where
surface runoff is concentrated in quantities that exceed evaporative demands and soil
deficits.
The ‘harvesting’ of water for irrigation in these arid areas may have
important implications to the sustainable use of groundwater
•
Relatively large volumes of recharge may occur along river channels and landscape
depressions during large flow and inundation events. However, in semi-arid and arid
zones, if these volumes are averaged over the entire groundwater catchment area
and time, then the recharge estimates are typically very low (< 1 mm/year).
•
Sustainable irrigation with groundwater in semi-arid and arid regions will require a
recharge area that is several orders of magnitude greater than the irrigated area.
•
Where groundwater systems discharge to the surface environment in the seasonal
climates of northern Australia, it often provides an essential source of water to
surrounding flora and fauna. Because the discharge is normally reduced by new
groundwater abstraction a sustainable yield policy for groundwater systems would
need to consider ecological water requirements that may critically limit other uses. In
semi-arid and arid environments it may not be realistically possible to develop a
sustainable yield policy for groundwater systems if ecological constraints are applied.
•
The use of groundwater for irrigation in northern Australia may present management
challenges because of the uncertainties associated with estimating recharge,
discharge and lateral flow, and the associated time lags. Detailed information has
been collected in only a few areas within northern Australia.
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•
Although mining is a relatively small user of water in Australia (2%) it can impose
severe stresses on local water resources.
Many mines are located in areas with
igneous and metamorphic rocks that typically have very small specific yields. Water
volumes extracted from fractures in these rocks may be large relative to the stored
volume of groundwater, and may require long time periods to be replenished.
•
Groundwater-fed perennial river systems in northern Australia support unique natural
ecosystems that are dependent upon the quantity and quality of flow in the Dry
season.
Surface hydrology of northern Australia
•
Approximately 60% of Australia’s runoff is generated north of the Tropic of Capricorn.
•
Only a few rivers in northern Australia have perennial flow, with the exception of the
wet tropics zone where most of the major rivers are perennial. The few perennial
rivers in the wet-dry tropics have a strong connection to their underlying groundwater
systems, which maintain baseflow in the Dry season, and are usually set in
sedimentary environments.
•
In the semi-arid zone, many large rivers are ephemeral and large transmission losses
may occur when they flow due to the infiltration of river water through the river bank
and bed.
•
Flow durations are typically short and the rivers of northern Australia are considerably
more seasonal than rivers in other regions of the world with the same climate type,
and more seasonal than the rivers of southern Australia.
•
In the wet-dry tropics the largest flows occur during January to March. In the wet
tropics the highest flows occur approximately one month later during February to
April.
•
Large flow events have important ecological implications for in-stream, estuarine and
near-shore marine environments.
•
The highly seasonal streamflow in northern Australia means that permanent
settlements and irrigation during the Dry season require surface water storage
structures in the absence of suitable groundwater resources.
•
If adaptive water management practices are employed and water allocations are
based upon recent and predicted rainfall and streamflow trends, then the highly
seasonal character of streamflow in northern Australia might enable post wet season
water allocations to be granted with relative certainty.
•
Australia is the world’s driest continent with the smallest annual runoff volume (11%
of rainfall).
31
Runoff coefficients31 across northern Australia vary from almost zero
Ratio of runoff to rainfall
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during the dry season over much of the arid and semi-arid in-land regions to greater
than 0.8 in northern wet-dry tropical regions and the humid wet tropics.
•
The variation in annual flows has been observed to be considerably greater in rivers
in northern Australia than in rivers from the rest of the world of the same climate type
and slightly greater than the variation in annual flows in rivers in southern Australia
(Petheram et al. 2008). Inter-annual variability of streamflow is of great significance
to the design of water storage structures. Large variation in flow from season to
season and from year to year requires a large storage structure to accommodate
volume fluctuations and meet the required demand.
•
The northern Australian landscape has a low natural storage capacity and small lag
times between the peak precipitation month and peak streamflow month. There is
negligible carry-over of moisture in most catchments from one year to the next.
•
Streamflow drought severity is greater in northern Australia than in similar climatic
regions of the world due to a high drought magnitude and normal drought length.
Agricultural enterprises seeking to source water from rivers in northern Australia may
need special contingency measures to deal with the high likelihood of severe
droughts.
•
The rivers of northern Australia have very small slow flow components and their water
levels rise and fall very quickly. This limits the opportunity to ‘harvest’ surface water
for irrigation and other uses.
•
While 60% of Australia’s runoff is generated in northern Australia, the seasonality and
inter-annual variability of flow and the lack of suitable locations for large dams means
that only about 20-25% of this runoff can potentially be exploited. This equates to
about 40% of Australia’s total ‘exploitable’ water. These figures do not account for
economic, social, cultural or environmental needs.
•
If this volume of water were expressed as an area that could be irrigated, only 20 to
25% of Australia’s total potential irrigated area, 35% if Tasmania is excluded from the
analysis because of insufficient land area, could be irrigated in northern Australia
because of the higher irrigation demand in the North. These calculations ignore other
factors that constrain irrigation, such as the availability of suitable land and soil,
economics, crop type, etc.
Key knowledge gaps and challenges
•
Protecting unique aspects of tropical environments will present new challenges to
sustainable irrigation in the North. Aspects of irrigation design that need particular
attention include:
o
Drainage management in regions of large seasonal watertable fluctuations
o
Social, economic and biophysical costs and benefits of irrigation mosaics
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o
Aquifer Enhanced Recharge within an irrigation context in a highly seasonal
tropical environment.
o
Management of irrigation tail waters in highly ephemeral systems
o
Water harvesting in the wet-dry tropics of northern Australia.
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END OF REPORT
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