Regional groundwater flow modeling in Lower

Disaster Advances
Vol. 7 (12) December 2014
Regional groundwater flow modeling in Lower Bhavani
River basin, Tamil Nadu, India
Anandakumar S.1 and Subramani T.2*
1. Department of Civil Engineering, Kongu Engineering College, Erode, Tamil Nadu, INDIA
2. Department of Mining Engineering, CEG, Anna University, Chennai-600025, Tamil Nadu, INDIA
*geosubramani@annauniv.edu; geosubramani@yahoo.com
test hypotheses regarding the behavior of particular facets
of groundwater systems.20 Groundwater model describes
various parameters such as groundwater flow, solute or
material transport on the basis of various simplifying
assumptions. Computer simulation has been widely used to
understand the responses of the aquifer system to changing
hydrological stresses.13 A number of groundwater modeling
studies have been carried out around the world for effective
groundwater management.9,12,13,23,24,26 The most widely
used numerical groundwater flow model is MODFLOW
which is a three-dimensional model originally developed
by the U.S. Geological Survey.19
Abstract
Groundwater flow models are beneficial for the
management of groundwater resources as they give
an approximate estimate about the various
hydrogeological parameters. They also help in
illustrating a clear picture of the flow pattern in an
aquifer. Such a numerical three-dimensional
groundwater modeling study was attempted in Lower
Bhavani River basin, South India with the main
objectives of simulating the regional groundwater
flow and identification of the distribution of heads for
improved understanding of the natural flow system.
Bhavani River is one of the important tributaries of
Cauvery River and originates in the Silent Valley
range of Kerala State, India.
In India, as in many parts of the world, considerable
research has been carried out to understand the aquifer
system and groundwater flow. In the study area, the depth
of groundwater is very shallow in the central part.
However, depth of occurrence of groundwater and its
fluctuation are considerably high in the north-eastern and
south-western parts of the basin. Some of the previous
studies carried out in this region are:
(i) Groundwater resources and development potentials in
Erode District, Tamil Nadu, India, by the Central Ground
Water Board6, Government of India,
(ii) Groundwater level monitoring, rainfall recording, pump
test analysis and estimation of concentrations of major ions
in groundwater by the PWD22, Tamil Nadu,
(iii) A case study on groundwater quality of hard rock
aquifers of Erode District by Chidambaram et al8,
(iv) Understanding of major ion groundwater chemistry by
Anandkumar et al1,3
(v) Seasonal behavior of rainfall pattern by Anandkumar et
al2 and
(vi) Identification of hydrogeochemical processes using
Netpath modeling by Subramani et al.27
The model simulates groundwater flow over an area
of 2,475 km2 with 55 rows and 45 columns, with a
single vertical layer. The model was simulated in
transient state condition using three-dimensional
partial differential equation of groundwater flow from
1995-2006. The model was calibrated for steady and
transient state conditions. There was a reasonable
match between the computed and observed heads. The
transient model will run until the year 2015 to
forecast the dynamic groundwater flow under various
scenarios of over pumping and less recharge. The
model predicts the behavior of this aquifer system
under various hydrological stress conditions. The
results indicate that the aquifer system is stable under
the present conditions. The model also predicts the
changes in groundwater head with changes in
hydrological conditions like drought occurring once
in three years and a normal run for another 8 years
without any major changes.
Study area
Physiography and land use: Bhavani River is one of the
important tributaries of Cauvery River and originates in the
Silent Valley range of Kerala State, India (Figure 1). The
Lower Bhavani River Basin lies between 11° 15‟ N and 11°
45‟ N latitudes and 77° 00‟ E and 77° 40‟ E longitudes.
Bhavani, Gobichettipalyam, Satyamangalam and Andiyur
are the major settlements in this region.17 The study area
includes reserve forest, built-up lands, agricultural fields
and barren lands. Tanks in the north east and south west
part of the study area are mainly rain-fed and remain dry
throughout the year except during rainy seasons. The
Bhavani River flows from west to east in the study area and
confluences with the Cauvery River at Bhavani Town.
Keywords: Groundwater flow, Groundwater head, Flow
modeling, Lower Bhavani River basin, South India.
Introduction
Groundwater has become a highly dependent source of
water. Low risk of contamination and its wide distribution
make it more preferable than surface water. Groundwater
modeling has emerged as an important tool for collecting
various databases of an aquifer. Hence, groundwater
modeling studies have enabled researchers to develop a
better understanding of the functioning of aquifers and to
41
Disaster Advances
Vol. 7 (12) December 2014
The area is comprised of hilly regions and plain terrain with
maximum and minimum altitudes of 1,487 m and 215 m
above mean sea level (MSL) respectively. The terrain
slopes towards south-east. Topography, in general, plays a
vital role in groundwater management for understanding
the slope of the terrain and surface runoff. The topography
of this region mainly controls the occurrence of
groundwater, land use and drainage pattern. Scattered
hillocks of moderate elevation occur within the uplands.
The plains area is characterized by gentle undulations with
a general gradient due east and south-east.
landform and bedrock type and also suggest soil
characteristics and site drainage condition. In addition, the
stream pattern is a reflection of the rate at which
precipitation infiltrates in comparison with the surface
runoff. The infiltration/runoff relationship is controlled
largely by permeability, which is, in turn, a function of the
type and fracturing of the underlying rock or surface
bedrock. When comparing two terrain types, the one that
contains the greatest drainage density is usually less
permeable.10
A well-developed dendritic to sub-dendritic drainage
system is generally noticed in the basin which indicates the
occurrence of rocks of uniform resistance.29 The map also
illustrates the major drainages and man-made canals in the
basin. The basin area is drained by the Bhavani River and
its tributaries. The Bhavani River, which has its origin in
the Silent Valley range of Kerala State, enters the study
area about 30 km west of Bhavanisagar Reservoir and
flows more or less in an easterly direction and confluences
with the Cauvery River at Bhavani Town. A number of
streams have their origin in the hill ranges of the Eastern
Ghats and have their flow direction towards south. The
drainage pattern in the area is also controlled by structural
features. Amongst the different drainage patterns
recognized in the study area, the dendritic, sub-dendritic,
radial and parallel patterns are noteworthy.
The major crops are paddy, banana, groundnut and
sugarcane. The term “land use” relates to the human
activity or economic function associated with a specific
piece of land. The land use pattern of the study area mainly
depends on topography, land form and soil cover. Thick
vegetation is seen in the hill ranges in the north-western
and northern parts of the study area. The areas adjacent to
Bhavani River are intensively irrigated. Sparsely irrigated
areas are noticed along some of the tributaries of Bhavani
River where the groundwater is effectively utilized. More
cultivable, fertile agricultural lands are noticed along the
canals. Banana and sugarcane are the common wet crops.
Paddy is also cultivated in some places during the monsoon
season. Dry crops are sowed in many places.
Climate and Rainfall: The climate of the study area is dry
except during the monsoon season. The area experiences
dry climate with maximum temperature of 40° C during
April and May and minimum temperature of 22° C during
November and December. The first two months of the year
are pleasant. During March, the sky is clear and the
mercury gains an upward trend which persists till the end of
May. Highest temperature is normally recorded during
May. During pre-monsoon period, the mercury reverses its
trend and by September the sky gets overcast. In spite of
the heavy overcast sky, the rains are meager during
September.22 The north-east monsoon gets vigorous only
during October or November.
Geological setting: The Archean basement of the study
area mainly consists of fissile hornblende-biotite gneiss
(mainly in plains) and charnockite (mainly in hills).14 The
study
area
also
comprises
of
garnetiferousquartzofeldspathic gneiss, hornblende-biotite gneiss,
quartzite, pyroxene granulite, ferruginous quartzite, talctremolite schist, amphibolite, gabbro/anorthosite, pink
migmatite, dolerite dykes and granite intrusions.
Garnetiferous-quartzofeldspathic gneiss and hornblende
biotite gneiss are the other major rock types in this region.
Soil characteristics of a terrain are important since they
meet the basic needs of all agricultural production.
Different soils that occur are derived from a wide range of
geological materials. Knowledge about the types of soils,
their extent and occurrence is of primary importance.
Basement rocks of the basin are covered by various types
of soils namely red non-calcareous soil, red calcareous soil,
brown soil and black soil. Red soils of calcareous and noncalcareous varieties occupy most of the basin area. Brown
and black soils occur as pockets in some places.3
The average annual rainfall of the basin is 617.7 mm. The
Palghat gap in the Western Ghats which has a soothing
effect in the climate of Coimbatore district, does not render
much help in bringing down the dry climate in this area.18
The basin receives more rainfall during Northeast monsoon
season period. The average annual contribution of this
monsoon is about 338 mm which is nearly 45% of the total
rainfall of the basin. Highest intensity of rainfall occurred
during this monsoon is 692 mm recorded in the year 2005.
The average annual rainfall received during the southwest
monsoon is 210 mm.2
Hydrogeomorphology: It is possible to delineate various
hydrogeomorphic units from satellite imageries through
visual interpretation.21 Groundwater recharge, transmission
and discharge of the basin are controlled by the basin
geomorphology, geology and structural patterns.15
Hydrogeomorphological maps help to identify the various
geomorphic units and groundwater occurrence in each unit.
Sreedevi et al25 used remote sensing and GIS techniques to
Drainage system: Drainage pattern is one of the most
important indicators of hydrogeological features since it is
controlled by underlying lithology.7 According to Lillesand
and Keifer17, the drainage pattern and texture seen on aerial
photographs/satellite imageries are the indicators of
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Disaster Advances
Vol. 7 (12) December 2014
study the occurrence of groundwater in various geomorphic
units of the Pageru River Basin, Cuddapah District, India.
Hydrogeomorphological map of the study area was
prepared from the IRS-1D-LISS III satellite imageries. The
various geomorphic units as identified from the studies are
structural hills, residual hills, valley fills, uplands,
pediments and bajadas. It is seen that structural and
denudational process controls influence the fluvial
processes in this region.
Boundary Conditions: The boundary conditions modeled
were as per the watershed boundary (Figure 3). The
northern and southern parts of the area had negligible flow
and hence considered as “no-flow boundaries”. Along the
western boundary, the existing hydrogeological boundary,
the Bhavanisagar Reservoir, was considered as the “general
head boundary”. The Bhavani River divides the basin into
two halves; it was considered as the “river boundary”. The
eastern part of the study area is also a part of the watershed
boundary with one outlet falling under the river boundary
itself. The aquifer top and bottom were derived mainly
based on the lithology of boreholes and by intensive field
surveys. The single, unconfined layer is comprised of the
top soil and weathered rocks. The thickness of the
unconfined aquifer varies from 6 to 21 m.
Hydrogeological setting: Generally, the entire area of
study is traversed by metamorphosed gneisisic rocks of
Archean age. The occurrence and movement of
groundwater in hard rock formations are restricted to open
systems of fracture like fissures and joints in nonweathered portions and also in the porous zone of
weathered formations. In hard rock regions, the weathered
thickness is discontinuous both in space and depth. Hence,
the recharge of groundwater in hard rock formation is
influenced by the intensity of weathering. The subsurface
condition was analyzed from borehole lithology and pumps
test data. Borehole lithology revealed that the thickness of
the aquifer in the study area is highly erratic and varies
between 6 m and 20 m below ground level.
Grid design: The geographic boundaries of the model grid
covering 2,475 km2 of the study area were determined
using the map module. The map was projected using the
metric coordinates in the map module and then imported
into the Visual MODFLOW v.4.0. The finite-difference
grid superimposed on the study area was constructed based
on the conceptual model representing the physical
properties of the groundwater system. The grid network
had a constant spacing of 1 km by 1 km. The model grid
was discretized into 2,475 cells with 55 rows and 45
columns with one vertical layer (Figure 3). The length of
model cell was 1 km each along the east-west and northsouth directions of the study area.
Inter-granular porosity is essentially dependent upon the
intensity and degree of weathering and fracture
development in the bedrock. Deep weathering has been
noticed in the gneissic formation and moderate weathering
in the charnockite. Pump test data revealed that
permeability varies between 0.0031 and 11.4642 m/day and
transmissivity between 0.1142 and 100.3536 m2/day.
Spatial variation map of transmissivity prepared from the
pump test data of 19 bore wells indicates that the values are
relatively higher in the central part of the basin. Increasing
trends of transmissivity (Figure 2) are also seen in the
south-eastern and south-western part.
Input parameters
Initial groundwater head: The initial groundwater head of
the study area is shown in fig. 4. After detailed analysis of
the hydrographs (rainfall and water level fluctuation
studies), it was decided that the groundwater head data of
January 1995 represented the initial groundwater head
distribution of the study area. The duration of the rainfall
was normal and the groundwater fluctuation was also
representative of the normal year.
Results and Discussion
A numerical three-dimensional groundwater flow model
was developed for the Lower Bhavani River Basin with the
objectives: (i) To simulate the regional groundwater flow
and (ii) To identify the distribution of heads for improved
understanding of the natural flow system in the study area.
Few scenarios were also developed for proper
understanding of the aquifer system.
Aquifer characteristics: Aquifer properties such as
hydraulic conductivity, horizontal transmissivity and
specific capacity used in the model were derived from the
pumping test results available at 19 sites and are listed in
table 1.
Groundwater abstraction: The groundwater of the study
area is abstracted for irrigation and domestic purposes.
Agricultural activity in this area is mainly dependent on
surface water resources as there is a good canal network in
this region. The land use pattern, drainage pattern and
period of canal flow show that groundwater is used only
during the summer period (one season). The domestic and
drinking water requirement of the study area were
calculated based on population statistics.28
Model conceptualization: The conceptual model of the
system was arrived at from the detailed studies of geology,
borehole lithology and water level fluctuations in wells.
Groundwater of the study area was found to occur in
weathered rocks. The groundwater head in wells
penetrating only up to the weathered formation as well as in
wells penetrating up to the hard rock formation was more
or less the same. Hence, the top soil and the lower
weathered and fractured rocks could be considered as a
single, unconfined aquifer.
Groundwater recharge: The recharge varies considerably
due to differences in land use pattern, soil type, geology,
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Disaster Advances
Vol. 7 (12) December 2014
topography and relief. The recharge to the aquifer system is
from rainfall, irrigation and inflow from the river and
storage tanks. Rainfall is the principal source of
groundwater recharge in this region. A comparison between
the monthly rainfall values and consequent variations in
groundwater levels over a span of 30 years revealed that
groundwater was replenished whenever the monthly
rainfall exceeded 60 mm. The aquifer gets recharged and
the groundwater level shoots up immediately after rainfall
of above 60 mm. The major portion of the study area is
geologically covered by fissile hornblende-biotite gneiss.
The infiltration capacity of rocks in their weathered portion
ranges from 08–12%. The recharge values range from 8 to
11% of the rainfall.28
observation wells distributed throughout the aquifer, the
transient models were considered to be calibrated
satisfactorily. The sensitivity of the model to input
parameters was tested by varying only the parameter of
interest over a range of values and monitoring the response
of the model by determining the root mean square error of
the simulated heads compared to the measured heads.
Simulation results: The model was simulated in transient
condition for a period of 12 years from 1995 to 2006. There
was fairly good agreement between the computed and
observed heads (Figure 5). A study of the simulated
potentiometric surface of the aquifer indicated that the
highest heads are found on the western side of the study
area which is a general reflection of the topography. The
regional groundwater flow direction is towards the center.
Groundwater flows from the north towards the center of the
Bhavani River.
The rate of leakage between the river and aquifer was
estimated using the difference between the river and
groundwater heads. The Bhavanisagar Reservoir is situated
in the western part of the study area. Its contribution to
groundwater recharge was calculated based on the
difference between the head in the adjoining wells and the
reservoir head. The reservoir level data was also inputted in
the model.
The simulated and the observed regional heads for
December 2005 are shown in figure 6. The computed and
observed groundwater heads of well nos. 1 and 19 of the
study area are shown in figure 7. The computed head
values mimic the observed head values. At present the
aquifer is stable. There is a very gradual decline of
groundwater head over 10 years in the northern and
southern parts of the study area; this might be mainly
attributed to the flow towards the river while in the central
part near the river, the groundwater level increases nearer
to the surface.
Model calibration: The calibration strategy was to initially
vary the best known parameters as little as possible and
vary the poorly known or unknown values most to achieve
the best overall agreement between simulated and observed
results. Steady state model calibration was carried out to
minimize the difference between the computed and field
water level conditions. Steady state calibration was carried
out with the water level data of January 1995 in 25 wells
distributed over the basin. Of all the input parameters, the
hydraulic conductivity value was the only “poorly known”
parameter as only 12 pumping tests had been carried out in
this area. Lithological variations in the area and borehole
lithology of existing large diameter wells were studied.
Model forecast: The aquifer response for different input
and output fluxes was studied in order to sustainably
manage the Lower Bhavani River Basin aquifer system.
The model was run for a further period of 8 years from
2007 to 2015. Before commencement of this simulation,
the data pertaining to average rainfall, abstraction, tank
water, river flow and recharge were provided to the model
up to 2015.
Based on these, it was decided to vary the hydraulic
conductivity values up to 10% of the pumping test results
for the aquifer in order to get a good match of the computed
and observed heads (Figure 5). Figure 5 indicates that there
is very good match between the calculated and observed
water heads in most of the wells. Root mean square error
and mean error were minimized through numerous trial
runs.
Two prediction runs were planned to evolve optimal
management schemes:
i) for normal rainfall condition and
ii) for drought (once in three years) condition.
(i) Normal rainfall condition: The model was run to
predict the regional groundwater head in this area until the
year 2015. For these runs, the monthly average rainfall
calculated from 60 years rainfall data was used. The present
level of groundwater abstraction was considered for this
simulation. The simulated regional groundwater head for
September 2015 is shown in figure 8. There is not much
increase or decrease in groundwater level. Such observation
is made in most of the wells. In the years when the flow in
Bhavani River was considered, there is an increase in
groundwater level by about 0.5 m in the wells located near
the rivers.
Transient state simulation was carried out for a period of 12
years from January 1995 to December 2006 with monthly
stress periods and 24 hour time step. The trial and error
process by which calibration of transient model was
achieved comprised of several trials until a good match
between computed and observed heads was obtained over
space and time. The hydraulic conductivity values
incorporated in the transient model were modified slightly
from those calibrated by the steady state model. Based on
the close agreement between the measured and computed
heads from January 1995 to December 2006 at 25
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Disaster Advances
Vol. 7 (12) December 2014
(ii) Drought (once in three years) condition: Analysis of
the past 10 years rainfall data indicates that in three years,
the rainfall was less than the average of 617 mm/year. The
average of these low rainfall years (drought period) was
found to be 573.43 mm/year. In order to study the effect of
drought years in this area, the model was run by assuming
deficit rainfall once in three years until 2015. The monthly
average of deficit rainfall years was calculated and used for
this purpose. The groundwater level decline is by about 0.3
to 0.73 m during the assumed drought years (Figure 9).
However, the groundwater level recovers to the level
observed during normal rainfall within the next year.
However, due to the contribution of the reservoir and the
river, the deficit in rainfall subdues to normal within one to
two seasons. The contribution of the reservoir to the aquifer
system maintains the system in a stable condition.
Figure 1: Location map of the study area
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Disaster Advances
Vol. 7 (12) December 2014
Figure 2: Spatial variation of transmissivity
No flow boundary
No flow boundary
Figure 3: Discretization of the study area
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Disaster Advances
Vol. 7 (12) December 2014
Figure 4: Initial groundwater head of the study area during January 1995
Figure 5: Comparison of computed and observed groundwater heads under steady state condition
47
Disaster Advances
Vol. 7 (12) December 2014
Computed-December 2005
Observed-December 2005
Figure 6: Computed and observed groundwater heads of December 2005
48
Vol. 7 (12) December 2014
Groundwater head (m)
Disaster Advances
Groundwater head (m)
Time
Time
Figure 7: Simulated and observed groundwater heads of well 1 to 19
Table 1
Pumping test results
Well No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Accepted Hydraulic
Conductivity
(m/day)
4.3636
0.0651
11.4642
0.6818
0.0455
0.0271
0.0448
0.3968
0.0647
0.1093
0.2565
0.1415
1.2222
1.4462
0.0105
0.0031
0.0501
0.0522
0.8165
Accepted
Transmissivity
(m2/day)
100.3536
3.8904
39.2517
22.4922
3.1238
1.8649
3.0628
26.2760
2.0707
6.3823
10.6838
5.8945
3.3956
59.9761
0.6871
0.2460
3.3705
2.0884
0.1142
49
Specific Capacity
(m2/sec)
0.0001266
0.0001847
0.0086319
0.0001167
0.0001391
0.0000113
0.0000144
0.0001426
0.0000612
0.0007762
0.0000790
0.0001557
0.0000903
0.0001683
0.0001035
0.0000140
0.0001943
0.0000232
0.0000103
Vol. 7 (12) December 2014
Groundwater head (m)
Disaster Advances
Time
Groundwater head (m)
Figure 8: Computed groundwater head of well number 19 until 2015 under normal rainfall conditions
Time
Figure 9: Computed groundwater head of well number 19 until 2015 under drought condition
The simulated results indicate that this aquifer system is
stable under the present condition. The spatial groundwater
head follows the topography and the groundwater water
flows from the northern part towards the central portion.
The reservoir and the river contribute to the stable
maintenance of the aquifer system. The model predicts
changes in groundwater head with changes in hydrological
conditions like drought occurring once in three years and a
normal run for another 8 years without any major changes.
The aquifer system is stable with few of concern areas near
the river and canal in the eastern part of the basin with
increasing water level. Thus, an integrated approach may
be necessitated in this region to avoid water logging
conditions in the future.
Conclusion
Detailed modeling study was attempted in Lower Bhavani
River basin, Tamil Nadu, India to understand the
groundwater flow mechanism. The topography of the basin
mainly controls the occurrence of groundwater, land use
and drainage pattern. The Archean basement of the region
mainly consists of fissile hornblende-biotite gneiss and
charnockite. The occurrence and movement of groundwater
in the basin are restricted to open systems of fracture like
fissures and joints in non-weathered portions and also in
the porous zone of weathered formations. The thickness of
the aquifer is highly erratic and ranges between 6 and 20 m.
Inter-granular porosity is essentially dependent upon the
intensity and degree of weathering and fracture
development in the bedrock. Deep weathering is observed
in the gneissic formation and moderate weathering in the
charnockite. A single-layered, finite-difference flow model
was used to simulate the groundwater head in the Lower
Bhavani River basin for a period of 12 years (1995-2006)
for better understanding of the aquifer system.
Acknowledgement
The corresponding author thanks the Department of
Science and Technology (DST), Government of India for
providing necessary funds under „Young Scientist‟ Scheme
to carry out the work.
50
Disaster Advances
Vol. 7 (12) December 2014
14. GSI., Geological and Mineral map of Tamil Nadu and
Pondicherry. Published by the Director General Geological
Survey of India on 1: 500,000 scale (1995)
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