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CASE STUDY 
A watershed covering an area of 650 sq. km in 
southern part of Tamil Nadu, India was selected for 
study with the following objectives. 
The main objective of the study was to model the 
watershed by assessing various land and water 
requirements necessary for the watershed and test the 
sustainability of watershed considering the various 
resources and constraints available in the watershed. 
This was achieved by : 
(i) quantifying the total quantity of water required for 
various activities like agriculture, industry and 
domestic consumption etc. 
(ii) quantifying the total quantity of surface water 
available in the watershed using USDA-SCS 
method. 
(iii) quantifying the total quantity of ground water 
available by developing ground water model for 
the watershed. 
(iv) optimize the agricultural production through linear 
programming model and assess the sustainability 
status of the watershed. 
Even though the study covers a wide spectrum of 
sustainability factors, this paper deals with the methods 
and results obtained on quantifying the ground water 
through the modelling techniques. 
By using the satellite data of Landsat 5 TM FCC 
and IRS-1A LISS 11 FCC the resource maps were 
prepared. The collateral data like daily rainfall data for 
10 years, meteorological data like temperature, 
humidity, sunshine, wind velocity, well census, pumping 
data, demographic data, agricultural data on cropping 
pattern, crops cultivated, bore hole details, well 
extraction details, fodder requirement for animal 
population, water requirement for different crops were 
used in the modelling study. The area was divided into 
grids of 1 km x 1 km size for the analysis. 
Computation of available water resources consists 
of (i) computation of surface run off using land use, land 
cover and hydrological soil group maps (ii) computation 
of sub surface potential by developing a mathematical 
model through partial differential equation using mainly 
field measurements (iii) computation of various inflow 
components in the hydrologic process like rainfall 
recharge, river bed recharge, return flow from irrigation, 
sub surface inflow etc. (iv) computation of various 
outflow components include extraction for human and 
animal population, agricultural extraction, sub surface 
outflow, loss due to evapotranspiration through natural 
vegetation. 
After computation of various inflows and outflows 
to each grid, the net discharge or recharge to each grid 
has been computed using the equation : 
Qnet = I - Q where Qnet is the net charges in 
storage of all inflows and outflows, 1 is the total volume 
of inflow components and Q is the total volume of 
outflow components. 
The simulation of model was carried out at IRS, 
Chennai using basic aquifer simulation program with 
various inputs of net storage (NQ) with time interval of 
15 days (t), the boundary conditions, the available 
piezometric heads and the aquifer parameters. 
The model was calibrated with the data for the year 
1983-84 to 1988. The calibrated model was tested with 
the input values pertaining to the period of 1988-89 to 
1992-93. The results from the test runs were compared 
with the observed values and the difference between 
them was obtained. After calibrating the model it was 
used for assessing the quantity of ground water 
available. The watershed boundary was super imposed 
over the ground water level contour drawn taking into 
account the minimum and maximum ground water 
available in the acquifer. Final computation of total 
quantity of water was calculated for each micro 
watershed by adding surface runoff or overland flow 
with the available groundwater. 
RESULTS OBTAINED 
The available groundwater in each micro water 
shed was computed from the distributed groundwater 
model by aggregating the potentials available in all the 
grid segments in a particular watershed for period of ten 
years 1983-92. The minimum quantity of ground water 
that can be extracted is 31.62 mm\ The maximum and 
average quantity of ground water that can be harnessed 
during the ten year period was assessed as 238.24 and 
136.30 mm 3 respectively for the entire aquifer of the 
study area. The average surface water quantity of 
surface water that can be harnessed are 2 mm’ and 25 
mm 3 respectively during thg 10 years period. The 
average total quantity of water i.e. both surface and sub 
surface available in the entire watershed was assessed as 
147.65 mm 3 . The minimum quantity of water available 
during the year 1988-89 works out to be around 33 mm 3 
and the maximum quantity of water available during the 
year 1987-93 for both surface and ground water in the 
watershed is given in Table 1.