Geology Reference
In-Depth Information
The major part of the groundwater withdrawal from the system is due to
pumping from the wells. Various means were used to cross-check the
estimates; two quite independent methodologies, viz. a direct one based
upon the well inventory including the location of the wells, their discharge
and pumping duration etc., and an indirect one based upon the demand using
data on irrigated areas, cropping pattern and crop water requirements were
applied.
Evaluation of the discharge rates due to pumping from the well inventory
survey is a direct method of estimation and also provides the grid-wise
information as the wells pumping the groundwater can be plotted on the
grids. For the years 2000, 2001 and 2002, all the wells drilled up to 2000,
2001 and 2002 respectively were taken into account. The total yearly amount
of groundwater withdrawal from the aquifer is given in Table 2. The
groundwater pumping is mainly from the 2nd layer due to water level declines
but the return flow from the irrigated fields occurs in the 1st layer. It was
deduced from the pumping according to:
Andhra Pradesh Groundwater Department (APGWD) (personal
communication, 2000) report on Maheshwaram land use, and
APGWD (1977) report on hydrologic parameters of groundwater recharge
in Andhra Pradesh.
that 86% of the pumped water is used for the rice and 14% for other crops.
In the APGWD report (1977), experiments on the return flow from the
rice in a nearby watershed showed that 55 to 88% of the irrigation water
returned to the aquifer. After calibration, 60% of return flows from the rice
and 20 % from the other crops were assumed. In the model, we do not
distinguish between the locations of the rice and the other crops, because
they are not known. The average return flow of one crop (rice/other) is then
taken as:
0.86 × 60 + 0.14 × 20 = 54.4 or D55% of return flow from the
withdrawals.
Simulation and Calibration of the Model
The time-steps of the hydrodynamic calculation in transient flow are 14 days
during the dry season and one week in the rainy season (they are daily when
it rains). The MARTHE code uses GARDENIA to calculate the hydroclimatic
balance; this balance is calculated with a weekly time-step during the dry
season and a daily one during rainy days throughout the year.
The fitting of the model was carried out by calibrating the following
parameters:
the hydraulic conductivities of the two layers; and
the return flow from withdrawals.
