Geoscience Reference
In-Depth Information
years. The calibration is performed assuming specified transmissivities for
each of the zones obtained from several pump tests in the region
6
along with
an assumption of no flux boundary along the entire subbasin. In this case,
the calibration provides an estimate of recharge factor and specific yield
for each of the zones, along with the lateral groundwater fluxes between
the zones. The fit between the observed and simulated groundwater levels
during the calibration period is found to be good (Fig. 3). Before apply-
ing MODFLOW for the subbasin a lumped ground water balance model
is considered for zone 7. In this model, the lateral inflow and outflow are
maintained constant throughout the entire simulation period. The model is
calibrated with suitable inflows and outflows, and recharge for the period
between 1977 and 1979. The system is simulated using either a constant
draft (based on the draft of 1979) or a variable draft for the entire period
up to year 2000. Figure 4 shows the simulations for these cases and their
comparison with the observed water levels in OW7. It is clear from the plot
that though the lumped model provides a good fit up to 1984, thereafter
both the constant and variable draft cases fail to match the observed water
level responses. The constant draft produces a response as if the ground-
water levels are quite stable over a long period from 1984 to 2000. On the
other hand, the variable draft produces a monotonically decreasing ground-
water levels as expected. The simulated water levels for the variable draft
case show a trend similar to observed pattern up to 1990 with a marginally
lesser decline. However, the simulation is not able to reproduce the observed
raise and subsequent fall in the later periods. Thus, these variations might
20.00
15.00
R
2
= 0.9705
10.00
5.00
0.00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
Observed (m)
Fig. 3.
Model performance for the subbasin during calibration period (1977-1979).
