Geology Reference
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groundwater storage will be directly equivalent to the recharge. The monsoon
will allow the aquifer to recharge but the irrigation will, within one year or
less, empty the aquifer until the next monsoon season. In these conditions,
the farmers will be more vulnerable to dry years than at present and may not
be able to grow two or even one crop in these years. This will lead to
economic instability and hardship for the farmers.
CONCLUSION
The flow in a weathered-fractured coupled system of aquifers was simulated
using two layers model. Such situation is quite representative of the aquifers
in granitic terrains. The special condition in this particular case has been that
in most of the simulation period the aquifer in the weathered layer has been
dry. In the two-layer aquifer model the upper layer being the weathered
rocks was simulated as porous medium with comparatively low variability
in hydraulic property but in the lower layer the weathered-fractured rocks
was simulated as equivalent porous medium with highly variable hydraulic
properties and contrast. The special case is also due to the fact that no
separating layer exists nor is simulated between the weathered and fractured
layers. For a majority of wells, hydraulic head simulations are in accordance
with the water levels observed in the project wells for the years 2001 and
2002. Hydraulic heads that are not in accordance with the field observations
are located in areas where the withdrawals have been intensive and are to
be verified. The average hydraulic conductivity and the specific yield of the
weathered-fissured layer are in accordance with those found with the “Discrete
Fracture Network” model used to interpret the pumping tests in the aquifer
(Bruel et al., 2002). The model has to be further improved and calibrated for
the heterogeneities in the hydraulic properties by accurately estimating other
input parameters. The return flow from the irrigation water is very difficult
to validate. It is an approximate parameter that can also be calibrated with
the model. Although the model was calibrated only for two years for which
the data existed, some predictive scenarios were tested for further 20 years.
The results of the predictive model with the anticipated scenario show
that if the water demand increases, the water levels in the watershed will
decrease drastically everywhere. More and more wells will become dry and
the groundwater storage at the beginning of a given year will be entirely
dependent on the recharge of that year. In other words, there will not be any
more reserves in the aquifer (Custodio, 2002) to even out the temporal
variability of recharge from year to year. This will lead to difficulties for the
farmers and economic instability.
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