Abstract:
The Thal aquifer starts from Salt Range in the north which is surface water divide and drains at the confluence of Indus and Chenab rivers at Mithankot in the southern Punjab. It is regional aquifer covering ~32790 km2 area. On the basis of numerical computer model Visual MODFLOW 3.3, mass balance components were derived from simulation for each stress period indicating the average annual recharge as 2.11 BCM in 1991 which increased to 5.06 BCM in 1996 and 11.83 BCM in 2004. Moreover, the numerical computer model showed induced recharge in the Thal aquifer by multiple increased pumping. The Thal Doab is triangular shaped area forming regional groundwater system. A grid size of 2.5 km2 was selected for this study; the layering of this aquifer was based on pumping depth as it is useless to develop layering on the basis of lithology due to uniformity in geological formation. Predevelopment or steady state conditions were simulated for the year 1984. The steady state calibration yielded maximum residual of 3.789 m and minimum residual of -0.153 m with mean of 1.535 m. Whereas the transient simulation was calibrated for three stresses for the years 1991, 1996 and 2004. The output of regional groundwater modeling in Thal Doab reveals that Indus, Jhelum, Chenab rivers play an important role in recharging the Thal aquifer. The upper Thal Doab needs more careful exploitation of aquifer as the decrease in heads during simulation period is high (2.25 m) between the years 1994-2004. The average declination of hydraulic head in middle Thal is about 1 m during the years 1994-2004. The hydraulic heads in lower Thal during the year 1994-2004 are not affected by pumping rather it increased by 0.5 m because of development of hydraulic gradient and replenishment and cleaning of aquifer formation. Finally based on the computer modeling, it is concluded that the safe yield of the Thal Doab area is higher than pumping. However, a detailed groundwater simulation is recommended beyond 2004 for different stress periods to assess the effects of pumping in various parts of the Thal Doab aquifer.
