Assessment of Sustainable Groundwater Extraction rate for Quetta city using MODFLOW

Quetta is the biggest population center in Baluchistan province. The city population entirely depends upon the groundwater beneath its aquifer. The increasing population of the city and unplanned use have depleted water table in the recent decades. The city population has increased from 0.26 million in 1975 to 1.452 million in 2014. This decline in the water table was first noticed by WAPDA in 1989 as 0.25 m/year due to over draft so they increased the observatory wells network. USGS groundwater flow model MODFLOW (MODFLOW Pro) was applied for the Quetta city to simulate the behavior of aquifer under stresses. The model was calibrated and validated for the available data.  The calibration results show that 20% of the total precipitation is ultimately going to the groundwater. The results show that the water table under Quetta city has declined at the average rate of 0.91 m/year since 1995 to 2014. The analysis shows that the abstraction rate for the city has increased from 32.25MGPD to 57.76 MGPD upon these 20 years. The Vermont Storm Water Management (VSWM) method of volume recharge was used to calculate the fraction of recharge that is retained by the imperviousness caused by the urbanization of the city. This fraction comes out 0.6 MGPD to 2.9 MGPD per year, depending upon the amount of precipitation and the amount of imperviousness for the same year. The water budget calculated by the model shows that the average recharge per year in this period is 37.04 MGD and the average abstraction from the aquifer is 84.20 MGD, so there is annual deficit of 47.11MGD is occurring at the city. The inflow through the boundaries is increasing with the passage of time due to fall of heads inside the model area, whereas the outflow through the boundaries is almost zero during our study period because of higher heads outside the model boundary. The model was then used to predict different future scenarios by giving the same average recharge rates and varying abstraction rates, in order to predict the future behavior of the aquifer <