Spatio-Temporal Estimation of Groundwater Recharge in the Islamabad, Pakistan Watershed Using GIS-Based Soil Water Balance Modelling

The primary source of freshwater in the Islamabad watershed is the groundwater which has been used in the domestic, agricultural, and industrial sectors, and recharge estimation has to be done properly in order to manage the aquifers sustainably. This paper uses the USGS Soil water balance (SWB) model in a GIS framework to simulate spatially distributed groundwater recharge, 2000–2020 using daily precipitation, evapotranspiration, land-use/land-cover, soil hydraulic properties and curve-number-based runoff. The annual precipitation was between 950 and 1700 mm and simulated recharge was between 178 and 308 mm with a long-term mean of 250–260 mm. Recharge has great spatial variations, as there are high values in forested Margalla and Murree foothills, and there are low values in the urban corridor in Islamabad-Rawalpindi. Sensitivity analysis indicates that soil hydraulic conductivity (12–18% variation) and root-zone depth (6–9% variation) are most effective in recharge whereas surface roughness and initial soil moisture exert lesser effects (less than 3%). The climatic responsiveness of the basin is endorsed by a strong relationship between the rainfall and recharge (r = 0.838, R2 = 0.702). The interpretation of land-use shows that most of the high-recharge areas are located in the forest and agricultural area, but most urban regions fall within the low-recharge areas. The 21 years of combined recharge data becomes a solid basis of groundwater vulnerability mapping and a significant point of view in sustainable ground water management of the fast-developing urban Islamabad watershed.