Estimation of groundwater recharge variability using a GIS-based distributed water balance model in Makutupora basin, Tanzania

Groundwater recharge estimation in the Makutupora basin (1500 km2) is vital considering the potentiality of the basin to Dodoma city. This study aims to apply the WetSpass model to estimate the long-term average seasonal (dry and wet) and annual groundwater recharge for the Maku-tupora basin. Data required for this study were biophysical data (topography, land use, soil, slope, and depth to the groundwater) and long-term hydro-meteorological data (2000-2020). Data were collected by using both field visits and disk transfer from respective institutions and websites. Hydro-meteorological data were prepared for dry and wet seasons. Raster maps were prepared in ArcMap 10.4 using the Inverse Distance Weighting (IDW) interpolation technique followed by resampling into a 200 x 200 m grid size. Resampled raster maps were converted from raster to ASCII format suitable to input in the WetSpass model. The findings indicated that more recharge was dominating in the wet season ranging between 0 and 120 mm/year with a mean value of 24.65 mm (99%) while less recharge occurs in the dry season ranging between 0 and 4.35 mm/ year with a mean value of 0.24 mm/year (1%) and annually recharge ranges between 0 and 120.88 mm/year with a mean value of 24.88 mm. Mean annual precipitation computed from data for twenty (20) years was 694 mm/year out of which recharge accounted for 3.6%, surface runoff 33.9% and evapotranspiration 62.5%. The groundwater table receives total average volumetric recharge of 37.3 million m3 annually from precipitation for the entire basin area. The model was employed to realize the area's capacity for groundwater recharge to manage the water supply effectively, use it wisely, and plan for the future. Sustainable groundwater exploitation can be feasible only when there is knowledge of the rate at which groundwater is replenished annually. Therefore, the results of this study are useful in sustainable management plans, it may also be used as a benchmark for water supply authorities, policymakers and researchers to set proper protection measures and pumping policies.