Integrated Groundwater Potential Assessment and Numerical Modeling for Sustainable Water Access in Pastoralist Areas of the Lower Omo Subbasin, Ethiopia

Surface water availability is highly variable across the Lower Omo Subbasin of Ethiopia, affecting the livelihoods of pastoralist communities. This challenge is expected to intensify under climate change. As a result, groundwater serves as a critical water source and an important buffer against climate variability. However, groundwater information in the region remains limited. This study employed an integrated approach combining geographic information system (GIS), remote sensing (RS), and the analytic hierarchy process (AHP) within a multi-criteria decision analysis (MCDA) framework to delineate groundwater potential zones (GWPZs). Ten thematic layers—lineament density, drainage density, distance from rivers, slope, topographic wetness index (TWI), rainfall, soil type, geology, land use/land cover (LULC), and elevation—were standardized and weighted based on their influence on groundwater occurrence. Lineament density exerted the highest influence (weight = 0.341), followed by drainage density (0.176) and river proximity (0.113). The resulting map classified the basin into five potential categories, with moderate (49.26%), high (22.22%), and very high (13.68%) zones collectively covering over 85% of the area. Validation using 51 borehole yield records yielded an overall accuracy of 90.19%, confirming the model’s reliability. Numerical groundwater modeling revealed a single-layer unconfined aquifer system with recharge rates highest in Surma (864.61 mm/yr), lowest in Hammer 39.10 mm/yr, and an overall average of 451.85 mm/yr. The model achieved a near-perfect mass balance (error < 0.02%). Sensitivity analysis identified recharge and hydraulic conductivity as key uncertainty drivers, particularly under drought conditions. Simulated scenarios indicated that combined recharge reduction and abstraction increase could lower water levels by over 15 m in vulnerable zones. The integrated GIS RS, AHP, and MODFLOW approach effectively guides sustainable groundwater development and well siting to support improved water access to pastoralist regions.