Comparative assessment of groundwater recharge using stable isotope and WetSpass-M modeling in the Omo Gibe River Basin, Ethiopia

Groundwater recharge remains a critical yet poorly quantified component of the hydrological cycle, particularly in data-scarce, heterogeneous basins. This study employs a novel comparative framework that integrates stable isotope mass balance with the spatially distributed WetSpass-M hydrological model to assess groundwater recharge dynamics across three climatically and topographically distinct watersheds (Gilgel Gibe, Gombora, and Neri) within the Omo Gibe River Basin, Ethiopia. We analyzed stable isotopes of Oxygen (δ 18 O) and Hydrogen (δ 2 H) in precipitation and groundwater to establish Local Meteoric Water Lines (LMWLs) and quantify seasonal recharge contributions. Concurrently, the WetSpass-M model was calibrated and validated against stream flow data to simulate high-resolution spatiotemporal components of the water balance. Key findings include: (1) groundwater recharge is primarily driven by wet-season precipitation, with isotopes revealing that 88.1% (Gilgel Gibe) and 83.7% (Gombora) of annual recharge occurs during the wet season; (2) in the bimodal Neri watershed, a more balanced seasonal distribution is observed (64.7% wet season), reflecting its distinct rainfall regime; and (3) the WetSpass-M model corroborates these seasonal proportions while providing high-resolution spatial mapping of recharge hotspots. The WetSpass-M model provided detailed spatial mapping, confirming that recharge is highest in vegetated highlands with permeable soils and is significantly reduced in urbanized areas. The strong agreement between the two methods underscores the robustness of our synergistic framework, in which isotope data validate the model's temporal dynamics and the model elucidates the spatial context of the isotopic signatures. This study suggests that sustainable groundwater management in the basin should prioritize wet-season water harvesting, land-use planning to protect infiltration zones, and climate adaptation strategies.