QUANTIFYING SEASONAL GROUNDWATER RECHARGE USING ENVIRONMENTAL ISOTOPES IN ETHIOPIA

Understanding the relationship between seasonal precipitation and groundwater recharge is vital for assessing groundwater resources' source, quantity, and sustainability, especially in regions vulnerable to climate variability like Ethiopia. In this study, we conducted a comprehensive water isotope analysis in the Abay, Omo-Gibe, and Wabishebele basins to establish the link and evaluate the seasonal groundwater recharge pattern. By using stable isotopes of Hydrogen (δD) and Oxygen (δ¹⁸O), as a conservative tracer, from a collection of 751 samples, we developed local meteoric waterlines (LMWL) and employed isotope mass balance equations. Our findings indicated that precipitation in the Abay and Omo-Gibe basins predominantly originated from recycled moisture sources, while Wabishebele obtained from direct ocean advection, as evidenced by d-excess. The preliminary quantitative estimation using isotope mass balance quantitative demonstrated that wet season precipitation contributed most of the groundwater recharge. In the Abay and Omo-Gibe basins, over 95% of the groundwater recharge occurs during the main rainy season between June and September, accounting for 50-80% of the total precipitation. Similarly, the groundwater in the relatively more arid Wabishebele basin is dominantly recharged between September and November. The ¹⁸O depleted and low d-excess groundwater in Wabishebele suggested recharge from monsoon and associated high-intensity rainfall systems. In contrast, dry season light-intensity precipitations are subjected to sub-cloud evaporation and have little contribution to groundwater recharge. The outcome of this study provides valuable insights into the recharge, availability, and management of groundwater resources in the studied basins. In addition, establishing the link between the source of seasonal precipitation to regional groundwater recharge patterns in Ethiopia provides invaluable information for sustainable development and climate change adaptation strategies. Further research on water movement from source to sink would elucidate the unprecedented impact of climate change on water resources. Ultimately, this will assist water resource management, planning, and resilience.

Keywords: groundwater, Ethiopia, stable isotope, season, deuterium excess