Evaluating Runoff Potential for Managed Aquifer Recharge for Sustainable Groundwater Management in Coastal Kenya: A Case Study of the Tiwi Aquifer

Increasing reliance on groundwater, declining water tables and saltwater intrusion have intensified water stress along the coastal regions. This study evaluates the availability of source water for Managed Aquifer Recharge (MAR) in the Tiwi coastal aquifer, Kenya. Runoff, as a key potential source for artificial recharge, was quantified for the Pemba–Shimbo and Kongo/Mwachema watersheds using the Soil Conservation Service Curve Number (SCS-CN) method within a Geographic Information System (GIS) environment. Ten years (2015–2024) of rainfall data from three meteorological stations were spatially distributed using the Thiessen polygon method, while land use and soil characteristics informed curve number estimation under varying antecedent moisture conditions. Results indicate that excess runoff and thus potential recharge water is highly seasonal, occurring mainly during the long and short rainy seasons (March–May and October–December). The Pemba–Shimbo watershed exhibited significantly higher recharge potential, with runoff volumes ranging from 0.1 to 77.7 million cubic meters (MCM), compared to 0.1 to 5.5 MCM in Kongo/Mwachema. On average, approximately 13 MCM and 0.7 MCM of water were available annually for recharge in Pemba–Shimbo and Kongo/Mwachema, respectively. The findings demonstrate substantial, yet underutilized, opportunities for MAR, particularly in larger catchments. Strategic planning, including real-time flow monitoring and integration of weather forecasts, is recommended to optimize recharge interventions and enhance groundwater sustainability under changing climatic conditions.