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. Managed aquifer recharge (MAR) has served as an effective solution for groundwater management and for enhancing water storage during wet seasons, which is recovered during dry periods. Despite the increasing frequency of flooding in the region, excess runoff harvesting has been underutilized as a potential water resource. This study evaluates the availability of runoff as a source of water for MAR in the Tiwi coastal aquifer, Kenya. 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 within the aquifer exhibited significantly higher recharge potential, with runoff volumes ranging from 0.1 to 81.7 million cubic meters (MCM), compared to 0.1 to 5.5 MCM in Kongo/Mwachema. On average, approximately 13 MCM and 0.6 MCM of water were available among the wettest months 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.