Modeling Land Management Impacts in Kenya : Asymmetric Reductions of Sediment and Phosphorus versus Legacy Nitrogen in the Enkare Narok Catchment.

Agricultural intensification in East Africa, particularly within the Enkare Narok Catchment in Kenya, poses significant threats to water quality and hydrological stability, yet the efficacy of conservation practices is often complicated by legacy nutrients and water balance trade-offs. This study aims to use a SWAT model to simulate and calibrate hydrology and water quality, quantify the impacts of various land management scenarios on sediment, nitrogen (N), and phosphorus (P) loads, and evaluate the trade-offs between water quality improvements and water yield, with a specific focus on legacy nitrogen dynamics. The rigorously calibrated model (NSE = 0.67, R 2 = 0.73, PBIAS = 2.97%) simulated a Baseline scenario, a Conservation scenario (featuring no-till, filter strips, and riparian buffers), and an Intensification scenario. Results demonstrated that conservation practices dramatically reduced sediment and phosphorus yield by 80% and 76%, respectively, but were markedly less effective for dissolved nitrogen, achieving only an 8% reduction due to legacy N in groundwater. Furthermore, the Conservation scenario significantly reduced water yield (Q/P from 0.37 to 0.11) by enhancing evapotranspiration, revealing a critical trade-off between water quality and quantity. Conversely, intensification increased all pollutant loads by 30-50% and resulted in a flashier hydrological regime. We conclude that while surface-level best management practices are effective for sediment and phosphorus control, sustainable watershed management in such regions requires a dual strategy that couples these practices with targeted subsurface interventions to address the legacy nitrogen problem and carefully managed water use.