SWAT-Based Assessment of Sediment Sources, Non-Stationary Delivery, and Management Scenarios in a Semi-Arid Reservoir Catchment: Kiri Dam, Nigeria

Reservoir sedimentation poses an escalating threat to water supply reliability and dam sustainability in semi-arid regions, where climate variability and land-use changes intensify sediment delivery. This research employs a calibrated, uncertainty-constrained Soil and Water Assessment Tool (SWAT) model to reconstruct sediment generation, transfer, and mitigation within the Kiri Dam catchment in northeastern Nigeria, covering the period from 1982 to 2024. The results disclose significant spatial heterogeneity, with approximately 50–60% of total sediment inflow emanating from merely five sub-basins that constitute about 15% of the catchment area, indicating strong geomorphic connectivity and Pareto-type sediment-source concentration. Temporally, sediment delivery exhibits non-stationarity and is event-driven, transitioning from a vegetation-buffered regime to a phase dominated by disturbances, and more recently, evolving into a climate-driven regime influenced by extreme runoff events. The coupling between hydrology and sediment transport is primarily transport-limited, with a distinct runoff threshold regulating sediment mobilization. Scenario analyses indicate that individual Best Management Practices (BMPs) can reduce sediment inflow by 20–38%, whereas a combined, hotspot-focused BMP package can achieve approximately 51% reduction, surpassing international standards for a significant extension of reservoir lifespan. Despite limited data availability, uncertainty analysis confirms that the model provides robust comparative decision support. These findings highlight the importance of targeted, integrated catchment management strategies in mitigating reservoir sedimentation in data-scarce semi-arid environments.