Modeling Of Sediment Yield Using Soil And Water Assessment Tool: The Case Study Of Humbo Weyne Dam, Somaliland

Abstract

Reservoir sedimentation presents a critical threat to water security in the semi-arid Horn of Africa, where infrastructure lifespan is often compromised by extreme soil erosion. The Humbo Weyne Dam, a vital water source for Hargeisa, Somaliland, is currently facing severe dysfunction due to rapid siltation, which has drastically reduced its 500,000 m³ design capacity. This study aimed to quantify the annual sediment yield entering the reservoir and identify critical erosion hotspots using the Soil and Water Assessment Tool (SWAT) model. Due to the absence of local hydrological data, a spatial proximity regionalization approach was employed, transferring calibrated parameters from the spatially proximate gauged Gode catchment (Wabi Shebelle basin) to the ungauged Humbo Weyne watershed. The model was set up using high-resolution 30m SRTM DEM, FAO soil maps, and Copernicus Land Use data, driven by bias-corrected CHIRPS satellite rainfall and ERA5 climate reanalysis products. The model performance on the donor catchment was reliable, achieving Nash-Sutcliffe Efficiencies (NSE) of 0.86 for streamflow and 0.81 for sediment yield. For the ungauged Humbo Weyne catchment, the SWAT+ model was utilized to simulate sediment dynamics at the Landscape Unit (LSU) level. The results indicate an average annual sediment yield of 4.9 t/ha/yr, translating to a total influx of approximately 660,000 tons of sediment per year a volume that explains the rapid loss of reservoir storage. Spatial analysis revealed that erosion is not uniform; critical hotspots in the northeastern escarpment, covering only 19.8% of the area, generate severe yields exceeding 35 t/ha/yr. These findings provide the first quantitative evidence to guide targeted soil conservation measures, such as hillside terracing and check dams, which are essential to rehabilitate the existing structure and safeguard future water infrastructure development at this strategic site.