GIS-BASED ASSESSMENT OF SOIL EROSION RISK IN GUBI DAM WATERSHED, NORTHEASTERN NIGERIA, USING THE REVISED UNIVERSAL SOIL LOSS EQUATION (RUSLE)

Soil erosion within reservoir watersheds can degrade land resources and mobilise sediment through drainage networks, making spatial identification of erosion-prone areas important for watershed management. This study assessed the magnitude and spatial distribution of soil erosion in the Gubi Dam watershed, northeastern Nigeria, using the Revised Universal Soil Loss Equation (RUSLE) integrated with Geographic Information System (GIS) and remote-sensing techniques. A 30m Shuttle Radar Topography Mission (SRTM) digital elevation model was used for watershed delineation and terrain analysis, while rainfall records, FAO soil information and September 2021 Landsat 8 OLI imagery were used to derive rainfall erosivity (R), soil erodibility (K), slope length and steepness (LS), cover-management (C), and support-practice (P) factors. Vegetation protection was represented using a continuous NDVI-derived C-factor surface. The delineated watershed covered approximately 18,067.39 ha (180.67 km²). RUSLE estimated soil loss ranged from approximately 0 to 715.3 t ha⁻¹ yr⁻¹, with a watershed-wide mean of 5.590t ha⁻¹ yr⁻¹. The archived study-specific classification indicated that 98.6% of the watershed occurred within the two lower soil loss classes, whereas the three upper classes collectively occupied approximately 252.94 ha (1.4%). Elevated estimates were spatially concentrated, particularly in areas associated with steep terrain and major drainage pathways. The assessment provides a spatial screening basis for prioritising field investigation and soil conservation planning. The estimates represent potential sheet and rill soil loss rather than sediment delivery to Gubi Reservoir, and field validation is recommended.