An Integrated Assessment of Soil Erosion Risk in the Bilate Sub-Basin of Southern Ethiopia Using the SWAT Model and Multi-Criteria Decision Analysis

Soil erosion poses a significant environmental threat in Bilate sub-basin, where steep terrain, heavy rainfall, and unsustainable land use practices intensify the problem. This study employs an integrated approach combining the Soil and Water Assessment Tool (SWAT) model and Multi-Criteria Decision Analysis (MCDA) to evaluate soil erosion risk, overcoming the limitations of standalone methods. Geospatial data (e.g., LULC, slope, soil type) and hydrological modeling were used to identify erosion hotspots and quantify sediment yield. The Analytic Hierarchy Process (AHP) weighted 11 erosion factors in MCDA, while SWAT simulated sediment yield using local hydro-meteorological data. Key findings indicated that 34.5% of the sub-basin is covered by erosion-prone Leptosols and Luvisols, while 29.9% has slopes exceeding 15%, intensifying erosion susceptibility. NDVI analysis indicated 72.4% of the area faces high erosion risk. Combined factors classified 65.16% of the sub-basin under moderate erosion severity, with 27.02% experiencing severe to extreme erosion. The SWAT result showed 51.2% of the area yielding 15-25 ton ha⁻¹ y⁻¹ (severe erosion), while 11.9% exceeded 25 ton ha⁻¹ y⁻¹ (extreme erosion). The SWAT-MCDA integrated analysis identified 53.46% of the sub-basin as severe and 9.12% as extremely severe erosion hotspots, particularly in downstream and upstream parts, threatening agricultural productivity and downstream ecosystems like Lake Abaya. In conclusion, the anthropogenic activities, especially agricultural expansion, were primary drivers of soil erosion, exacerbated by natural factors. The study recommends agroforestry, terracing, and check dams in high-risk areas, alongside policy measures to ensure sustainable land use practices in the sub-basin.