Introduction: Soil erosion is a major land degradation problem in southeastern Nigeria. The Idemili drainage area is one of the most degraded landscapes in the country. This study evaluated the spatial variability of water-induced soil erosion risk in the Idemili watershed. The aim was to identify erosion hotspots, understand the role of key drivers, and provide insights for targeted and sustainable land management practices.

Methods: A GIS-based terrain modelling approach was employed, based on a 10 m resolution Copernicus DEM. Three hydrologically informed indices were computed: Slope, Stream Power Index (SPI) and Topographic Wetness Index (TWI). Spatial analysis in ArcGIS Pro 3.4 involved terrain preprocessing, index computation, reclassification to standardised classes and weighted overlay fusion into a single erosion risk map (Very Low to Very High). Model validation was based on field-surveyed gully erosion points collected in the field using GPS.

Results: Spatial variability of erosion risk was high, with more than 47% of the watershed characterised by moderately steep to steep slopes, with a high correspondence to observed gully occurrence. The erosion risk map categorised 34.2% of the watershed into High and Very High-risk regions, forming a distinct belt in the central/southeastern part of the study area. Validation showed that the model performs well, with about 58.3% of observed gullies located within the High and Very High-risk regions and a further 20.8% in the Moderate-risk category and only 20.9% in the Low/Very Low risk.

Conclusions: The integration of SPI, TWI and slope presented a reliable geospatial approach to assess soil erosion risk in the data-scarce tropical environment under study, with the results demonstrating a strong spatial relationship between high-risk areas, steep slopes, hydrological convergence and gully erosion occurrence. The method provided a valuable tool to prioritise erosion control interventions, specifically in the upland and valley convergence areas of Idemili.