Spatially Explicit Erosion Severity as a Proxy for Landslide Susceptibility in a Mountainous Watershed

Mountainous watersheds often suffer from incomplete and spatially biased landslide inventories, which limit the reliability of conventional susceptibility modelling. This study examines whether the erosion coefficient Z of the Gavrilović Erosion Potential Method can provide a process-oriented indicator of slope-instability predisposition in the Portaikos watershed, Central Greece. The Z coefficient was derived from geospatial layers representing vegetation protection, lithological erodibility, erosion-process expression and slope gradient, using Copernicus land-cover products, tree-cover density data, Sentinel-2 imagery, FABDEM and national soil–geological information. A landslide inventory of 46 mapped occurrences from the Hellenic Survey of Geology and Mineral Exploration was then used as an independent reference layer. Erosion severity was classified into five classes and compared with the landslide distribution through Frequency Ratio analysis. Most of the basin was assigned to moderate, very slight and slight erosion classes, covering 34.7%, 30.1% and 28.2% of the area, respectively. By contrast, severe and excessive erosion occupied only 6.7% and 0.4% of the watershed, but contained a much larger proportion of the mapped landslides: 58.7% and 10.9%, respectively. This disproportion was also reflected in the Frequency Ratio analysis. When the severe and excessive classes were considered together, they occupied approximately 7.1% of the watershed but contained 69.6% of the mapped landslides, corresponding to an FR value of 9.79. The separate excessive class showed the highest FR, but it was interpreted cautiously because of its very limited spatial extent and small landslide count. These results indicate that high Z values coincide with terrain sectors where lithological weakness, steep slopes, reduced surface protection and erosion-related sediment-source conditions jointly favour slope instability. The Gavrilović Z coefficient should therefore not be interpreted as a substitute for rainfall-threshold analysis or inventory-based predictive models. Rather, it may serve as a useful first-order screening layer for field verification, spatial prioritization and ecosystem-based mitigation planning in data-scarce Mediterranean mountain watersheds.