Natural susceptibility and anthropogenic pressures jointly shape county-scale gully density in southern China

Gully erosion is a severe form of land degradation threatening regional ecological security and sustainable land management. However, the relative and shared explanatory contributions of natural susceptibility and anthropogenic pressures to its regional spatial distribution remain insufficiently quantified. Using a county-scale gully inventory of seven provinces in southern China, this study investigated the dominant natural and anthropogenic controls on gully erosion within an integrated multi-model framework. Boruta and Geodetector consistently identified granite weathering crust thickness (GWCT), granite area (GA), and mean annual rainfall (MAR) as the dominant natural drivers. Certainty factor (CF) analysis further revealed that susceptibility increased markedly under conditions of GWCT > 32.94 m, GA > 1695.80 km², and MAR > 1827.15 mm, indicating that thick weathering mantles provide a highly erodible substrate, while intense rainfall represents a key climatic forcing. Human-related variables also showed nonnegligible associations. Lower rural employment proportion (REP < 51.43%) and intensive land-use transitions, especially forestland-to-cropland area (36.91 < FTCA < 104.68 km²) and cropland-to-forestland area (CTFA > 167.42 km²), were associated with elevated gully erosion susceptibility. Variance partitioning analysis (VPA) identified a substantial shared explanatory fraction between natural and human factors, while Geodetector revealed bi-factor enhancement among dominant predictors. These results indicate overlapping and interaction-enhanced statistical associations with county-level gully density. Structural equation modeling (SEM) suggested that soil-related factors exhibited the strongest positive association with county-level gully density, whereas economic factors showed the strongest negative association. These findings highlight the importance of coupled human-natural controls on gully erosion and provide a county-scale basis for prioritizing targeted mitigation and ecological restoration strategies in southern China.