Geomorphic memory and monsoon forcing in gully erosion: Evidence from upper Western Ghats river catchments

Gully erosion is an important mechanism of landscape change in tropical mountain environments, yet the interactions among hydro-climatic forcing, soil erodibility, hydrological connectivity, and inherited landscape conditions remain insufficiently understood. This study investigates gully erosion susceptibility in the upper catchments of the Western Ghats, India, with particular emphasis on the role of geomorphic memory under monsoon-dominated climatic conditions. A Random Forest (RF) based Gully Erosion Susceptibility Index (GESI) was developed using topographic, hydrological, climatic, soil, and geomorphic variables, including slope, plan and profile curvature, stream power index, slope length and steepness, stream head density, rainfall erosivity, soil erodibility, and distance to palaeo-landslide initiation locations. The RF model achieved strong predictive performance (accuracy = 0.82; AUC = 0.87), indicating reliable discrimination between gully and non-gully locations. GESI values range from 0 to 0.98 (mean ≈0.30), with approximately 36% of the study area classified as having moderate to very high susceptibility. High-susceptibility zones are concentrated in areas characterized by elevated rainfall erosivity (>10,000 MJ mm ha ⁻¹ h ⁻¹ yr ⁻¹ ), high soil erodibility, and efficient drainage connectivity. Variable importance analysis identifies soil erodibility and rainfall erosivity as the dominant controls, while topographic and hydrological factors regulate runoff concentration and sediment transfer pathways. Rainfall–erosivity analysis reveals strong exponential relationships during peak monsoon months, demonstrating that relatively small increases in rainfall produce disproportionately large increases in erosive energy and promote the exceedance of critical erosion thresholds. August exhibits the highest erosivity values and the strongest rainfall–erosivity relationship, indicating that this period represents the principal window of erosive activity and gully initiation under peak monsoonal conditions. The contribution of distance to palaeo-landslide initiation locations further indicates that inherited geomorphic disturbances influence present-day susceptibility, supporting the importance of geomorphic memory in shaping erosion responses. The results suggest that gully erosion in the upper Western Ghats is governed by the interaction of monsoon-driven hydro-climatic forcing, soil susceptibility, hydrological connectivity, and geomorphic inheritance rather than by any single environmental factor. More broadly, the study highlights how legacy effects and contemporary climatic forcing interact to regulate erosion processes in humid tropical mountain catchments and provides a framework for investigating gully development under increasing rainfall extremes.