Spatially distributed water content thresholds for rainfall-induced landslide initiation

Rainfall-induced shallow landslides are among the most widespread natural hazards in mountainous regions, where intense precipitation, steep topography, and subsurface hydrological processes interact to trigger slope failures. Physically based approaches commonly derive rainfall-triggering thresholds using the framework proposed by Montgomery and Dietrich (1994), which defines instability conditions as a function of groundwater table position. However, this formulation neglects the stabilizing contribution of matric suction in unsaturated soils, potentially limiting its applicability. This study introduces a complementary metric, the Critical Soil Moisture (CSM), which, together with the classical Critical Wetness Index (CWI), provides a continuous hydro‑mechanical description of stability across the full range of hillslope moisture states. The methodology is applied to the 28.6 km² Pontaiba basin in the Carnic Alps (northeastern Italy), a region characterized by steep terrain, high precipitation, and documented shallow landslides. Spatially distributed analyses based on topographic, soil, and landslide inventory data are combined with sensitivity analyses and an ensemble calibration procedure using Receiver Operating Characteristic (ROC) metrics to constrain uncertain parameters. Results delineate three stability regimes, unconditionally stable terrain, groundwater-controlled instability (CWI), and moisture-controlled instability (CSM), and identify slope-dependent hydrological thresholds that can support landslide early warning by focusing on state variables (groundwater, soil moisture) rather than rainfall alone.