A spatial-distributed soil erosion analysis model was applied to a small-sized mountain catchment of Southern Italy, where shallow deposits of loose, cohesionless unsaturated air-fall volcanic (pyroclastic) soils exist. The physically-based LISEM model was used to analyze a past event, taking into account the potential role of both the soil suction and water content, and the spatial variability of soil thickness. The water discharge and the solid concentration at the outlet of the catchment were computed, as well as the spatial distribution of the eroded soil thickness was discussed for the uppermost areas of the catchment. The results of the paper show that the soil suction and the soil conductivity are key factors for the spatial-temporal evolution of both ground infiltration and runoff in the catchment.