Leveraging SMAP-based soil moisture to identify runoff thresholds on flash flood-prone basins in the Mediterranean Region

Catchment response to precipitation is a complex, highly nonlinear, threshold-driven process influenced by various factors, including rainfall characteristics, topography, soil type, and antecedent soil moisture conditions. Identifying specific rainfall and soil moisture values beyond which runoff is generated—potentially leading to hazardous river discharges—is crucial in hydrology and fundamental for flash flood forecasting. However, this is not always straightforward, especially in data-scarce regions, due to ground data's limited availability and representativeness. Satellite-based soil moisture retrievals provide a viable alternative for defining these thresholds, particularly in data-scarce catchments, though their application still needs to be explored. This study examines the ability of SMAP Level 4 data to define surface runoff (SR) generation thresholds in six small catchments (i.e., smaller than 250 km2) with varied lithologies and land uses in the hilly and mountainous regions of Central Italy—catchments representative of many in the Mediterranean. We analyze responses to 435 rainfall-runoff events, focusing on the antecedent soil moisture within the top 30 cm, estimated by SMAP L4, up to three hours before rainfall onset. Our findings show that the threshold-driven behavior of runoff generation can be effectively captured only by considering SMAP L4 antecedent soil moisture and rainfall depth together. In contrast, the use of rainfall or soil moisture alone is insufficient. This study demonstrates that SMAP L4 is a viable alternative to in-situ observations for SR threshold identification, with practical applications for issuing flash flood warnings and alerts.