Monitoring the role of soil hydrologic conditions and rainfall for the triggering of torrential flows in the Rebaixader catchment (Central Pyrenees, Spain)

Abstract

Abstract Torrential flows (debris flows and debris floods) are mainly triggered by precipitation and soil hydrological processes. Most early warning systems in torrential catchments are rainfall-based. However, this approach can result in frequent false positives, due to its pure black-box nature, in which soil water conditions are neglected. We aim to contribute to the understanding of the conditions required for triggering torrential flows by considering also in situ measurements of soil water content. Herein, monitoring data of 12 years of rainfall and torrential flow occurrence (2009–2020) and 8 years of soil hydrologic conditions (2013–2020) in the Rebaixader catchment (Central Pyrenees, Spain) are analyzed. The dataset includes more than 1000 rainfall events and 37 torrential flows. First, rainfall thresholds using maximum rainfall intensity (Imax) and mean intensity (Imean) are defined. For the 2013–2020 dataset, which includes 15 torrential events, the Imean threshold predicted 2 false negatives and 73 false positives (positive predictive value, PPV, of 15.1%) and the best Imax threshold predicted also 2 false negatives but only 11 false positives (PPV of 54.2%). However, our observations confirmed quantitatively that the lower is the soil moisture the higher is the rainfall intensity to trigger torrential flows. Then, we combined Imax and volumetric water content at 15 and 30 cm depth to define a hydro-meteorological threshold. This latter threshold reduced false negatives to 1 and false positives to 8 and increased the PPV to 63.6%. These results confirm that soil hydrological conditions are key factors for torrential flow triggering and may improve early-warning predictions.