Frequency and area affected by critical rainfall conditions triggering debris flows increase in a changing climate

Critical rainfall conditions for the initiation of debris flows are typically investigated in connection with landslides. Here, we approach this issue from the channel perspective. We derive and analyze critical rainfall conditions triggering debris flows and fluvial floods using hourly rainfall estimates based on radar-rain gauge data for over 3600 torrent events in Austria between 2003 and 2022, and assess their spatiotemporal changes in a future climate. We observe that the rainfall patterns leading to either process type are similar, but exhibit statistically significant differences. A general dependence of the magnitude or intensity of the triggering rainfall on the antecedent rainfall cannot be confirmed. In a warming climate, both the probability of occurrence and the affected area in Austria will increase for both process types and across all emission scenarios. However, these changes are not uniformly distributed throughout Austria. The results indicate a strong geomorphological control for debris-flow initiation and provide a basis for an improved adaptation and risk management of torrent processes in a changing climate.