Debris flows represent frequent hazards in mountain regions. Though significant effort has been made to predict such events, the trigger conditions as well as the hydrologic disposition of a watershed at the time of debris flow occurrence are not well understood. Traditional intensity-duration threshold techniques to establish trigger conditions generally do not account for distinct influences of rainfall, snowmelt, and antecedent moisture. To improve our knowledge on the connection between debris flow initiation and the hydrologic system at a regional scale, this study explores the use of a semi-distributed conceptual rainfall-runoff model, linking different system variables such as soil moisture, snowmelt, or runoff with documented debris flow events in the inner Pitztal watershed, Austria. The model was run on a daily basis between 1953 and 2012. Analysing a range of modelled system state and flux variables at days on which debris flows occurred, three distinct dominant trigger mechanisms could be clearly identified. While the results suggest that for 68% (17 out of 25) of the observed debris flow events during the study period high-intensity rainfall was the dominant trigger, snowmelt was identified as the dominant trigger for 24% (6 out of 25) of the observed debris flow events. In addition, 8% (2 out of 25) of the debris flow events could be attributed to the combined effects of low-intensity, long-lasting rainfall and transient storage of this water, causing elevated antecedent soil moisture conditions. The results also suggest a relatively clear temporal separation between the distinct trigger mechanisms, with high-intensity rainfall as a trigger being limited to mid- A nd late summer. The dominant trigger in late spring/early summer is snowmelt. Based on the discrimination between different modelled system states and fluxes and, more specifically, their temporally varying importance relative to each other, this exploratory study demonstrates that already the use of a relatively simple hydrological model can prove useful to gain some more insight into the importance of distinct debris flow trigger mechanisms. This highlights in particular the relevance of snowmelt contributions and the switch between mechanisms during early to mid-summer in snow-dominated systems.

Debris flows are typically triggered by rainfall-related weather conditions—including short-duration storms and long-lasting rainfall, in cold climates sometimes in connection with intensive snowmelt. Given the considerable observational uncertainties of rainfall, we tested if other hydrometeorological variables carry enough information content to compensate for these uncertainties and if the combined information of hydrologic catchment state and rainfall can be used to predict the regional temporal susceptibility for debris flow initiation. For this we carried out a probabilistic analysis of variables derived from a conceptual hydrological model for the Montafon region, Austria, where debris flows were recorded on 41 days between 1953 and 2013. Exclusively from hydrological characteristics and, importantly, neglecting precipitation itself, we quantitatively determined different trigger types for historical debris flows. Subsequently, we used four Naive Bayes classifier models, ranging from a simple rainfall-only model to a multiparameter hydrometeorological model differentiating between trigger types, to predict days susceptible for debris flow occurrence in the region. The results suggest that debris flows were triggered by convective rainstorm events on 23 days, on 12 days due to gradual soil moisture buildup in the course of long-lasting rainfall events and on six further days snowmelt played an important role. We find that the differences between the trigger types are statistically significant and that a susceptibility prediction differentiating between trigger types and including hydrological information can outperform simple rainfall-only models. This study thereby contributes to an improved understanding of the hydrometeorological impact on debris flow initiation in a mountain watershed.