Testing the predictive capability of the Index of Connectivity for debris-flow coupling under varying forcing conditions: Insights from two consecutive events in the Horlachtal catchment, Austria

This study examines the relationship between structural connectivity, forcing conditions and functional connectivity of debris flows in an alpine catchment in the Austrian Alps. We investigate two consecutive rainfall events in the Horlachtal valley in 2022 that triggered 163 and 69 debris flows, respectively, providing a unique opportunity to assess connectivity under different rainfall forcing magnitudes. Using the Index of Connectivity (IC) to represent structural connectivity, spatially distributed precipitation data for forcing and a debris flow–channel proximity metric to quantify functional connectivity, we evaluate how well the IC predicts debris flow–channel coupling with and without incorporating observed forcing information. Our results demonstrate that the IC serves as a robust predictor of debris flow connectivity across different forcing conditions, with strong correlations for both events. While observed rainfall forcing showed moderate correlation with functional connectivity, their inclusion in predictive models provided only marginal improvement (2% additional variance explained) over IC alone. This suggests that topographic and morphological constraints, rather than precipitation patterns, predominantly control debris flow propagation in this setting. Notably, the predictive capability of the IC proved relatively stable despite substantial differences in rainfall magnitude between events. Various regression models were evaluated, with quadratic and beta regression approaches performing best. The proximity metric used in this study offers advantages over binary coupling classifications by providing more nuanced information about functional connectivity, especially valuable when most observed processes do not reach the channel network. These findings empirically validate the IC as a meaningful descriptor of system structure in alpine catchments and suggest that challenges in spatial transferability of IC models likely stem from factors other than forcing variability.