Spatio-Temporal Non-Stationarity of Flood Risk in the European Alps over the last 1,450 Years

Abstract. Floods are a major source of losses from natural hazards, yet modelling their occurrence and severity is challenging due to complex spatial dependencies and non-stationary behaviour over time, both of which are increasingly affected by climate change. In this study, we characterise spatial and temporal non-stationarity in flood risk and quantify the dependence of flood occurrences under minimal modelling assumptions. We analyse a unique dataset of flood records from 27 Alpine lakes spanning 1,450 years, applying penalised additive mixed models to capture the empirical spatio-temporal dependence structure of flood events. Our results reveal pronounced regional and temporal variations in flood risk and highlight periods of elevated susceptibility. The model further allows extrapolation of flood occurrence probabilities to unobserved locations across the European Alps, providing a robust tool for hazard assessment under changing climatic conditions.