The effects of rising temperatures on flood quantiles in the Italian Alps can be amplified by different catchment morphologies

This study quantifies potential rates of change in flood quantiles for Alpine basins in Italy under rising temperatures, focusing on the influence of catchment elevation and morphology on natural processes leading to the maximum annual flood. The approach undertaken develops the concepts presented in a previous methodology, that allows to build flood frequency curves by accounting for seasonal variations in the snow-covered portion of the catchment area. We have significantly refined the original methodology by improving the analytical representation of the basin hypsometric curve. This enhanced formulation provides a more accurate characterization of the elevation distribution within catchments, addressing the limitations of the simplified function originally used. This revised approach, named here FloodAlp, allows us to systematically assess, for the first time, the role of the basin’s hypsometric properties on the sensitivity of current 10-year and 100-year floods to gradual changes of the snowline elevation over a large scale, i.e. the entire Alpine chain of Italy. Our findings indicate that basins above 2000 m a.s.l. exhibit a pronounced sensitivity to temperature increases, with potential increments of up to 18.5 % and 21 % in the 10-year and 100-year floods, respectively, for a 2 ◦C rise. A 4 ◦C increase is found to raise these values by 35 % and 43 %, respectively. A paired-catchment analysis reveals that flood increases due to temperature rise strongly depend on the shape of the elevation distribution: even slight variations in the sinuosity of the hypsometric curve can lead to differences of up to 50 % in the 100-year flood, a finding that would have been missed using the original formulation of the methodology. Given the pronounced morphological heterogeneity of basins in the Italian Alps, the results of this study allow to significantly differentiate, over large areas, flood sensitivity to global warming in mountain basins.