Hydrological Drought Generation Processes and Severity Are Changing in the Alps

Streamflow droughts are governed by different hydro-meteorological processes, whose relative importance may change over time, with potential impacts on drought severity. Here, we assess changes in the importance of different hydrological drought generation processes in the European Alps by applying a standardized drought type classification scheme to two time periods—one in the distant (1970–1993) and one in the recent past (1994–2017). Our findings show that changes in the relative importance of different drought generation processes are stronger in high-elevation catchments, where we detect clear changes in drought seasonality, than in low-elevation catchments. Furthermore, they suggest that changes in drought severity and generation processes are related because increasingly frequent snowmelt-deficit droughts in high-elevation catchments have larger deficits than droughts caused by decreasingly frequent cold temperatures. These changes might persist into the future, because of continuing decreases in snow cover and increases in evapotranspiration, with potential implications for water management.