Classification reveals varying drivers of severe and moderate hydrological droughts in Europe

Streamflow droughts are generated by a variety of processes including rainfall deficits and anomalous snow availability or evapotranspiration. The importance of different driver sequences may vary with event severity, however, it is yet unclear how. To study the variation of driver importance with event severity, we propose a formal classification scheme for streamflow droughts and apply it to a large sample of catchments in Europe. The scheme assigns events to one of eight drought types – each characterized by a set of compounding drivers - using information about seasonality, precipitation deficits, and snow availability. Our findings show that drought driver importance varies regionally, seasonally, and by event severity. More specifically, we show that rainfall deficit droughts are the dominant drought type in western Europe while northern Europe is most often affected by cold snow season droughts. Second, we show that rainfall deficit and cold snow season droughts are important from autumn to spring, while snowmelt and wet to dry season droughts are important in summer. Last, we demonstrate that moderate droughts are mainly driven by rainfall deficits while severe events are mainly driven by snowmelt deficits in colder climates and by streamflow deficits transitioning from the wet to the dry season in warmer climates. This high importance of snow-influenced and evapotranspiration-influenced droughts for severe events suggests that these potentially high-impact events might undergo the strongest changes in a warming climate because of their close relationship to temperature. The proposed classification scheme provides a template that can be expanded to include other climatic regions and human influences.