Hydrological droughts are generated by different processes including precipitation deficits, snowmelt deficits, or positive evapotranspiration anomalies, and various combinations of these factors. In this talk, we discuss how the importance of different hydro-meteorological driver sequences varies in space over Europe and the Alps, in time over the last five decades, and by event magnitude, i.e. for moderate and severe droughts. To study variations in drought generation processes, we developed an automated classification scheme for streamflow drought events, which assigns events to one of eight drought event types - each characterized by a set of single or compounding drivers. Our results show that (1) drought events caused by rainfall deficits are most common in Western Europe and events caused by abnormally low temperature in the snow season in Eastern & Northern Europe; (2) temporal changes in both drought intensity, deficit, duration, and generation processes are stronger in high elevation catchments than in low-elevation ones; (3) in high-elevation catchments, snowmelt-deficit-induced droughts became more frequent over the past 50 years, which led to increases in drought deficits; and (4) moderate droughts are mainly driven by rainfall deficits while severe events are mainly driven by snowmelt deficits. We conclude that climate impact assessments on droughts can profit from assessing changes in drought generation processes to improve the understanding of how drought magnitudes are changing in a warming world.
