Hydrological drought but not flood synchronicity increases over Europe

Spatially synchronized drought or flood events, that is the co-occurrence of drought/flood in multiple locations, can have severe impacts that challenge water and emergency management because they require resources in multiple places at once. Climate change can affect the frequency of such compound events because of its influence on drought and flood generation processes. While the impacts of climate change on local hydrological extreme events are well studied, its impact on event synchronicity remains uncertain. Here, we investigate how hydrological drought and flood synchronicities have changed in Europe during the period 1981-2020 using observations from 4299 streamflow stations. Our results show that drought synchronicity has grown significantly, most strongly in Central Europe, and that years with spatially extensive drought tend to follow one another. In contrast, flood synchronicity has remained relatively stable. Regionally, regions of growing drought synchronicity show decreasing flood synchronicity, and vice versa. Synchronicity trends are mostly in line with those of local frequencies suggesting that trends in synchronicity are mainly driven by overall frequencies, rather than by the spatial distribution of events. The observed growth in drought synchronicity highlights the need to develop adaptation measures to more frequent large-scale droughts.