From floods to droughts: Climate change impact on compounding streamflow flood and drought in Europe

Droughts and floods have large impacts on a wide range of sectors and their frequency is expected to increase in a warming climate. While droughts and floods individually have distinct impacts, the occurrence of compound flood and drought (CFD) events, or vice versa, can cause greater impacts than when these events occur in isolation. This study examines changes in the characteristics and return period of single flood and drought events, as well as changes in CFD characteristics, by analyzing daily streamflow data from the CWatM (CommunityWaterModel) model for four European rivers during both historical and future periods under two climate scenarios (SSP1-2.6 and SSP5-8.5). Floods and droughts were identified using threshold methods and CFD events were determined when floods and droughts occurred within a 7-month interval. Flood and drought characteristics were defined as flood/drought frequency, flood/drought duration, flood magnitude, flood volume, and drought volume. On the other hand, CFD characteristics were analyzed based on frequency, duration, transition time, and empirical compound severity index. Flood and low flow return periods were estimated based on Gumbel’s extreme value distribution. Results show that floods will generally become more frequent and severe under SSP1-2.6, whereas under SSP5-8.5, they will become less frequent but more severe. Drought severity is projected to increase substantially under both scenarios, though the frequency will vary across different basins. Changes in return periods of high- and low-flow events also vary greatly between basins, with more extreme both high and low flows in the Rhone basin. CFD events will be more frequent and severe in the Rhine and Rhone basins, while their frequency will decrease in the Danube and Tagus basins. Rivers with lower baseflow are expected to experience more frequent and severe CFD, due to more extreme variations in rainfall. The Rhone basin, in particular, will experience shorter transitions between flood and drought events, indicating that CFD will be most impacted by climate change.