Projections of climate change impacts on floods and droughts in Germany using an ensemble of climate change scenarios

Under a warming climate, changes in hydrological extremes may be more significant than changes in hydrological mean conditions. Due to the high risk of damage and the increasing trends of floods and droughts in Germany, the potential changes in hydrological extreme events are of high importance. However, projections of extreme events particularly for floods are associated with large uncertainties and depend on climate scenarios. If only a few scenarios are applied, there is a danger that the impact assessment is biased. This study aims to evaluate the performance of a set of climate scenarios from the ENSEMBLES project for flood and drought projections and to detect the robust changes using the eco-hydrological model SWIM in five large river basins covering 90 % of the German territory. The study shows that there is a moderate certainty that most German rivers will experience more extreme 50-year floods and more frequent occurrences of 50-year droughts. Projected changes with a high certainty include an increasing trend of floods in the Elbe basin and more frequent extreme droughts in the Rhine basin in 2061–2100. Wetter conditions, i.e., more extreme floods and less frequent droughts, are projected for the alpine rivers in 2021–2060. Using only those RCMs for impact assessments that perform best in the reference period does not guarantee more consistent and certain future projections. Hence, the use of the whole ensemble of available scenarios is necessary to quantify the “full” range of uncertainties corresponding to the current state of knowledge and assuring the robustness of projected change patterns.