Downward Counterfactual Analysis of Historical Rainfall Events in Germany

In the last 20 years a variety of heavy precipitation events (HPEs) have caused severe floods and large damages in Germany. However, the impact of an HPE is not solely determined by the event itself, but also by the geomorphologic characteristics of the location where it occurs.Previous studies have shown that HPEs can happen anywhere in Germany. To find out where in Germany historical HPEs could have caused a potential hazard, we extracted the 10 most extreme HPEs by using the cross-scale weather extremity index (xWEI) from the last 20 years of radar data (RADKLIM) and shifted these events to every mesoscale subbasin in Germany.We use the geomorphological instantaneous unit hydrograph as a simple screening tool to investigate the runoff concentration at the mesoscale and the following flood wave propagation in these subbasins as response to historical HPEs. While this method might not be sufficient to model precise discharge, it can be used to spot rapid increase in direct runoff and shed light on the peak development further downstream, depending on the spatiotemporal characteristics of the HPE. By using historical HPEs as benchmarks, our method can help to identify areas in Germany which are prone to flood hazard and assist to adjust mitigation measures accordingly.