An increase in the spatial extent of European floods over the last 70 years

Abstract. Floods regularly cause substantial damage worldwide. Changing flood characteristics, for instance due to climate change, pose challenges to flood risk management. The spatial extent of floods is an important indicator for potential impacts, as consequences of widespread floods are particularly difficult to mitigate. The highly uneven station distribution in space and time, however, limits the ability to quantify changes in flood characteristics, and in particular flood extent, over large regions. Here we use observation-driven routed runoff simulations over the last 70 years in Europe from a state-of-the-art hydrological model (mHM) to identify large spatio-temporally connected flood events. Our identified spatio-temporal flood events compare well against an independent flood impact database. We find that flood extents increase by 11.3 % on average across Europe. This increase occurs over most of Europe, except for parts of eastern Europe (e.g., Ukraine, Belarus) and southern Europe (e.g., Spain). Over northern Europe, the increase in flood extent is mainly driven by the overall increase in flood magnitude caused by increasing precipitation and snowmelt. In contrast, the increasing trend in flood extent over central Europe can be attributed to an increase in the spatial extent of heavy precipitation. Overall, our study illustrates the opportunities of combining long-term consistent regional runoff simulations with a spatio-temporal flood detection algorithm to identify large-scale trends in key flood characteristics and their drivers. The identified change in flood extent poses challenges to flood control and water resource management.