An extremeness threshold determines the regional response of floods to changes in rainfall extremes

Precipitation extremes will increase in a warming climate, but the response of flood magnitudes
to heavier precipitation events is less clear. Historically, there is little evidence for
systematic increases in flood magnitude despite observed increases in precipitation
extremes. Here we investigate how flood magnitudes change in response to warming, using a
large initial-condition ensemble of simulations with a single climate model, coupled to a
hydrological model. The model chain was applied to historical (1961–2000) and warmer
future (2060–2099) climate conditions for 78 watersheds in hydrological Bavaria, a region
comprising the headwater catchments of the Inn, Danube and Main River, thus representing
an area of expressed hydrological heterogeneity. For the majority of the catchments, we
identify a ‘return interval threshold’ in the relationship between precipitation and flood
increases: at return intervals above this threshold, further increases in extreme precipitation
frequency and magnitude clearly yield increased flood magnitudes; below the threshold, flood
magnitude is modulated by land surface processes. We suggest that this threshold behaviour
can reconcile climatological and hydrological perspectives on changing flood risk in a
warming climate.