It is increasingly acknowledged that the acceleration of the global water cycle, largely driven by anthropogenic climate change, has a disproportionate impact on sub-daily and small-scale hydrological extreme events such as flash floods. These events occur thereby at local scales within minutes to hours, typically in response to high-intensity rainfall events associated with convective storms. Despite their local scale and rapid onset, the effects of flash floods can be devastating, making their prediction and mitigation of critical importance. However, the modeling and analysis of such events in data-scarce regions present a unique set of challenges. In the present work, we show that by employing physically based representative hillslope models that resolve the main gradients controlling overland flow hydrology and hydraulics, we can get reliable simulations of flash flood response in small data-scarce catchments. To this end, we use climate reanalysis products and transfer soil parameters previously obtained for hydrological predictions in an experimental catchment in the same landscape. The inverted mass balance of flood reservoirs downstream is employed to derive a target data set for model evaluation in these nearly ungauged basins. We show that our approach using representative hillslopes and climate datasets can provide reasonable uncalibrated estimates of the overland runoff response in three of the four catchments considered. Given that flash floods typically occur at scales of a few km2 and in ungauged places, our results have implications for operational flash flood forecasting and the design of small and medium flood retention basins around the world.
