Global sensitivity of inundation extent and population exposure to flood magnitude

Understanding global river flood risk is fundamental for impact assessment of future climate and socio-economic change. There is a growing interest in understanding future flood risk using alternative methods that are independent of the uncertainties associated with the common approach based on a model cascade of global climate models coupled with hydrological and inundation models. Here, we propose a new sensitivity index that quantifies whether river reaches are more sensitive to flooding from low or high return periods using flood hazard data from a global flood model. We assess the sensitivity of flood extents and population exposure to increasing river flow magnitudes of 1.1 million river reaches globally. The dominant control on the sensitivity of reaches is the local topography and upstream drainage area. We find that steep bedrock and low slope alluvial streams are sensitive to high return periods, while intermediate and transitional streams are sensitive to low return periods. We find a clear spatial pattern in where the largest proportions of populations have settled on floodplains, which are found in North Africa, South America and South and East Asia. This analysis allows us to identify regions where river reaches and populations might be most affected by climate change and an increase in frequency and magnitude of flood events.