Modelling the spatial structure of flood events in ungauged basins

Abstract:

A basic yet unanswered question in global hydrology is to the extent to which different sites within an ungauged region can experience flooding simultaneously. We know relatively little about how extreme discharges vary in space during flood events, and the limited knowledge that we do have comes from a small number of densely gauged regions in north America and Europe. As a result, the nature of flood footprints in many parts of the world is completely uncharacterized. We address this question in this paper using a novel hybrid of global hydrological model output and regional flood frequency relationships based on global data sets. We show the efficacy of modelled river discharge reanalyses in the characterization of flood spatial dependence in the absence of a dense stream gauge network. While global hydrological models may have limited skill in simulating absolute observed river flows, we find that the rate at which they can simulate the joint occurrence of relative flow exceedances at two given locations is broadly similar to when a gauge-based statistical model is used. Evidenced over the US, flood events simulated using observed gauge data from the US Geological Survey versus those generated using modelled streamflows have similar: (i) distributions of site-to-site correlation strength, (ii) relationships between event size and return period, and, importantly, (iii) loss distributions when incorporated into a continental-scale flood risk model. Extremal dependence is generally quantified less accurately on larger rivers, in arid climates, in mountainous terrain, and for the rarest high-magnitude events. However, local-scale errors are shown to broadly cancel each other out when combined, producing an unbiased flood spatial dependence model. The results suggest that the spatial patterns of relative flow exceedance simulated by a global hydrological model can predict the rate of occurrence and correlation in space of flood events. Further, global-scale regionalized flood frequency relationships can be used to estimate absolute discharge based on these modelled relative flow exceedances. This provides a clear route to a global method for understanding differences in flood footprints across a wider range of climate regimes and landscape settings than has hitherto been possible.