Radar hydrology modifies the monitoring of flash‐flood hazard

Abstract

Implementation of flood warning systems and community self-help programs is one of the most effective ways to mitigate the flash-flood risk. In many instances, these are the only affordable and sustainable mitigation approaches. Unfortunately, flash-flood warning is still hampered by changes in societal needs and by our limited knowledge of atmospheric and hydrologic generating mechanisms of this type of flood. On the one hand, flood warning has traditionally focused on localized targets for the protection of communities and for the management of hydraulic reservoirs. Given the general development of dispersed urbanization and transportation, and the expansion of green tourism and water sports in mountainous regions, human lives and property are increasingly exposed to flash-flood hazard in a scattered manner. This means that every river section of a monitored region can be considered as a potential target for flood warning. On the other hand, our poor knowledge of this phenomenon comes from the fact that, in most flash-flood cases, the rain accumulations and the discharges are simply unknown over most of the watersheds of concern. Flash floods develop at space and time scales that conventional observation systems of rain and discharge in rivers are not able to monitor. As these events are locally rare, they also escape the realm of field-based experimentation. Consequently, the atmospheric and hydrologic generating mechanisms of flash floods are poorly understood. Progress in flash-flood research is hampered by three main problems: (i) the downscaling problem due to the incoherent space and time scales between atmospheric models and the flash-flood triggering processes; (ii) the ungauged basin problem due to the fact that the small basins prone to flash-floods are seldom gauged and must be modelled without calibration; and (iii) the problem of knowing the limits of the soil retention control of runoff under the range of rain accumulations considered. Our view is that weather radar networks deeply modify the flash flood monitoring problem. Combining radar detection of rain and distributed hydrological modelling shows considerable potential for improving both the understanding of fundamental scientific questions, including extreme storms and runoff production mechanisms, and the 'distributed' monitoring of the flash-flood hazard. This short commentary addresses some of these developments and identifies some new research needs in radar hydrology.