How do drought conditions and meteorology shape subsequent floods? Insights from quasi-global stress-test experiments

Floods can have enhanced impacts when they occur in close succession with streamflow droughts. Despite the increased likelihood of impacts during such drought-to-flood transition events, substantial gaps remain in the understanding of the hydrological processes and drivers leading to their development. Specifically, it remains unclear how changes in hydrological conditions (IHCs) after drought and meteorological forcings during the transition towards flood conditions propagate to flood characteristics, such as duration and peak flow. We address this research gap by running three stress test experiments at the event scale: (1) changes in IHC based on perturbations applied to the meteorological forcing (e.g., temperature and precipitation) during the drought; (2) perturbation of observed forcing during the transition; and (3) modifications to both IHC and forcings during the event (i.e., a combination of cases 1 and 2). To do so, we calibrate the bucket-type GR4J hydrological model across a quasi-global dataset encompassing 2003 catchments. Using the results of all synthetic experiments conducted across all catchments, we estimate the isolated and joint sensitivities of both transition and flood characteristics to changes in drought characteristics (e.g., duration, intensity, severity) and to meteorological forcings during the transition. By analyzing the joint sensitivity of a given attribute (e.g., peak flow) to IHCs and meteorological forcings, we estimate the relative importance of each in driving the attribute's overall sensitivity (i.e., total effect of all individual sensitivities). Our findings indicate that, regardless of whether the system is stressed in isolation (i.e., experiment 1 or 2) or jointly (i.e., experiment 3), meteorological forcings - primarily temperature - are the main drivers affecting changes in transition and flood characteristics across hydroclimatic regions. Overall, clear differences between sensitivities, e.g., of peak flow to precipitation during the transition, emerge when comparing snowmelt- and rainfall-influenced catchments. Furthermore, hydroclimatic and streamflow regimes, along with catchment storage dynamics, are the main proxies for understanding the sensitivity of transition attributes to changes in IHC and meteorological forcings. Additionally, we show that IHCs are more important than meteorological forcing in explaining the variability of transition characteristics. Consequently, although IHCs control the baseline state, small changes in variables such as temperature during transitions can significantly alter flood characteristics, largely independently of the IHCs. Ultimately, we show that warming, rather than variations in drought severity (i.e., antecedent conditions) or precipitation, is likely to shape future transitional floods.  Our results enhance our understanding of drought-to-flood transitions and their sensitivities to hydrometeorological conditions, thereby generating clearer insights into how droughts and meteorology influence floods and their impacts.