Understanding the responses of river floods to warming climate across the United States

Abstract

<p>How river floods respond to warming climate has drawn increasing attention in recent years. One concern is whether floods would respond similarly to global warming as extreme precipitation, since extreme precipitation is usually the main trigger of flooding. Another argument is flood investigation should not solely focus on precipitation, while antecedent soil moisture condition and snowmelt are also important issues in flood generation. To address these concerns, we first implemented scaling analysis to illustrate the sensitivity of extreme precipitation and flood to warming climate across the Continental United States (CONUS). We found both similar and opposite scaling patterns in extreme precipitation and streamflow to temperature. Generally, the similar scaling was found in regions where extreme precipitation and flood timing is correspondent, e.g., the west coast and southern plains. For regions with dissimilar timing distribution of extreme precipitation and floods, the scaling patterns are much different. To further explain the contrasting scaling patterns, we used interpretable machine learning to investigate the extent to which different variables/processes contribute to the floods across the CONUS. We formulated four common flood generating mechanisms, including (1) precipitation, (2) precipitation excess, considering precipitation infiltration upon unsaturated soil, (3) snowmelt, (4) effective precipitation, considering both snowmelt and infiltration. Tree-based regression models (XGBoost, random forest) combined with interpretable machine learning (SHAP, ALE) were then used to quantify the relative contributions of these mechanisms to floods. The results show that the relative contributions of these mechanism show clear regional patterns. The scaling patterns of extreme precipitation and floods across the CONUS are in line with the contribution patterns of the four flood generating mechanisms. It demonstrates that the sensitivity of floods to warming climate is determined by local flood generating mechanisms. Therefore, more attention should be given to flood generating mechanisms when exploring flood responses to global warming.</p>