Attribution of Changes in small and large Floods across Brazil

Abstract. In tropical regions, flood changes are driven by a combination of event rainfall characteristics and antecedent wetness changes. However, how the interactions between storage capacity, event rainfall, and antecedent wetness influence flood changes across event magnitudes is elusive. Here, we explore the causes of changes in small and large floods by combining flood elasticities with trends in event rainfall peak and pre-event antecedent wetness of 765 catchments in Brazil. Our results suggest that large floods are increasing more than small ones, corresponding to 80 % of substantial flood increases. While those changes in large events are usually rainfall-driven, changes in small floods are mostly aligned with changes in antecedent wetness. We find that in regions with high water storage capacity, antecedent wetness drives changes in both small and large floods. Conversely, in regions with low water storage capacity, changes in small floods are driven by antecedent wetness, whereas large floods are mainly rainfall-driven, as rainfall outweighs antecedent wetness in those fast-saturating catchments. Our findings highlight that reliable predictions of flood responses to climate change should account for both event magnitude and catchment storage capacities, as climatic drivers alone are insufficient to fully explain flood changes.