Contrasting changes in the type-dependent river floods over the past 40 years

Abstract:

Global warming continues to exacerbate extreme rainfall events. However, whether and to what extent the increased extreme rainfall translated to changes in floods remain controversial in literautre, posing difficulties in assessing flood risks under future warming. Here, we assessed changes in flood magnitudes with respect to different flood types (i.e., intense rainfall floods, IRFs, snowmelt floods, SMFs, and rain-on-snow floods, RoSFs) over the past four decades at 4927 unimpaired catchments across North America and Europe. We find that increased extreme rainfall magnitudes (on average by 5.4%) has led to increases in IRFs in 68% of the catchments with an average increase of 13%, confirming a positive and amplified response of IRFs to extreme rainfall increase. By contrast, decreased SMFs and RoSFs are found in 66% and 67% of the catchments, with an average decrease of 13% and 12%, respectively. As a result, the increases in IRFs are almost entirely offset by the concurrent decreases in snow-related floods, resulting in a more or less unchanged total flood (increased by 1.2% on average) over the past four decades. This offsetting effect is especially evident in high-latitudes and mountainous regions, where snow-related floods contribute significantly to the total flood changes. Our results provide direct observational evidence to the response of river floods to warming-induced extreme rainfall increases and highlight the needs of distinguishing different flood types when assessing flood changes and future flood risks in a changing climate.