Comment on hess-2023-46

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Floods are a major natural hazard in the Mediterranean region, causing deaths and extensive damages. Recent studies have shown that intense rainfall events are becoming more extreme in this region, but paradoxically without leading to an increase in the severity of flood events. Consequently, it is important to understand how flood events are changing to explain this absence of trends in flood magnitude despite increased rainfall extremes. A database of 98 stations in Southern France with an average record of 50 years of daily river discharge data between 1958 and 2021 was considered, together with a high-resolution reanalysis product providing precipitation and simulated soil moisture. Flood events, corresponding to an average occurrence of one event per year (5317 events in total), were extracted and classified into excess rainfall, short rainfall and long rainfall event types. The evolution through time of the flood event characteristics and seasonality were analyzed. Results indicated that, in most basins, floods tend to occur earlier during the year, the mean flood date being on average advanced by one month. This seasonal shift can be attributed to the increased frequency of southern-circulation weather types during spring and summer. An increase in total and extreme event precipitation has been observed, associated with a decrease of antecedent soil moisture before rainfall events, linked to a smaller contribution of base flow during floods. The majority of flood events are associated with excess rainfall on saturated soils, but their relative proportion is decreasing over time with a concurrent increased frequency of short rain floods. Therefore, this study shows that even in the absence of trends, flood properties may change over time and these changes need to be accounted for when analyzing the long-term evolution of flood hazards.