Tramblay Yves, Arnaud P., Artigue G., Lang M., Paquet E., Neppel L., Sauquet E. (2023). Changes in Mediterranean flood processes and seasonality. Hydrology and Earth System Sciences, 27 (15), 2973-2987. ISSN 1027-5606.
Auteurs
Tramblay Yves, Arnaud P., Artigue G., Lang M., Paquet E., Neppel L., Sauquet E.
Source
Hydrology and Earth System Sciences, 2023,
27 (15), 2973-2987 ISSN 1027-5606
Floods are a major natural hazard in the Mediterranean region, causing deaths and extensive damages. Recent studies have shown that intense rainfallevents are becoming more extreme in this region but, paradoxically, without leading to an increase in the severity of floods. Consequently, it isimportant to understand how flood events are changing to explain this absence of trends in flood magnitude despite increased rainfall extremes. Adatabase of 98 stations in southern France with an average record of 50 years of daily river discharge data between 1959 and 2021 wasconsidered, together with a high-resolution reanalysis product providing precipitation and simulated soil moisture and a classification of weatherpatterns associated with rainfall events over France. Flood events, corresponding to an average occurrence of 1 event per year (5317 events intotal), were extracted and classified into excess-rainfall, short-rainfall, and long-rainfall event types. Several flood event characteristics havebeen also analyzed: flood event durations, base flow contribution to floods, runoff coefficient, total and maximum event rainfall, and antecedentsoil moisture. The evolution through time of these flood event characteristics and seasonality was analyzed. Results indicated that, in mostbasins, floods tend to occur earlier during the year, the mean flood date being, on average, advanced by 1 month between 1959-1990 and1991-2021. This seasonal shift could be attributed to the increased frequency of southern-circulation weather types during spring and summer. Anincrease in total and extreme-event precipitation has been observed, associated with a decrease of antecedent soil moisture before rainfallevents. The majority of flood events are associated with excess rainfall on saturated soils, but their relative proportion is decreasing over time,notably in spring, with a concurrent increased frequency of short rain floods. For most basins there is a positive correlation between antecedentsoil moisture and flood event runoff coefficients that is remaining stable over time, with dryer soils producing less runoff and a lowercontribution of base flow to floods. In a context of increasing aridity, this relationship is the likely cause of the absence of trends in floodmagnitudes observed in this region and the change of event types. These changes in flood characteristics are quite homogeneous over the domainstudied, suggesting that they are rather linked to the evolution of the regional climate than to catchment characteristics. Consequently, thisstudy shows that even in the absence of trends, flood properties may change over time, and these changes need to be accounted for when analyzing thelong-term evolution of flood hazards.
