Impact of rainfall variability on sedimentary and hydropower dynamics in a dam reservoir of southern France

Abstract. Hydropower is the leading renewable energy source, but its vulnerability to environmental pressures remains a critical concern, particularly under climate change. While climate impacts on hydropower are well studied, the interplay between precipitation variability, sediment dynamics, and dam operation strategies has received comparatively less attention. This study investigates these interactions in the French Mediterranean region, focusing on the Mont d’Orb dam reservoir. An integrated approach was adopted, combining (1) sediment core analysis using fallout radionuclide dating to assess the impact of extreme rainfall on sediment yield; (2) statistical analysis of meteorological records to detect long-term trends and seasonal shifts; and (3) hydrological and operational data analysis to evaluate how sedimentation, precipitation variability, and reservoir management influence hydropower generation. Results show that extreme rainfall events (top 1 %) contribute disproportionately—20–50 %—to the annual sediment delivery. Although annual precipitation has not significantly changed since 1950, a clear seasonal shift from winter- to fall-dominant precipitation emerged. This change is driven by a significant increase in fall event frequency (+87 % to +400 % for 100 to 150 mm/day thresholds) and intensity (+31 %), alongside a -20 % decrease in winter precipitation, which is key to reservoir recharge. During thefirst two management periods, hydropower generation was more impacted by multipurpose dam objectives (e.g.,flood control, water supply) than by direct climatic forcing. A high sedimentation rate of 19.3 mm/year was observed. While sedimentation did not directly affect electricity production yet, it poses long-term risks to storage capacity and turbine functioning. Hydropower output declined by 25 % between 1976–1997 and 2006–2022, underlining the growing sensitivity of hydropower to hydroclimatic constraints. Thesefindings highlight the need to better anticipate future environmental impacts in the context of rising electricity demand and climate uncertainty.