Multi-Day Precipitation and Compound Events Challenge the Flood Attenuation Ability of Reservoirs

Abstract. Reservoirs are among the most widespread anthropogenic modifications of river systems and serve multiple purposes, including flood attenuation. Despite their importance, few studies explicitly account for reservoir regulation when examining flood drivers. Here, we investigate 175 catchments in Peninsular India spanning a broad gradient of regulation intensity, using catchment-scale, event-based, and long-term analyses. Event coincidence analysis and an event classification framework were used to examine how the relative importance of hydrometeorological flood drivers changes with regulation. We found that regulated catchments were able to attenuate floods generated by short-duration extreme rainfall, whereas multi-day precipitation and compound events increasingly challenged this attenuation ability. The relative importance of antecedent catchment wetness as a flood driver also increased with regulation. To assess changes in long-term streamflow, we applied the Budyko framework and found that catchment factors dominated streamflow change in 61.36 % of catchments, with increasing regulation associated with greater streamflow reductions. These findings highlight the importance of accounting for prolonged rainfall sequences and antecedent catchment wetness when assessing flood risk in regulated catchments, with implications for design flood estimation and reservoir operation strategies that primarily focus on short-duration extreme storms.