Global patterns of reservoir fullness and fluctuations during droughts

Abstract Reservoirs are designed to mitigate water and energy crises during droughts, yet their conditions—such as fullness and fluctuations during droughts remain underexplored. In particular, our understanding of how these conditions are related to reservoir function, size, location, climate, and socio-economic context remains limited. Here, we assess the storage conditions of 6634 global reservoirs during major river basin-scale droughts using normalized storage (NS; storage-to-capacity ratio). River basin-scale drought spells were identified using a 6 month Standardized Precipitation Evapotranspiration Index (SPEI) during 1999–2018. Our results showed that reservoirs in Asia had the lowest NS levels during droughts compared to other continents. Reservoirs in Africa and Asia exhibited the largest fluctuations, with median NS fluctuations of 34% and 37%, respectively. Large capacity reservoirs demonstrated greater resilience, whereas smaller reservoirs fluctuated more. Irrigation reservoirs had the lowest NS levels (49%) and the greatest NS fluctuations (33%). Reservoirs in dry climate remained the least filled, with irrigation reservoirs consistently showing the largest fluctuations across all climate zones. We found that reservoirs located in economically stronger countries tend to maintain higher storage levels (61% vs. 46%) and lower fluctuations (23% vs. 33%) during droughts. Furthermore, almost 40% of all reservoirs exhibited a significant ( p -value < 0.05) correlation between NS and the Oceanic Niño Index (ONI), highlighting the association between large-scale climate variability and reservoir storage dynamics. Overall, our study provides valuable observational insights that can guide targeted management strategies and contribute to sustainable, long-term water governance.