Global irrigation reservoirs are at a higher risk of water shortages.

Water shortages induced by Reservoir-Based Droughts (RBD) pose a significant threat to global food, water, and energy security. However, a global-scale assessment of RBD, identification of reservoirs at higher risk of water shortages, and the dominant drivers (climate vs. human) remain underexplored. Addressing these gaps is critical for identifying where overreliance on reservoirs may undermine long-term sustainability and where targeted policy and management reforms are needed. Here, we evaluate RBD at 1203 large global reservoirs, representing 81% of the total global reservoir storage capacity. We further evaluate drought propagation time from meteorological drought (MD) to RBD, recovery time, and severity. We then use these metrics to classify reservoirs into low- and high-risk categories. The results suggest that reservoirs in tropical regions face faster drought propagation, whereas those in dry climates witness longer recovery times and greater severity. Recovery time and severity are substantially higher for irrigation and water supply reservoirs. Nearly 24% of all reservoirs are at high risk of water shortages with long recovery times and high severity. Irrigation reservoirs have the highest proportion (37%) of high-risk reservoirs, suggesting that overreliance on them could exacerbate future water shortages. Notably, 834 of the 1,203 reservoirs-nearly 70% of the total-have a linkage probability (i.e., percentage of RBD linked to upstream MD) greater than 80%, indicating that climate is the dominant driver of RBD. Through the lens of RBD, this study provides a deeper understanding of water shortages and facilitates more efficient water resource planning and management.