Global Assessment of Reservoir Drought Patterns

Reservoirs are a cornerstone of global water management, providing critical buffering capacity for irrigation, municipal supply, energy production, and environmental flows. However, intensifying drought under warming climate is increasingly challenging the reliability of reservoir storage worldwide. Unlike meteorological or hydrological droughts, reservoir droughts emerge from nonlinear and lagged interactions between climate forcings, catchment processes, and storage dynamics, and therefore remain poorly characterized in global drought assessments. In particular, consistent metrics that capture the intensity, spatial extent, and frequency of reservoir storage deficits across regions are lacking.

Here, we present a global assessment of reservoir drought patterns using a storage-centric framework based on standardized reservoir storage anomalies (SRSA). We analyze monthly storage variations for approximately 7,000 large reservoirs worldwide over the period 1999–2018, spanning 44 IPCC reference regions. SRSA is computed using long-term, month-specific climatologies for each reservoir. Trends in SRSA are evaluated using Mann–Kendall tests at the reservoir scale and aggregated to assess field-significant regional patterns. To characterize the spatiotemporal structure of reservoir droughts, we extend the Intensity–Extent–Frequency (IEF) framework to reservoir storage. Drought intensity is defined as the normalized storage deficit within affected reservoirs, extent as the fraction of regional storage capacity impacted, and frequency as the return interval of such events. Capacity-weighted metrics are used to quantify regional drought behavior and distinguish localized from system-wide storage failures.

Results reveal a pronounced latitudinal divide in global reservoir drought behavior. Northern regions generally exhibit stable or increasing storage trends and experience frequent but low-severity fluctuations that are spatially coherent across reservoirs. In contrast, tropical and subtropical regions show declining storage trends and are dominated by episodic, high-intensity droughts that affect a limited fraction of regional capacity. IEF curves further demonstrate that extreme reservoir drought risk is primarily driven by localized storage failures in lower-latitude regions, whereas droughts in northern regions tend to expand more uniformly across systems. These findings highlight substantial regional heterogeneity in reservoir drought dynamics and emphasize the importance of storage-based diagnostics for understanding drought risk in managed water systems. This study provides a global, capacity-weighted assessment of reservoir drought intensity, extent, and frequency and establishes a transferable framework for evaluating reservoir vulnerability under ongoing and future climate change.