Declining water storage and rising risk: considerations on implications for hydropower and water security

Water storage is a critical component of global water security, yet its future availability is increasingly uncertain under climate change and infrastructure constraints. This study synthesises current evidence on global freshwater-storage trends, with a particular focus on implications for hydropower systems and their reliability. We discuss changes in both natural storage—glaciers, snowpack, soil moisture, and groundwater—and artificial storage provided by reservoirs and dams. Multiple lines of evidence indicate that terrestrial water storage has been declining globally over recent decades, driven by climate-induced hydrological shifts, groundwater depletion, and glacier retreat. At the same time, the expansion of artificial storage has slowed as dam construction plateaus in many regions because of environmental, social, and economic constraints, while aging infrastructure and sedimentation progressively reduce effective reservoir capacity. These trends undermine the common assumption in water and energy planning that storage capacity will continue to grow or remain stable. The review highlights how over-estimation of future storage leads to systematic under-estimation of water and hydropower risk, particularly under increasing climate variability. Significant uncertainties persist in measuring and modelling global water storage, with discrepancies between satellite observations, ground data, and model projections complicating robust assessments. Finally, we discuss adaptive pathways and conclude that realistic representation of declining storage, coupled with integrated climate-adaptive management strategies, is essential to safeguard future water security and hydropower resilience.