Observational Constraint Reveal Overestimation of Required Water Storage Expansion under Climate Change

Effective water storage strategies are essential for reducing drought and flood risks, enhancing agricultural productivity, and fostering socioeconomic development. However, estimating future required water storage capacity (RWSC) is subject to great uncertainty due to varying model predictions of runoff variability (Rv). Here we integrate observations with the identified emergent constraint framework to refine global ΔRv estimates and reassess ΔRWSC across global basins. Under the SSP5-8.5 scenario for 2070-2099, the constrained RWSC increases by 24.39-29.93% across all three return periods (30, 50, and 100 years) compared to the historical period (1980-2014). Notably, the constrained ΔRWSC exhibits a significant decrease, particularly in lower-middle-income basins (by 11.66-22.12%) and low-income basins (by 7.95-14.69%), due to overestimations in ΔRv (by 26.98%). Our findings suggest a lower risk associated with Rv and a diminished need for water storage expansion, especially in basins with lower income levels, as shown by raw model projections.