Joint effects of storage-runoff limitation amplify global stress in water availability

Nearly 80% of the global population faces severe water stress, yet changes in global water availability are not well quantified. Two essential factors for identifying water stress and promoting sustainable development include runoff, a flux variable reflecting water use, and freshwater storage, representing the potentially maximum water resources. Despite their importance, integrated analysis of changes in runoff/storage and their impacts on society remains in its infancy at the global scale. By leveraging the strengths of remote sensing techniques, advanced land surface models, and reanalysis data, here we quantified global changes in total water storage (TWS) and runoff over the past two decades. In addition, we proposed new indices to evaluate the relative vulnerability in water availability normalized at the global scale. The results show that 80% of global areas experienced declines in TWS and/or runoff for the past two decades, and 39% of areas suffered from water loss in both storage and runoff. The joint effects of storage-runoff limitation amplified vulnerability in water availability, reflected by not only a larger spatial domain but also higher severity relative to individual stress caused by either TWS or runoff changes. The most vulnerable regions in water availability were found across the north and central South America, south Asia, and Europe. Specific threats include the loss of solid water storage, groundwater extraction for irrigation, and climate-induced extreme heat and drought over the past two decades. Our findings provide valuable insights not only for understanding hydrologic responses to a changing climate and fast-developing society, but also for developing adaptive strategies for water-stressed hotspots.