Environmental flow requirements can largely reshape global water scarcity assessment

Environmental flow requirements (EFRs) are critical to maintaining the health of river ecosystems. Incorporating EFRs in water scarcity assessment is necessary for appropriately reflecting the status of the scarcity. There are various methods for estimating EFRs from local to global scales. The quantification of EFR subject to large uncertainties on a global scale due to complex river ecological conditions and unavailability of local information. So far, it is unclear how much EFRs from different methods would affect the water scarcity assessment on a global scale. In this study, the differences in EFRs by several commonly used methods are examined and their impacts on water scarcity assessment are quantified on a global scale. The calculated water scarcity index (WSI, a ratio of water withdrawal to water availability) indicate that different EFR methods could result in quite different water scarcity assessment with respect to magnitude and extent over the world. The global mean EFRs range from 96 m³/s to 428 m³/s for different EFR quantification methods, which result in largely different WSIs. Using these different EFRs, the increase of area under water scarcity (WSI > 1) could vary from by 3% to more than 40% of global land areas compared to that without considering EFR. Accordingly, distinct estimates of affected population ranging from 30% to 60% of the global total are indicated by WSIs with inclusion of different EFRs. We emphasize the sharp differences in EFRs derived from different methods and call for more attention on their impacts on water scarcity assessment.