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The dataset presents a first attempt to estimate the volume of water required for the maintenance of freshwater-dependent ecosystems at the global scale. This total environmental water requirement (presented as a percentage of Mean Annual River Flow - MAR) consists of ecologically relevant low-flow and high-flow components, and depends upon the objective of environmental water management. Both components are related to river flow variability and are estimated by conceptual rules from discharge time series simulated by the global hydrology model.
A water stress indicator is further defined, which shows what proportion of the utilizable water in world river basins is currently withdrawn for direct human use and where this use is in conflict with environmental water requirements. Approximately 20 to 50 percent of the mean annual river flow in different basins needs to be allocated to freshwater-dependent ecosystems to maintain them in fair conditions. This is unlikely to be possible in many developing countries in Asia and North Africa, in parts of Australia, North America, and Europe, where current total direct water withdrawals (primarily for irrigation) already tap into the estimated environmental water requirements. Over 1.4 billion people currently live in river basins with high environmental water stress. This number will increase as water withdrawals grow and if environmental water allocations remain beyond the common practice in river basin management.