A Global Groundwater Stress Index from Estimates of Groundwater Withdrawal and Recharge

Over 2 billion people rely on groundwater as their primary source of fresh water for consumptive and agricultural use, yet most research on characterizing water stress indices has not included groundwater aquifers. This study explicitly evaluates groundwater use compared to groundwater recharge to create a global groundwater scarcity index in order to determine which regions of the world face groundwater stress. Per capita groundwater use date were compiled by country. These were combined with higher-resolution global population maps to determine global patterns of groundwater use. Groundwater recharge was estimated by taking the long-term average from the output of a global hydrological model. A groundwater stress index was created by dividing rates of groundwater withdrawal by rates of groundwater recharge and applying a standard scarcity scale established by the United Nations. The resulting map correctly reflects known regions of water stress. The global map of the groundwater stress index was compared to trends in net groundwater recharge derived from a combination of GRACE data and hydrological model output. The comparison highlights important differences between the long-term average approach based on by-country reporting, and direct observation of the recent past from satellite data. Furthermore, it underscores the need for an accelerated effort to collect and compile accurate global groundwater datasets.