Historic assessment of groundwater sustainability

Abstract Groundwater, the largest accessible store of freshwater, plays a critical role in sustaining ecosystems and supporting human activities. Increasing water demands and climate variability, however, are accelerating the exploitation of this resource in many regions, often beyond environmentally sustainable limits. In this study, we present the first global-scale assessment of sustainable groundwater head declines, defined as the maximum permissible drop in groundwater head that avoids environmental harm. With a global-scale hydrological-groundwater model coupling (VIC-WUR-MODFLOW) at 5 arcmin resolution, we simulated groundwater discharge and head under natural conditions. Using these values, we isolated mean groundwater levels during low-flow, intermediate-flow, and high-flow periods to estimate historic sustainable groundwater head declines. We compared our estimates with observations and simulations to assess the historic sustainability of groundwater use practices. Globally, the median sustainable groundwater head decline was 4.5 m between 1984–2018. Tropical regions (e.g. Amazon Basin, southeast Asia) exhibited greater sustainably available groundwater due to high recharge, while arid regions (e.g. western United States, South Africa) exhibited limited capacities. Comparisons with historic groundwater head revealed that several key agricultural regions (e.g. Indo-Gangetic Plain, North China Plain, Central Valley) exceeded sustainable limits by more than 20 m, corresponding to overexploitations of sustainable resources by more than 150%. In contrast, much of Europe and North America remained within sustainable boundaries when assessed at the aquifer-scale, although localized overexploitation was still evident. We also identified emerging hotspots (e.g. Iran, northern Africa) where groundwater head declines approached sustainable thresholds over the study period. As freshwater demands continue to rise, these regions may become future hotspots of unsustainable groundwater use without intervention. This study provides a novel global methodology to quantify environmentally sustainable groundwater resources, offering insight into at-risk regions and supporting informed water resources management under future pressures.