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The world's largest freshwater resource is groundwater. A review of our understanding of groundwater depletion suggests that although the problem is global, solutions must be adapted to specific regional requirements at the aquifer scale. Groundwater—the world's largest freshwater resource—is critically important for irrigated agriculture and hence for global food security. Yet depletion is widespread in large groundwater systems in both semi-arid and humid regions of the world. Excessive extraction for irrigation where groundwater is slowly renewed is the main cause of the depletion, and climate change has the potential to exacerbate the problem in some regions. Globally aggregated groundwater depletion contributes to sea-level rise, and has accelerated markedly since the mid-twentieth century. But its impacts on water resources are more obvious at the regional scale, for example in agriculturally important parts of India, China and the United States. Food production in such regions can only be made sustainable in the long term if groundwater levels are stabilized. To this end, a transformation is required in how we value, manage and characterize groundwater systems. Technical approaches—such as water diversion, artificial groundwater recharge and efficient irrigation—have failed to balance regional groundwater budgets. They need to be complemented by more comprehensive strategies that are adapted to the specific social, economic, political and environmental settings of each region.

The flow of fresh groundwater may provide substantial inputs of nutrients and solutes to the oceans. However, the extent to which hydrogeological parameters control groundwater flow to the world’s oceans has not been quantified systematically. Here we present a spatially resolved global model of coastal groundwater discharge to show that the contribution of fresh groundwater accounts for ~0.6% (0.004%–1.3%) of the total freshwater input and ~2% (0.003%–7.7%) of the solute input for carbon, nitrogen, silica and strontium. However, the coastal discharge of fresh groundwater and nutrients displays a high spatial variability and for an estimated 26% (0.4%–39%) of the world’s estuaries, 17% (0.3%–31%) of the salt marshes and 14% (0.1–26%) of the coral reefs, the flux of terrestrial groundwater exceeds 25% of the river flux and poses a risk for pollution and eutrophication. The authors here present the global entry of nutrients into marine systems through fresh submarine groundwater discharge to be below 1%. However, they also identify hotspots and argue that about 25% of world’s estuaries are at danger of eutrophication.

The depletion of groundwater resources threatens food and water security in India. However, the relative influence of groundwater pumping and climate variability on groundwater availability and storage remains unclear. Here we show from analyses of satellite and local well data spanning the past decade that long-term changes in monsoon precipitation are driving groundwater storage variability in most parts of India either directly by changing recharge or indirectly by changing abstraction. We find that groundwater storage has declined in northern India at the rate of 2 cm/yr and increased by 1 to 2 cm/yr in southern India between 2002 and 2013. We find that a large fraction of the total variability in groundwater storage in north-central and southern India can be explained by changes in precipitation. Groundwater storage variability in northwestern India can be explained predominantly by variability in abstraction for irrigation, which is in turn influenced by changes in precipitation. Declining precipitation in northern India is linked to Indian Ocean warming, suggesting a previously unrecognized teleconnection between ocean temperatures and groundwater storage.

There are concerns that sea-level rise resulting from climate change could lead to saltwater intrusion into coastal aquifers. However, a study shows that groundwater extraction is the main driver of saltwater intrusion in the United States, highlighting the importance of sustainable water management. Climate change and human population growth are expected to have substantial impacts on global water resources throughout the twenty-first century 1 , 2 . Coastal aquifers are a nexus 3 of the world’s oceanic and hydrologic ecosystems and provide a water source for the more than one billion people living in coastal regions 4 , 5 . Saltwater intrusion caused by excessive groundwater extraction is already impacting diverse regions of the globe 5 , 6 , 7 . Synthesis studies 8 , 9 and detailed simulations 10 , 11 , 12 , 13 have predicted that rising sea levels could negatively impact coastal aquifers through saltwater intrusion and/or inundation of coastal regions. However, the relative vulnerability of coastal aquifers to groundwater extraction and sea-level rise has not been systematically examined. Here we show that coastal aquifers are more vulnerable to groundwater extraction than to predicted sea-level rise under a wide range of hydrogeologic conditions and population densities. Only aquifers with very low hydraulic gradients are more vulnerable to sea-level rise and these regions will be impacted by saltwater inundation before saltwater intrusion. Human water use is a key driver in the hydrology of coastal aquifers, and efforts to adapt to sea-level rise at the expense of better water management are misguided.

Our environment is heterogeneous. In hydrological sciences, the heterogeneity of subsurface properties, such as hydraulic conductivities or porosities, exerts an important control on water balance. This notably includes groundwater recharge, which is an important variable for efficient and sustainable groundwater resources management. Current large-scale hydrological models do not adequately consider this subsurface heterogeneity. Here we show that regions with strong subsurface heterogeneity have enhanced present and future recharge rates due to a different sensitivity of recharge to climate variability compared with regions with homogeneous subsurface properties. Our study domain comprises the carbonate rock regions of Europe, Northern Africa, and the Middle East, which cover ∼25% of the total land area. We compare the simulations of two large-scale hydrological models, one of them accounting for subsurface heterogeneity. Carbonate rock regions strongly exhibit “karstification,” which is known to produce particularly strong subsurface heterogeneity. Aquifers from these regions contribute up to half of the drinking water supply for some European countries. Our results suggest that water management for these regions cannot rely on most of the presently available projections of groundwater recharge because spatially variable storages and spatial concentration of recharge result in actual recharge rates that are up to four times larger for present conditions and changes up to five times larger for potential future conditions than previously estimated. These differences in recharge rates for strongly heterogeneous regions suggest a need for groundwater management strategies that are adapted to the fast transit of water from the surface to the aquifers.

Groundwater is important for energy and food security, human health and ecosystems. The time since groundwater was recharged—or groundwater age—can be important for diverse geologic processes, such as chemical weathering, ocean eutrophication and climate change. However, measured groundwater ages range from months to millions of years. The global volume and distribution of groundwater less than 50 years old—modern groundwater that is the most recently recharged and also the most vulnerable to global change—are unknown. Here we combine geochemical, geologic, hydrologic and geospatial data sets with numerical simulations of groundwater and analyse tritium ages to show that less than 6% of the groundwater in the uppermost portion of Earth’s landmass is modern. We find that the total groundwater volume in the upper 2 km of continental crust is approximately 22.6 million km 3 , of which 0.1–5.0 million km 3 is less than 50 years old. Although modern groundwater represents a small percentage of the total groundwater on Earth, the volume of modern groundwater is equivalent to a body of water with a depth of about 3 m spread over the continents. This water resource dwarfs all other components of the active hydrologic cycle. Publisher Correction (11 June 2018) Groundwater recharged less than 50 years ago is vulnerable to contamination and land-use changes. Data and simulations suggest that up to 6% of continental groundwater is modern—forming the largest component of the active hydrologic cycle.

Humans and ecosystems are deeply connected to, and through, the hydrological cycle. However, impacts of hydrological change on social and ecological systems are infrequently evaluated together at the global scale. Here, we focus on the potential for social and ecological impacts from freshwater stress and storage loss. We find basins with existing freshwater stress are drying (losing storage) disproportionately, exacerbating the challenges facing the water stressed versus non-stressed basins of the world. We map the global gradient in social-ecological vulnerability to freshwater stress and storage loss and identify hotspot basins for prioritization ( n  = 168). These most-vulnerable basins encompass over 1.5 billion people, 17% of global food crop production, 13% of global gross domestic product, and hundreds of significant wetlands. There are thus substantial social and ecological benefits to reducing vulnerability in hotspot basins, which can be achieved through hydro-diplomacy, social adaptive capacity building, and integrated water resources management practices. This work identifies the world’s most vulnerable basins to social and ecological impacts from freshwater stress and storage loss: a set of 168 hotspot basins for global prioritization that encompass 1.5 billion people, 17% of global food crops, 13% of global GDP, and hundreds of significant wetlands.

A newly developed concept called ‘groundwater footprint’ is used to reveal the degree of sustainable use of global aquifers by calculating the area relative to the extractive demands; globally, this footprint exceeds aquifer area by a factor of about 3.5, and excess withdrawal is centred on just a few agriculturally important aquifers. Striking a balance on groundwater usage In many parts of the world, groundwater is being extracted for agricultural use and human consumption at a greater rate than the Earth's natural systems can replace it. Tom Gleeson and colleagues estimate the true scale of the problem using a newly developed concept called the 'groundwater footprint' — defined as the area required to sustain groundwater use and groundwater-dependent ecosystem services. The authors find that globally, the groundwater footprint exceeds the aquifer area by a factor of about 3.5. Overexploitation centres predominantly on a few agriculturally important aquifers in arid or semiarid climates, especially in Asia and North America. The groundwater footprint could serve as a useful framework for analysing the global groundwater depletion data sets emerging from NASA's GRACE satellites. Groundwater is a life-sustaining resource that supplies water to billions of people, plays a central part in irrigated agriculture and influences the health of many ecosystems 1 , 2 . Most assessments of global water resources have focused on surface water 3 , 4 , 5 , 6 , but unsustainable depletion of groundwater has recently been documented on both regional 7 , 8 and global scales 9 , 10 , 11 . It remains unclear how the rate of global groundwater depletion compares to the rate of natural renewal and the supply needed to support ecosystems. Here we define the groundwater footprint (the area required to sustain groundwater use and groundwater-dependent ecosystem services) and show that humans are overexploiting groundwater in many large aquifers that are critical to agriculture, especially in Asia and North America. We estimate that the size of the global groundwater footprint is currently about 3.5 times the actual area of aquifers and that about 1.7 billion people live in areas where groundwater resources and/or groundwater-dependent ecosystems are under threat. That said, 80 per cent of aquifers have a groundwater footprint that is less than their area, meaning that the net global value is driven by a few heavily overexploited aquifers. The groundwater footprint is the first tool suitable for consistently evaluating the use, renewal and ecosystem requirements of groundwater at an aquifer scale. It can be combined with the water footprint and virtual water calculations 12 , 13 , 14 , and be used to assess the potential for increasing agricultural yields with renewable groundwaterref 15 . The method could be modified to evaluate other resources with renewal rates that are slow and spatially heterogeneous, such as fisheries, forestry or soil.