PNNL

RICHLAND, Wash.—A new study finds that, by mid-century, nearly half the global population could live in areas where groundwater will become so costly as to raise regional food prices and significantly alter the geography of trade and crop production. Nine percent of the world’s water basins appear to have already reached such a state of near depletion. The new research, led by scientists at the Department of Energy’s Pacific Northwest National Laboratory, suggests an additional 11.5 percent could reach this point by 2030, with another 22 percent joining by mid-century.

The authors of the new work sought to identify when and where water withdrawals from many of the world’s aquifers could “peak,” as in, when external forces could drive groundwater extraction to reach its maximum. Similar peaks have been observed in other resources, like fossil fuels or minerals.

But, until now, no study has observed and quantified the same behavior in groundwater. This new work marks the first time anyone has projected the peak and decline of water withdrawals in relation to demand from human-driven systems. It represents “the most extensive, large ensemble experiment focused on future global groundwater extraction to date,” said lead author and Earth scientist Hassan Niazi.

Though water is conserved within Earth’s hydrosphere and never truly “lost,” the new work affirms that groundwater does behave as a non-renewable resource, constrained in part by the cost of retrieval. Those who study groundwater recognize this, said coauthor and Earth scientist Tom Wild.

“While it’s not new for us to show that groundwater behaves like a nonrenewable resource,” said Wild, “our work provides a very stark and visible reminder of groundwater’s finite nature, given how similar our projected groundwater patterns are to other resources that are in the process of being exhausted.”

Yet the rate at which groundwater is extracted globally, said Niazi, runs counter to this idea. Withdrawal rates, after all, continue to rise.

“About a fifth of the world’s food is grown using groundwater,” said Niazi. “So, it’s important to recognize that groundwater, particularly that in deep aquifers, is finite, much like oil or copper. And understanding when this depletable resource will peak and decline can help us carve out an informed path forward, as many regions face the imminent challenge of reducing their groundwater reliance. That’s important because groundwater is tied to so many essential functions of society, from irrigation to energy and, inseparably, to the well-being of our environment.”

The new work was published recently in the journal Nature Sustainability.

What drives a peak?

Though vast stores of water persist beneath Earth’s surface, the cost of extracting it climbs as water tables fall lower. More energy must be spent to overcome gravity when pumping groundwater to the surface.

When an aquifer’s level falls so low that its water can no longer be reached or the toll of doing so grows too costly, groundwater extraction reaches a point of either physical or economic infeasibility. Past this point, groundwater withdrawals typically decline as people look toward alternatives.

Other forces, too, can influence when such a peak may come. New technology like more efficient irrigation systems could dial down the amount of water people need in a region, offsetting a potential peak. Or groundwater-reliant populations could grow smaller, meaning there’s less demand. A complex system of factors stands to affect when basins peak and decline.

And that is exactly the complexity the researchers behind this new work sought to capture. Much of the research on groundwater depletion investigates when and where aquifers may physically “run dry.” Yet this is only one part of a complicated picture.

Peak water: when and where

Niazi and his coauthors examined 235 water basins across the globe. Complex simulations allowed them to probe the many factors that could drive basins to depletion across a range of hypothetical futures. Population growth, electricity demand and water use efficiency, among other socioeconomic factors, stand to shape when and where basins could peak.

To capture the uncertainty at play, the authors considered 900 total scenarios. They identified hotspots where peaks are most likely to pop up. In nearly all simulations, global groundwater reserves show a distinct peak and decline signature this century.

The timing and location of peaks differ depending on the nature of each scenario. Some basins, however, show a peak and decline signature across almost all scenarios.

Which are on track to peak in the coming decades? Those basins land within the Western United States, Mexico, India, Pakistan, China and several countries in the Middle East and Mediterranean. These are the same regions at the center of the Green Revolution, where crop production swelled in the early 20th century.

These same basins still serve as breadbaskets for most of the world. Yet, at the same time, the authors point out that these regions are responsible for most of the world’s unsustainable groundwater withdrawal. They are positioned to be on the receiving end of groundwater stress, should current withdrawal trends continue.

“At the very least, key basins in many of the nations responsible for the past 50 years of global agro-economic productivity could be forced into a transition away from groundwater use,” said Wild. “Our findings highlight imminent transformations in the way these regions conduct trade and manage water.”

Roughly half the basins under study do not peak and decline in any scenario. These include regions like the Amazon, where inexpensive surface water is sufficiently plentiful to meet projected demand, or areas where water needs are anticipated to be quite low, like in the higher latitude areas of Canada. Twenty-one basins have already peaked or are in the midst of peaking, including those in California and Missouri.

Peak and decline: what follows

What happens when a region peaks? With water supplies becoming increasingly limited, climbing water costs can cascade across sectors, causing food prices to rise—this could spark shifts in international trade as nations seek to import crops or water from less expensive regions. While not specifically addressed in this study, many prior studies have shown that as groundwater demand increases, aquatic ecosystems could face greater stress, water contamination could spread, and the land above diminished aquifers could sink into the earth more often—a phenomenon known as land subsidence.

The authors add that competing interests for water stem from many sectors: energy, manufacturing, agriculture, livestock, etc. Each of these can face unforeseen stress due to increasing demand for water within a region, driving a resultant rise in groundwater extraction.

“There’s great value in considering multiple sectors and their interactions in an integrated way,” Niazi added. “These analyses can help us to make more informed decisions when we face challenges not only in the realm of groundwater, but also challenges related to sustainable agriculture systems, or with planning a resilient grid.”

The authors highlight that nations with previously untapped ground or surface water resources could help to meet a gap in demand. Afflicted regions may need to expand rainfed croplands, or import crops through international trade, or seek less water-intensive power plant cooling technology, among other strategies.

Niazi and his coauthors point out some limitations of their work. More thorough records of groundwater withdrawals would strengthen future analyses. Considering other forms of adaptation, too, could better capture future paths on offer.

“Water is, of course, among the most important resources we manage,” said Wild. “Understanding our future with groundwater—all water, really—requires that we must understand it holistically. That means understanding how water withdrawal interacts with energy demand, with food production, with the extraction of raw materials, and more. And that’s what we’ve set out to show in this work.”

This work stems from the Joint Global Change Research Institute, a partnership between the University of Maryland and PNNL. There, researchers from a wide range of disciplines collaborate to model human and Earth systems, from those that harness and deliver energy into our homes to those that govern extreme weather. Their research helps provide decision-relevant information for management of emerging global risks and opportunities.

In addition to Niazi and Wild, authors of this work include Neal Graham, Son Kim, Mengqi Zhao, Sean Turner, Mohamad Hejazi, Siwa Msangi, and Jonathan Lamontagne. This work was supported by DOE's Office of Science.