Groundwater is rapidly declining across the globe, often at accelerating rates. Writing in the journal Nature, UC Santa Barbara researchers present the largest assessment of groundwater levels around the world, spanning nearly 1,700 aquifers. In addition to raising the alarm over declining water resources, the work offers instructive examples of where things are going well, and how groundwater depletion can be solved. The study (Rapid groundwater declines in many aquifers globally but cases of recovery, DOI, 10.1038/s41586-023-06879-8) is a boon for scientists, policy makers and resource managers working to understand global groundwater dynamics.
Many parts of the world are experiencing a rapid depletion in the subterranean reserves of water that billions of people rely on for drinking, irrigation and other uses, according to new research that analyzed millions of groundwater level measurements from 170,000 wells in more than 40 countries.
It is the first study to piece together what is happening to groundwater levels at a global scale, according to the researchers involved.
The study, published in the journal Nature on Wednesday, will help scientists better understand what impact humans are having on this valuable underground resource, either through overuse or indirectly by changes in rainfall linked to climate change.
Groundwater, contained within cracks and pores in permeable bodies of rock known as aquifers, is a lifeline for people especially in parts of the world where rainfall and surface water are scarce, such as northwest India and the southwest U.S.
Reductions in groundwater can make it harder for people to access freshwater to drink or to irrigate crops and can result in land subsidence.
The work revealed that groundwater is dropping in 71% of the aquifers. And this depletion is accelerating in many places: the rates of groundwater decline in the 1980s and ’90s sped up from 2000 to the present, highlighting how a bad problem became even worse. The accelerating declines are occurring in nearly three times as many places as they would expect by chance.
“This study was driven by curiosity. We wanted to better understand the state of global groundwater by wrangling millions of groundwater level measurements,” said co-lead author Debra Perrone, an associate professor in University of California’s Santa Barbara’s Environmental Studies Program, in a news release on the study.
The scientists found that groundwater levels declined between 2000 and 2022 in 71% of the 1,693 aquifer systems included in the research, with groundwater levels declining more than 0.1 meter a year in 36%, or 617, of them.
The Ascoy-Soplamo Aquifer in Spain had the fastest rate of decline in the data they compiled–a median decline of 2.95 meters per year, said study coauthor Scott Jasechko, an associate professor at the Bren School of Environmental Science and Management at University of California Santa Barbara.
Several aquifer systems in Iran were among those with the fastest rate of groundwater decline, he added.
The team was not able to gather data from much of Africa, South America and southeast Asia because of a lack of monitoring, but Jasechko said the study included the countries where most global groundwater pumping takes place.
Declines Are Not Universal
The study also highlighted some success stories in Bangkok, Arizona and New Mexico, where groundwater has begun to recover after interventions to better regulate water use or redirect water to replenish depleted aquifers.
“I was impressed by the clever strategies that have been put into action to address groundwater depletion in several places, though these ‘good news’ stories are very rare,” Jasechko said via email.
To understand whether the declines seen in the 21st century were accelerating, the team also accessed data for groundwater levels for 1980 to 2000 for 542 of the aquifers in the study.
They found that declines in groundwater levels sped up in the first two decades of the 21st century for 30% of those aquifers, outpacing the declines recorded between 1980 and 2000.
“These cases of accelerating groundwater-level declines are more than twice as prevalent as one would expect from random fluctuations in the absence of any systematic trends in either time period,” the study noted.
Donald John MacAllister, a hydrologist at the British Geological Survey who was not involved in the research, said it was a really “impressive” set of data, despite some gaps.
“I think it is fair to say this global compilation of groundwater data has not been done, certainly on this scale, at least to my knowledge before,” he said.
Groundwater is an incredibly important resource but one of the challenges is because we cannot see it, it is out of mind for most people. Our challenge is to constantly bang the drum for policymakers–that we have this resource that we have to look after, and that we can use to build resilience and adapt to climate change.
300 Million Water Level Measurements
A news release by the University of California—Santa Barbara (news.ucsb.edu) said:
The team compiled data from national and subnational records and the work of other agencies. The study took three years, two of which were spent just cleaning and sorting data. That is what it takes to make sense of 300 million water level measurements from 1.5 million wells over the past 100 years.
Next came the task of translating the deluge of data into actual insights about global groundwater trends. The researchers then scoured over 1,200 publications to reconstruct aquifer boundaries in the regions of inquiry and evaluate groundwater level trends in 1,693 aquifers.
Their findings provide the most comprehensive analysis of global groundwater levels to date, and demonstrate the prevalence of groundwater depletion.
Groundwater deepening is more common in drier climates, with accelerated decline especially prevalent in arid and semi-arid lands under cultivation–“an intuitive finding,” said co-lead author Scott Jasechko.
But it is one thing for something to be intuitive. It is quite another to show that it is happening with real-world data.
On the other hand, there are places where levels have stabilized or recovered. Groundwater declines of the 1980s and ’90s reversed in 16% of the aquifer systems the authors had historical data for. However, these cases are only half as common as would be expected by chance.
“This study shows that humans can turn things around with deliberate, concentrated efforts,” Jasechko said.
Take Tucson, Arizona for instance. Water allotted from the Colorado River is used to replenish the aquifer in the nearby Avra Valley. The project stores water for future use. “Groundwater is often viewed as a bank account for water,” Jasechko explained.
Intentionally refilling aquifers allows us to store that water until a time of need.
Communities can spend a lot of money building infrastructure to hold water above ground. But if you have the right geology, you can store vast quantities of water underground, which is much cheaper, less disruptive and less dangerous. The stored groundwater can also benefit the region’s ecology. In fact, while preparing a research brief in 2014, Perrone found that aquifer recharge can store six times more water per dollar than surface reservoirs.
Tucson’s groundwater recharge is a boon for the local aquifer; however, withdrawals have caused the mighty river to dwindle above ground. The Colorado rarely reaches its delta in the California Gulf anymore. “These groundwater interventions can have tradeoffs,” Jasechko acknowledged.
Another option is to focus on reducing demand. Often this involves regulations, permitting and fees for groundwater use, Perrone explained. To this end, she is currently examining water law in the western U.S. to understand these diverse interventions. Regardless of whether it comes from supply or demand, aquifer recovery seemed to require intervention, the study revealed.
The authors complemented measurements from monitoring wells with data from the Gravity Recovery and Climate Experiment (GRACE). The GRACE mission consists of twin satellites that precisely measure the distance between them as they orbit the Earth. In this way, the crafts detect small fluctuations in the planet’s gravity, which can reveal the dynamics of aquifers at large scales.
“The beauty of GRACE is that it allows us to explore groundwater conditions where we do not have in-situ data,” Perrone said. “Our assessment complements GRACE. Where we do have in-situ data, we can explore groundwater conditions locally, a crucial level of resolution when you are managing depletion.” This local resolution is critical, as the authors found out, because adjacent aquifers can display different trends.
That said, groundwater level trends do not present the whole picture. Even where aquifers remain stable, withdrawing groundwater can still affect nearby streams and surface water, causing them to leak into the subsurface, as Perrone and Jasechko detailed in another Nature paper in 2021.
The authors also analyzed precipitation variability over the past four decades for 542 aquifers. They found that 90% of aquifers where declines were accelerating are in places where conditions have gotten drier over the last 40 years. These trends have likely reduced groundwater recharge and increased demand. On the other hand, climate variability can also enable groundwater to rebound where conditions become wetter.
This study of monitoring wells complements a paper Perrone and Jasechko released in 2021. That study represented the largest assessment of global groundwater wells, and made the cover of the journal Science. “The monitoring wells are telling us information about supply. And the groundwater wells are telling us information about demand,” Perrone said.
“Taken together, they allow us to understand which wells have run dry already, or are most likely to run dry if groundwater-level declines occur,” Jasechko added.
Perrone and Jasechko are now examining how groundwater levels vary over time in the context of climate change. Connecting these rates of change to the depths of actual wells will provide better predictions of where groundwater access is at risk.
“Groundwater depletion is not inevitable,” Jasechko said. Fine resolution, global studies will enable scientists and officials to understand the dynamics of this hidden resource.
Arizona Announces Limits On Construction In Phoenix Area As Groundwater Disappears
A CNN report said:
Arizona officials announced Thursday the state will no longer grant certifications for new developments within the Phoenix area, as groundwater rapidly disappears amid years of water overuse and climate change-driven drought.
A new study showed that the groundwater supporting the Phoenix area likely cannot meet additional development demand in the coming century, officials said at a news conference. Gov. Katie Hobbs and the state’s top water officials outlined the results of the study looking at groundwater demand within the Phoenix metro area, which is regulated by a state law that tries to ensure Arizona’s housing developments, businesses and farms are not using more groundwater than is being replaced.
The study found that around 4% of the area’s demand for groundwater, close to 4.9 million acre-feet, cannot be met over the next 100 years under current conditions—a huge shortage that will have significant implications for housing developments in the coming years in the booming Phoenix metro area, which has led the nation in population growth.
State officials said the announcement wouldn’t impact developments that have already been approved. However, developers that are seeking to build new construction will have to demonstrate they can provide an “assured water supply” for 100 years using water from a source that is not local groundwater.
The CNN report said:
Under state law, having that assured supply is the key to getting the necessary certificates to build housing developments or large industrial buildings that use water. Many cities in the Phoenix metro area, including Scottsdale and Tempe, already have this assured water supply, but private developers also must demonstrate they can meet it.
Thursday’s announcement is an example of the law working as intended, according to an analysis by Arizona State University’s Kyl Center for Water Policy. Growth in the Phoenix area will likely continue under the new restrictions, the analysis said, but the rate of growth will likely change.
“It is going to make it harder for developments to spring up on raw desert in the far-flung parts of town where developers like to develop,” Sarah Porter, the director of the Kyl Center for Water Policy, told CNN.
It is another impediment to that kind of development, like new subdivisions out in Buckeye or Queen Creek.
Porter said the change would not necessarily curtail development in the booming Phoenix metro area, but it could push it towards bigger and older cities like Tempe and Scottsdale. Nor is it expected to curtail water use for industry and manufacturing—an important distinction given Arizona is quickly becoming a hub for advanced manufacturing of technology, including semi-conductor chips.
“It really is only impacting housing subdivisions,” Porter said.
There will continue to be new homes built because they have already proved up their 100-year water supply using groundwater, and they were figured into the model. There is this runway of continued development.
But Porter likened Thursday’s announcement to a “big, flashing billboard” telling private developers to find a new, more sustainable source of water—or build elsewhere.
Besides conserving water and projects recycling water, Arizona elected officials—including Sen. Mark Kelly, a Democrat—have advocated for creating new water supply through desalination, where ocean water is treated to remove the salt.
Desalination is used in some water-scarce countries, but it’s been criticized for being expensive and energy-intensive.
Arizona and other Southwest states are facing water shortages on a number of fronts. In addition to Arizona’s groundwater crisis, the state has also faced significant shortages of its surface water allocation from the Colorado River, which it shares with six other states.
And while the groundwater supplies around Phoenix and other Arizona cities are regulated under state law, much of rural Arizona is unregulated—allowing large corporate farms to use unlimited groundwater for crops.
One of those farms, owned by a Saudi company, has gotten increased scrutiny from state officials, including Arizona’s new Democratic attorney general Kris Mayes.
Some rural areas of the state have passed groundwater regulations themselves or have successfully persuaded the Arizona Department of Water Resources to grant them some protections that stop unlimited water use.
Kathleen Ferris, a former state water official and one of the architects of Arizona’s landmark 1980 groundwater management law, told CNN last year that groundwater is akin to a “savings account” for those who live in the desert.
Especially with a precarious situation on the Colorado River, “it’s all the more important that we’re conscious of using our groundwater,” Ferris said.