Global groundwater depletion is accelerating but not inevitable, researchers say

Groundwater is declining rapidly around the world, often at an accelerating rate. Write in the journal NatureUC Santa Barbara researchers present the largest assessment of groundwater levels in the world, covering nearly 1,700 aquifers.

In addition to sounding the alarm about declining water resources, the book offers instructive examples of where things are going well and how groundwater depletion can be addressed. The study is a boon to scientists, policymakers and resource managers striving to understand global groundwater dynamics.

“This study was driven by curiosity. We wanted to better understand the state of the world’s groundwater by examining millions of groundwater level measurements,” said lead author Debra Perrone, associate professor in the study program Environmental Studies at UC Santa Barbara.

The team compiled data from national and subnational registers and the work of other agencies. The study lasted three years, two of which were spent cleaning and sorting the data. That’s what it takes to make sense of the 300 million water level measurements from 1.5 million wells over the past 100 years.

Then came the task of translating the deluge of data into concrete information about global groundwater trends. The researchers then combed through more than 1,200 publications to reconstruct aquifer boundaries in the study regions and assess groundwater level trends in 1,693 aquifers.

Their results provide the most comprehensive analysis of global groundwater levels to date and demonstrate the prevalence of groundwater depletion.

The work found that groundwater is declining in 71% of aquifers. And this depletion is accelerating in many places: Rates of groundwater decline in the 1980s and 1990s have accelerated from 2000 to today, underscoring what a serious problem has become even worse. Accelerated declines are occurring in nearly three times as many places as would be expected by chance.

Groundwater deepening is more common in drier climates, with accelerated decline particularly prevalent in arid and semi-arid cultivated lands — “an intuitive finding,” said co-senior author Scott Jasechko, an associate professor at the Bren School of Environmental Science & Management. “But it’s one thing for something to be intuitive. It’s another to show that it happens with real-world data.”

On the other hand, there are places where levels have stabilized or recovered. The groundwater declines of the 1980s and 1990s were reversed in 16% of the aquifer systems for which the authors had historical data. However, these cases are only half as common as would be expected by chance.

“This study shows that humans can make a difference through deliberate, focused effort,” Jasechko said.

Take Tucson, Arizona, for example. Water 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 thought of as a bank account for water,” Jasechko explained. “Intentionally filling aquifers allows us to store that water until when we need it.”

Communities can spend a lot of money building infrastructure to hold water on the surface. But if you have the right geology, you can store large quantities of water underground, which is much cheaper, less disruptive and less dangerous. Stored groundwater can also benefit the ecology of the region. In fact, while working on a research paper in 2014, Perrone discovered that aquifer recharge can store six times more water per dollar than surface reservoirs.

Tucson’s groundwater recharge is a boon to the local aquifer; however, the withdrawals caused the mighty river to diminish on the surface. The Colorado rarely reaches its delta in the Gulf of California. “These groundwater interventions can result in trade-offs,” Jasechko acknowledged.

Another option is to focus on reducing demand. This often involves regulations, permits and fees for groundwater use, Perrone explained. To this end, she is currently examining water legislation in the Western United States to understand these various interventions. Whether coming from supply or demand, aquifer recovery appears to require intervention, the study finds.

The authors supplemented the monitoring well measurements 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 machines detect small fluctuations in the planet’s gravity, which can reveal the dynamics of aquifers on a large scale.

“The beauty of GRACE is that it allows us to explore groundwater conditions where we don’t have in situ data,” Perrone said. “Our comprehensive GRACE assessment. When we have in situ data, we can explore groundwater conditions locally, a crucial level of resolution when managing depletion.” This local resolution is essential, as the authors discovered, because adjacent aquifers can show different trends.

That said, groundwater level trends do not provide the full picture. Even where aquifers remain stable, groundwater withdrawal can still affect nearby streams and surface waters, causing them to leak underground, as Perrone and Jasechko explain in another article . Nature paper in 2021.

The authors also analyzed precipitation variability over the past four decades for 542 aquifers. They found that 90% of the aquifers whose decline was accelerating are in places where conditions have become drier over the past 40 years. These trends have likely reduced groundwater recharge and increased demand. On the other hand, climate variability can also allow groundwater to rise where conditions become wetter.

This monitoring well study complements an article by Perrone and Jasechko published in 2021. This study represented the largest assessment of groundwater wells in the world and made the cover of the journal. Science. “Monitoring wells give us supply information. And groundwater wells give us demand information,” Perrone said.

“Taken together, they allow us to understand which wells are already dry, or are most likely to become dry if groundwater levels drop,” Jasechko added.

Perrone and Jasechko are currently examining how groundwater levels vary over time in the context of climate change. Linking these rates of change to actual well depths will provide better predictions about where groundwater access is threatened.

“Groundwater depletion is not inevitable,” Jasechko said. At fine resolution, global studies will allow scientists and managers to understand the dynamics of this hidden resource.