Groundwater is declining rapidly around the world, often at an accelerating rate. Writing in the journal Nature, UC Santa Barbara researchers have conducted the largest assessment of groundwater levels in nearly 1,700 aquifers around the world. As well as sounding the alarm about dwindling water resources, the study provides some illuminating examples of where things are going well and how to address groundwater depletion.
The research is a boon to scientists, policymakers and resource managers working to understand global groundwater dynamics.
"This study was driven by curiosity. We wanted to use millions of groundwater level measurements to better understand the status of groundwater around the world," said lead author Debra Perrone, an associate professor in the Environmental Studies Program at UC Santa Barbara.
The research team compiled data from national and subnational records and the work of other agencies. The research took three years, two of which were spent cleaning and organizing the data. This is also the time it took to analyze 300 million water level measurements from 1.5 million wells over the past 100 years.
The next task is to translate the vast amounts of data into practical insights about global groundwater trends. The researchers then reviewed more than 1,200 publications, reconstructed aquifer boundaries in the survey area, and assessed groundwater level trends in 1,693 aquifers.
Their results provide the most comprehensive analysis of global groundwater levels to date and demonstrate how widespread groundwater depletion is. Research shows that 71% of aquifers are losing groundwater. Groundwater depletion is accelerating in many places: The pace of groundwater decline in the 1980s and 1990s has accelerated from 2000 to the present, highlighting how a dire problem is getting worse. Groundwater decline is accelerated almost three times more often than it would be by chance.
Solutions to accelerate recession and recovery
"Groundwater deepening is more common in drier climates, and accelerated groundwater decline is particularly common in arid and semi-arid arable lands—an intuitive finding," said co-first author Scott Jasechko, an associate professor in the university's Bren School of Environmental Science and Management. "But intuition is one thing. Using real-world data to prove it's happening is another."
On the other hand, there are places where groundwater levels have stabilized or recovered. Of the aquifer systems for which the authors have historical data, 16% experienced a reversal of groundwater decline in the 1980s and 1990s. However, these conditions occur only half as often as chance. This study shows that humans can turn things around with thoughtful, focused attention.
Take Tucson, Arizona, for example. Water allocated 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 the bank account for water," Jasechko explains. "Intentional replenishment of aquifers allows us to store water until it is needed.
Communities can spend large sums of money building infrastructure to store water above ground. But if the geology is right, large amounts of water can be stored underground, which is cheaper, less disruptive and less dangerous. Stored groundwater can also benefit regional ecology. In fact, while preparing a research brief in 2014, Perrone found that aquifers recharge six times more water per dollar of storage than surface reservoirs.
Tucson's groundwater recharge has been a boon to the local aquifer; however, groundwater pumping has caused this mighty river to shrink above ground. The Colorado River rarely reaches the delta of the Gulf of California. "These groundwater interventions may come at a cost," Jasico acknowledged.
Regulatory measures and technical implications
Another approach is to reduce demand. Perrone explained that this often involves regulations, permits and fees for groundwater use. To this end, she is currently studying water law in the western United States to understand these different interventions. Research shows that aquifer restoration appears to require intervention, whether from supply or demand.
The authors supplemented the monitoring well measurements with data from the Gravity Recovery and Climate Experiment (GRACE). The Gravity Recovery and Climate Experiment (GRACE) mission consists of two satellites that can accurately measure the distance between them as they orbit the Earth. In this way, satellites can detect tiny fluctuations in Earth's gravity, revealing the large-scale dynamics of aquifers.
"The beauty of GRACE is that it allows us to explore groundwater conditions where we don't have in situ data," Perrone said. "Our assessment complements GRACE. Local groundwater conditions can be explored where we have in situ data, which is a critical resolution when managing depletion. The authors found that this local resolution is critical because adjacent aquifers will exhibit different trends."
Still, trends in groundwater levels don't tell the whole story. Even where aquifers remain stable, pumping groundwater can still affect nearby streams and surface waters, causing them to seep into the ground, as Perrone and Jasico detailed in another Nature paper published in 2021.
The authors also analyzed precipitation changes in 542 aquifers over the past 40 years. They found that 90 percent of aquifers that were declining at an accelerating rate were located in locations that had become drier over the past 40 years. These trends are likely to result in reduced groundwater recharge and increased demand. On the other hand, climate variability can also allow groundwater to rebound where conditions become wetter.
This monitoring well study complements a 2021 paper by Perrone and Jasechko. The study is the largest assessment of groundwater wells around the world and was featured on the cover of Science magazine. "Monitoring wells tell us about supply. Groundwater wells tell us about demand," Perrone said. "Taken together, we can understand which wells have dried up, or which are most likely to dry up if water tables drop."
Researchers are currently observing how groundwater levels change over time in the context of climate change. Relating these rates of change to actual well depths allows for better predictions of where groundwater access is at risk. Groundwater depletion is not inevitable. Global studies at fine resolution will allow scientists and officials to understand the dynamics of this cryptic resource.
Compiled from /ScitechDaily