Where groundwater levels drop and rise worldwide
A study of nearly 1,700 aquifers in more than 40 countries found that groundwater levels have fallen in almost half since 2000. Only about 7 percent of the aquifers studied had groundwater levels that rose during the same period.
The new study is one of the first to collect data from monitoring wells around the world to try to construct a global picture of groundwater levels in detail.
The declines were most pronounced in regions with dry climates and lots of agricultural land, including California's Central Valley and the High Plains region of the United States. The researchers also found large areas of sharply declining groundwater in Iran.
“Groundwater decline has consequences,” said Scott Jasechko, associate professor at the University of California Santa Barbara's Bren School of Environmental Science and Management, and lead author of the study. “These consequences could include rivers leaking, land sinking, seawater contaminating coastal aquifers and wells running dry.”
Previous global studies have been based on satellite observations with much coarser resolution, and on models that calculate groundwater levels rather than measuring them directly.
The research, published Wednesday in the journal Natureconfirms the widespread decline in groundwater previously found with satellites and models, says Marc Bierkens, professor of hydrology at Utrecht University, who was not involved in the research. The paper also offers new findings about aquifers in recovery, he said.
The researchers compared water levels from 2000-2020 with trends from 1980-2000 in approximately 500 aquifers. This comparison with an earlier era revealed a more hopeful picture than simply looking at water levels since 2000. In 30 percent of the smaller group of aquifers, groundwater levels have fallen faster since 2000 than in the previous twenty years. But in 20 percent of them, groundwater decline has slowed compared to before, and in another 16 percent the trends have completely reversed and groundwater levels are now rising.
The improvements are taking place in aquifers around the world, in places as diverse as Australia, China, Saudi Arabia, South Africa, Spain, Thailand and the United States. These aquifers offer reason for cautious optimism, says Debra Perrone, an associate professor at the Environmental Studies Program at the University of California, Santa Barbara and co-author of the new study.
“We can be optimistic because our data reveal more than 100 aquifers where groundwater level decline has slowed, stopped or reversed. But be careful because groundwater levels are rising much less quickly than falling,” she said. “It's much easier to make things worse than it is to make things better.”
The study is based on data from approximately 170,000 monitoring wells that government agencies and researchers use to monitor groundwater levels. Well, data isn't available or doesn't cover enough years everywhere, so the researchers limited themselves to studying aquifers in about 40 countries and territories.
A recent New York Times investigation Analyzing more than 80,000 monitoring wells across the United States, broadly similar trends were found across the country.
The causes of groundwater subsidence vary from place to place. Some large cities depend on groundwater for domestic use. Outside of cities, agricultural irrigation is generally the largest user of groundwater.
“I wouldn't be surprised if many of the trends we're seeing globally are at least partially related to groundwater-fed irrigated agriculture,” said Dr. Yasechko.
One common correlation the researchers identified was a change in the amount of rain or snow that fell over a region. In 80 percent of the aquifers where groundwater decline accelerated, precipitation also decreased over a 40-year period.
Where aquifers recover, the causes vary. In some places, such as Bangkok and California's Coachella Valley, governments have established regulations and programs to reduce groundwater use. In other areas, such as several areas in the southwestern United States, communities are instead diverting more water from the rivers. In Arizona's Avra Valley, officials are actively recharging their aquifer with water from the Colorado River, a body of water that is itself under pressure. In Spain, water managers are recharging the Los Arenales aquifer using a combination of river water, reclaimed wastewater and roof runoff.
A valuable contribution of this new research is to reveal local differences, where ground-based well data deviates from larger regional trends that satellites can identify, says Donald John MacAllister, a hydrogeologist at the British Geological Survey who wrote the paper assessed.
“What we often hear is that groundwater subsidence is happening everywhere. And actually the picture is much more nuanced than that,” he said. “We need to learn lessons from places where things might be a little more optimistic.”