Well data from around the world show declines driven by water use and climate change.
by Katherine Bourzac
Farmers rely on groundwater to grow crops, especially when rainfall is scarce. Declines in this crucial resource can slash agricultural yields and have long-term effects on water quality, well levels, and even local geology—when groundwater is depleted, the land above can subside.
Groundwater level declines have accelerated in 30% of the world’s aquifers, according to an assessment published in Nature. The declines were particularly pronounced in agricultural regions with dry climates.
The assessment drew on 40 years of water level data collected from 170,000 monitoring wells in 1,693 aquifers.
In some places, the declines were dramatic. In 36% of the aquifers included in the study, groundwater levels fell faster than 0.1 meter per year, and in 12% of them, levels dropped by half a meter per year. The Rashtkhar Aquifer in Iran, for instance, had a median decline of 2.6 meters per year in the 21st century. In California’s Chowchilla Subbasin, water levels dropped by about 1 meter per year. Both are agricultural areas.
The researchers attributed global declines to both water use and climate change.
“Their ability to map these patterns at fine resolution is exciting.”
In 80% of the aquifers where groundwater declines were accelerating, precipitation also dropped, said Debra Perrone, a water resources engineer and scientist at the University of California, Santa Barbara (UCSB), and one of the study’s authors. When there’s less rain, farmers need to tap into groundwater. And without precipitation to recharge underground reservoirs, there’s no way to build them back up to compensate for increased use.
Precipitation isn’t the only issue. “As temperatures are rising, farmers are applying more irrigation,” said Meha Jain, an environmental scientist at the University of Michigan who was not involved in the study. Jain’s own work has shown that if critically depleted wells in India dry up, cropping intensity in the country will decrease by 20%.
The new study “fills long-standing gaps in the literature,” Jain said. Satellites such as NASA’s GRACE (Gravity Recovery and Climate Experiment) can provide a lot of information about the world’s water supply, but they don’t have the spatial resolution to see smaller underground reserves. The monitoring wells used in this study let the researchers analyze changes in individual aquifers. “Their ability to map these patterns at fine resolution is exciting,” Jain said.
Declines were not universal, however, and the data showed that pumping can sometimes be managed. In some areas, groundwater levels have even risen. After dropping in the 1980s and 1990s, groundwater beneath Bangkok, Thailand, recovered when the government increased fees on pumping water. An aquifer in New Mexico saw levels rise, thanks to water transferred from another overtaxed system, the Colorado River.
“There are far more places where things are bad than where things are good right now,” said UCSB water scientist Scott Jasechko, the first author on the study. He said he hopes the success stories “might inspire action.”
Citation: Bourzac, K. (2024), Groundwater levels are dropping around the world, Eos, 105, https://doi.org/10.1029/2024EO240071. Published on 14 February 2024.
