As the Colorado River’s giant reservoirs have declined during the last two decades, even larger amounts of water have been pumped and drained from underground, according to new research based on data from NASA satellites.
Scientists at Arizona State University examined more than two decades of satellite measurements and found that since 2003 the quantity of groundwater depleted in the Colorado River Basin is comparable to the total capacity of Lake Mead, the nation’s largest reservoir.
The researchers estimated that pumping from wells has drained about 34 cubic kilometers, or 28 million acre-feet, of groundwater in the watershed since 2003 — more than twice the amount of water that has been depleted from the river’s reservoirs during that time.
“The Colorado River Basin is losing groundwater at an alarming rate,” said Karem Abdelmohsen, the lead author and a researcher at ASU’s School of Sustainability.
The losses are being driven largely by heavy pumping to supply agriculture, he said. At the same time, prolonged drought and rising temperatures have sapped river flows and decreased the amount of water percolating underground and recharging aquifers.
“As surface water becomes less dependable, the demand for groundwater is projected to rise significantly,” the researchers wrote in the study, which was published Tuesday in the journal Geophysical Research Letters. “Groundwater is a crucial buffer … but it is rapidly disappearing due to excessive extraction.”
The Colorado River Basin covers parts of seven U.S. states, from Wyoming to Southern California, and northern Mexico. The river’s water sustains fast-growing cities including Phoenix and Las Vegas, as well as more than 5 million acres of farmland and ranchland.
The researchers found that most of the depletion of groundwater (about three-fourths of the total) is occurring in the river’s lower basin, largely in Arizona, where the bulk of the water is pumped from desert aquifers to irrigate farms.
They estimated that annual groundwater losses in the Colorado River Basin have averaged more than 1.2 million acre-feet — about four times larger than the amount of water the Las Vegas area is entitled to take from the Colorado River each year.
“If this trend continues, it could lead to severe water shortages that impact not only local farmers and residents but also broader agricultural markets and municipal water supplies throughout the southwestern U.S.,” Abdelmohsen said.
The declines in water supplies have worsened as climate change has intensified drought conditions, driving what scientists describe as the aridification of the Southwest.
Research shows that the past 25 years have probably been the driest quarter-century in western North America in 1,200 years. Scientists have found that global warming is intensifying this long megadrought and has caused roughly half of the 20% decrease in the Colorado River’s average flow this century.
“Climate change is only exacerbating the stress on groundwater,” said Jay Famiglietti, the study’s senior author and science director for ASU’s Arizona Water Innovation Initiative.
“If groundwater remains unprotected in large swaths of the southwestern U.S. and continues to disappear, it will dramatically limit food production,” Famiglietti said. “Groundwater is critically important in desert states like Arizona and desert cities like Phoenix and Tucson, and if it disappears, then it becomes an existential crisis.”
The researchers used data from a pair of NASA satellites, called GRACE Follow-On, which track changes in Earth’s gravity field to measure shifts in the total amounts of water, both above and below ground. They examined other data on snowpack, surface water and soil moisture to estimate how much groundwater has been depleted. They found that the losses of groundwater far exceed the declines in the river’s two largest reservoirs, Lake Mead and Lake Powell.
Despite this, Famiglietti noted that in large portions of the Colorado River Basin, groundwater pumping remains unregulated and unmanaged.
“The steady decline of water availability in the Colorado River Basin has been going on for decades,” he said. “That the bulk of these losses stem from groundwater overuse should put states like Arizona on high alert, and trigger more urgent dialogue about extending groundwater management across the entire state.”
Efforts to prevent the river’s reservoirs from reaching critically low levels have attracted widespread attention and become the focus of difficult negotiations among seven states. With Lake Mead and Lake Powell now two-thirds empty, officials representing California and other states are under growing pressure to negotiate a deal to take less water from the shrinking river.
Less widely known are the growing pressures on the region’s groundwater. Over the last decade, large farming companies have expanded in Arizona, planting hay and other water-intensive crops and drilling deep wells in desert areas where there are no regulations limiting groundwater pumping.
Some residents have been left with dry wells as water levels have dropped. In places, collapsing aquifers have caused the land to sink, creating fissures in the ground that have damaged roads.
The scientists found especially rapid groundwater losses in parts of northwestern and southeastern Arizona where large farms irrigate thirsty crops such as alfalfa, which is used to feed cattle locally and is also exported to countries such as China and Saudi Arabia.
These areas rely heavily on groundwater and largely have no access to water diverted from the Colorado River.
The study showed smaller, yet significant, declines in groundwater levels around Phoenix and Tucson. These areas receive imported water from the Colorado River via the Central Arizona Project canal, and they are required to manage groundwater under a 1980 state law.
In other research, Famiglietti and his colleagues have found similar but larger losses of groundwater driven by agricultural pumping in California’s Central Valley. There, local agencies are required under a 2014 state law, the Sustainable Groundwater Management Act, to curb overpumping and achieve a set of sustainability goals by 2040.
In Arizona, in contrast, groundwater pumping remains unregulated in 82% of the state. Proposals to protect declining aquifers have repeatedly faced opposition and died in the Legislature, but state regulators last year formed a new “active management area” in the Willcox basin in southeastern Arizona, where they have proposed measures to gradually limit agricultural pumping.
Hay is stored at the Fondomonte alfalfa farm in Vicksburg, Ariz., in 2023.
(Caitlin O’Hara / For Washington Post via Getty Images)
In parts of the river’s upper basin, groundwater pumping can reduce the flow of streams by lowering the water table, Abdelmohsen said. But in the lower basin, the groundwater lies deeper and is largely disconnected from the river.
The scientists didn’t offer specific recommendations, but they said their estimates could be used as science-based targets to help address overpumping. They said one way of reducing water usage would be to shift from water-intensive crops like alfalfa to other crops that use less.
In a separate analysis, the researchers found that total water losses across the Colorado River Basin have accelerated significantly, with the rate of depletion from 2015-2024 averaging roughly three times faster than from 2002-2014 — a trend driven partly by drying conditions in the Southwest. Groundwater accounted for two-thirds of the total losses.
“These scientists are bringing to light the sad reality that we’re losing more stored water underground than we are on the surface,” said Brian Richter, a researcher who was not involved in the study. “That tells us that our overconsumption of water in the Colorado River Basin is much worse than I think a lot of us perceived previously.”
The region’s desert aquifers contain water that has been underground for thousands of years. In many areas, once those water reserves are exhausted, they are effectively gone for good.
In addition to converting farms to crops that use less water, Richter said he believes “we’re going to have to start talking about permanent reductions in agricultural farmland.” He said legislation and federal funds would be needed to compensate agricultural landowners who agree to take cropland out of production.
Farmers in the Imperial Valley and other parts of Southern California have in recent years agreed to leave some fields dry temporarily to help conserve Colorado River water in exchange for cash payments. But they have strongly opposed permanent fallowing of land, which they say would harm food production and local economies.
Richter said the latest data suggest more farmland will need to be left dry to bring water consumption into balance with the limited supply.
“Climate warming is driving this drying of the Colorado River Basin for the long term, so we really need to come to grips with doing this great rebalancing act,” he said. “We need to start moving out of this danger zone.”
