A new study based on data from the NASA satellite shows that as the massive Colorado River reservoirs have declined over the past 20 years, more water has been drained underground and released underground.
Scientists at Arizona State University have investigated more than 20 years of satellite measurements and found that the amount of groundwater depleted in the Colorado River basin since 2003 is comparable to the total capacity of Lake Mead, the country’s largest reservoir.
Researchers estimated pumping from Wells has emitted about 34 cubic kilometres (28 million acre feet) of the basin’s groundwater since 2003.
“The Colorado River Basin is losing groundwater at an incredible rate,” says Karem Abdelmohsen, lead author and researcher at ASU’s School of Sustainability.
The losses are driven primarily by heavy pumps to supply agriculture, he said. At the same time, prolonged drought and rising temperatures led to river flow sinking, reducing the amount of water penetrating underground, and charging the aquifer.
“As surface water becomes less reliable, groundwater demand is projected to rise significantly,” the researchers wrote in the study, which will be published Tuesday in the journal Geophysical Research Letters. “Groundwater is an important buffer, but it’s disappearing rapidly because the extraction is overextracted.”
The Colorado River Basin covers parts of seven US states, from Wyoming to Southern California and northern Mexico. The river water supports fast-growing cities, including Phoenix and Las Vegas, including over 5 million acres of farmland and ranches.
Researchers found that the majority of groundwater depletion (approximately three-quarters of the total) occurs in the lower basin of the river. Mostly in Arizona, most of the water is pumped from desert aquifers to irrigate farms.
They estimated annual groundwater losses in the Colorado River basin average over acre feet of more than 1.2 million acres. That’s about four times the amount of water that the Las Vegas region consumes from the Colorado River each year.
“If this trend continues, it could lead to severe water shortages that affect not only local farmers and residents, but also the wider agricultural markets and local government water supplies in the southwestern United States,” Abdelmosen said.
Water supply declines have worsened as climate change has strengthened drought conditions and driven what scientists describe as their southwest motivations.
Research shows that the last 25 years have been perhaps the driest quarter century in western North America, perhaps 1, 200 years. Scientists have found that global warming is bolstering this long mega-druct, causing about half of a 20% reduction in the average Colorado River flow in this century.
“Climate change is only exacerbating groundwater stress,” says Jay Famiglietti, senior author and science director of research at ASU’s Arizona Water Innovation Initiative.
“Groundwater is not protected by large stripes in the southwestern United States and if it continues to disappear, it will dramatically limit food production,” Famiglietti said. “In desert states like Arizona and in desert cities like Phoenix and Tucson, groundwater is extremely important. If it disappears, it becomes an existential crisis.”
Researchers used data from a pair of NASA satellites called Grace Follow-On. This tracked changes in the Earth’s gravity field and measured the total water volume shifts both underground and below. They looked at other data on snowmen, surface water and soil moisture to estimate how much groundwater was depleted. They found that groundwater losses far outweigh the decline in the two largest reservoirs of the river, Lake Mead and Lake Powell.
Nevertheless, Famiglietti noted that groundwater pumps are not regulated or controlled in most of the Colorado River basin.
“The steady decline in water availability in the Colorado River Basin has been ongoing for decades,” he said. “The fact that a large portion of these losses stems from overuse of groundwater will lead to high alerts in conditions like Arizona and spark more urgent dialogue about expanding groundwater management across the state.”
Efforts to prevent river reservoirs from reaching very low levels have attracted widespread attention and are the focus of difficult negotiations among the seven states. Lake Mead and Lake Powell are currently two-half empty, with officials representing California and other states putting pressure on them to negotiate a deal to get less water from the reduced river.
Pressure on groundwater in lesser known regions is growing. Over the past decade, large farming companies have expanded in Arizona, planting hay and other water-intensive crops and drilling deep wells in desert areas without restrictions restricting groundwater pumping.
Some residents have dried wells left as water levels drop. At the location, impacting aquifers sink the land, creating clefts in the ground, and creating clefts that have damaged roads.
Scientists have discovered particularly rapid groundwater losses in parts of northwest and southeastern Arizona. There, large farms are used to supply cattle locally and irrigate thirsty crops such as alfalfa, which are exported to countries such as China and Saudi Arabia.
These areas rely heavily on groundwater and have little access to water decoupled from the Colorado River.
This study showed small groundwater levels around Phoenix and Tucson, but significant declines. These areas must receive imported water from the Colorado River via the Central Arizona Project Canal and manage groundwater under state laws of 1980.
In other studies, Famiglietti and his colleagues discovered similar but significant losses in groundwater driven by agricultural pumps in California’s Central Valley. There, local agencies are needed under the state law of 2014, the Sustainable Groundwater Management Act, to curb excess pumping and achieve a set of sustainability goals by 2040.
In contrast, in Arizona, groundwater pumping remains unregulated in 82% of the state. Proposals to protect the declining aquifer have repeatedly faced opposition and died in Congress, but last year, state regulators proposed measures to form a new “active management area” in the Wilcox Basin in southeastern Arizona, where they gradually restrict agricultural pumps.
Hay is kept in 2023 at the Von DeMonte Alfalfa Farm in Vicksburg, Arizona.
(Caitlyn O’Hara/For the Washington Post via Getty Images)
In parts of the river’s upper basin, groundwater pumping can reduce stream flow by lowering the water table. However, in the lower reaches, groundwater is deeper and most of it has been cut off from the river.
Scientists did not provide specific recommendations, but said their estimates could be used as a science-based goal to address overpumping. They said one way to reduce water usage is to move from water-intensive crops like alfalfa to other crops with less use.
In another analysis, the researchers found that total water losses across the Colorado River basin have been significantly accelerated, with depletion rates from 2015 to 2024 being three times faster on average between 2002 and 2014. Groundwater accounted for two-thirds of the total loss.
“These scientists are revealing the sad reality that we are losing water stored underground rather than the surface,” said Brian Richter, a researcher who is not involved in the study. “It says that overdose of water in the Colorado River basin is much worse than many of us previously knew.”
The area’s desert aquifers contain water that has been underground for thousands of years. In many regions, when these water reserves are exhausted, they will virtually disappear forever.
In addition to converting farms to crops that use less water, Richter said, “I think we need to start talking about permanent cuts to agricultural farmland.” He said laws and federal funds are needed to compensate farm landowners who agree to steal farmland from production.
Farmers in the Imperial Valley and other parts of Southern California have recently agreed to temporarily dry some fields in order to save the Colorado River water in exchange for cash payments. However, they are strongly opposed to permanent falls of land, which they say will harm food production and the local economy.
Richter said the latest data suggests that more farmland needs to be dried to balance it with limited supply.
“Climate warming drives this drying in the Colorado River basin over the long term, so we need to get used to doing this amazing rebalancing method,” he said. “We need to start moving out of this danger zone.”
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