By NASA’s Earth Observatory
By measuring the gravitational pull of water for more than two decades, NASA satellites have peered beneath the surface and measured changes in the groundwater supplies of the Colorado River Basin. In a recent analysis of the satellite data, Arizona State University researchers reported rapid and accelerating losses of groundwater in the basin’s underground aquifers between 2002 and 2024. Some 40 million people rely on water from the aquifers, which include parts of Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming.
The basin lost about 27.8 million acre-feet of groundwater during the study period. “That’s an amount roughly equal to the storage capacity of Lake Mead,” said Karem Abdelmohsen, an associate research scientist at Arizona State University who authored the study.
About 68 percent of the losses occurred in the lower part of the basin, which lies mostly in Arizona. The research is based on data collected by the GRACE (Gravity Recovery and Climate Experiment) and GRACE-FO (GRACE Follow-On) missions. The data were integrated with output from land surface models, such as NASA’s North American Land Data Assimilation System, and in-situ precipitation data to calculate groundwater losses.
The conclusions were similar to those arrived at by Arizona State University Global Futures Professor Jay Famiglietti in an analysis of the Colorado River Basin published in 2014, when his team was at the University of California, Irvine. “If left unmanaged for another decade, groundwater levels will continue to drop, putting Arizona’s water security and food production at far greater risk than is being acknowledged,” said Famiglietti, previously a senior water scientist at NASA’s Jet Propulsion Laboratory and the principal investigator of both studies.
The maps above underscore the accelerating rate of groundwater loss detected by the GRACE missions. In the first decade of the analysis, between 2002 and 2014, parts of the basin in western Arizona in La Paz and Mohave counties and in southeastern Arizona in Cochise County lost groundwater at a rate of about 5 millimeters (0.2 inches) per year. Between 2015 and 2024, the rate of groundwater loss more than doubled to 12 millimeters (0.5 inches) per year.
Two key factors likely explain the acceleration, the researchers said. First, there was a global transition from one of the strongest El Niños on record in 2014-2016 to a period when La Niña reasserted control, including the arrival of a “triple-dip” La Niña between 2020 and 2023. La Niña typically shifts winter precipitation patterns in a way that reduces rainfall over the Southwest and slows the replenishment of aquifers.
Second, there was an increase in the amount of groundwater being used for agriculture. “2014 was about the time that industrial agriculture took off in Arizona,” Famiglietti said, noting that large alfalfa farms arrived in La Paz and other parts of southern Arizona around that time. Dairies and orchards in southeastern Arizona likely impacted groundwater supplies as well, he added. Other “thirsty” crops grown widely in the state include cotton, corn, and pecans. Data from the U.S. Department of Agriculture’s Cropland Data Layer (CDL) shows that these crops are common in several parts of southern Arizona, including Maricopa, Pinal, and Cochise counties.
Irrigated agriculture consumes about 72 percent of Arizona’s available water supply; cities and industry account for 22 percent and 6 percent, respectively, according to Arizona Department of Water Resources data. Many farms use what Famiglietti described as “vast” amounts of groundwater in part because they use a water-intensive type of irrigation known as flood irrigation (or sometimes furrow irrigation), a technique where water is released into trenches that run through crop fields. The long-standing practice is typically the cheapest option and is widely used for alfalfa and cotton, but it can lead to more water loss and evaporation than other irrigation techniques, such as overhead sprinklers or dripping water from plastic tubing.
The satellite image above, captured by the OLI (Operational Land Imager) on Landsat 8, shows desert agriculture in La Paz and Maricopa counties on July 12, 2025. CDL data from the U.S. Department of Agriculture indicates that most of the rectangular fields around Vicksburg and Wenden are used to grow alfalfa, while the fields around Aguila are typically used for fruits and vegetables, such as melons, broccoli, and leafy greens. Some of the alfalfa fields in Butler Valley (upper part of the image) have gone fallow in recent years following the termination of leases due to concerns from the Arizona State Land Department about groundwater pumping.
The new analysis found some evidence that managing groundwater can help keep Arizona aquifers healthier. For instance, the active management areas and irrigation non-expansion areas established as part of the Arizona Groundwater Management Act of 1980 lessened water losses in some areas. The designation of a new active management area in the Willcox Basin in 2025 will likely further slow groundwater losses. “Still, the bottom line is that the losses to groundwater were huge,” Abdelmohsen said. “Lots of attention has gone to low water levels in reservoirs over the years, but the depletion of groundwater far outpaces the surface water losses. This is a big warning flag.”
NASA supports several missions, tools, and datasets relevant to water resource management. Among them is OpenET, a web-based platform that uses satellite data to measure how much water plants and soils release into the atmosphere. The tool can help farmers tailor irrigation schedules to actual water use by plants, optimizing “crop per drop” and reducing waste.
NASA Earth Observatory images by Wanmei Liang, using data from Abdelmohsen, K., et al. (2025), boundary data from Colorado River Basin GIS Open Data Portal, and Landsat data from the U.S. Geological Survey. Oceanic Niño Index chart based on data from the Climate Prediction Center at NOAA. Story by Adam Voiland.
