The Colorado River's hidden crisis: groundwater is vanishing faster than the reservoirs above

The pale bathtub ring around Lake Mead's shoreline has become the defining image of the American West's water crisis. But beneath the cracked earth of the Colorado River Basin, a second crisis has been unfolding out of sight — and a new study suggests it may be outpacing the one everyone can see.

Researchers at Virginia Tech, using NASA satellites that detect water by measuring tiny shifts in Earth's gravity, tracked groundwater across the entire Colorado River Basin from October 2003 through September 2023. Their findings, published in the journal Water Resources Research, show that groundwater has been declining across the basin for two decades, that the decline accelerated sharply after 2019, and that the region most people assumed was safest — the snowpack-fed Upper Basin — is now losing water underground faster than the desert below it.

We generally think of the Upper Basin as more resilient because of its snowmelt-driven recharge. I did not expect to see so much groundwater loss there during the recent drought. What surprised me even more was that groundwater did not track surface-water shortages in the same way in the Upper and Lower Basins.— Esther Oyedele, geosciences graduate student and lead author, Virginia Tech

During the severe drought of 2020 to 2022, the Upper Basin — which covers Colorado, New Mexico, Utah and Wyoming — lost about 4.6 trillion gallons of groundwater, nearly double the 2.5 trillion gallons lost in the Lower Basin of California, Arizona and Nevada. The upper basin was also losing groundwater at roughly 1.6 trillion gallons a year, twice the lower basin's pace. Those upper-basin losses alone could fill about 7 million Olympic swimming pools, according to earth.com.

The reversal matters because the Upper and Lower Basins have historically behaved very differently. The Lower Basin — hotter, drier, and more dependent on the river itself — has long been where groundwater stress showed up first. The Upper Basin benefited from melting snow coursing down the Rocky Mountains, which seeps into the ground and recharges aquifers. That recharge mechanism appears to be failing under the combined pressure of prolonged drought, higher temperatures and reduced snowpack.

The system has hit a turning point: The Upper Basin is now experiencing rapid groundwater losses.— Study authors

The Colorado River supplies water to roughly 40 million people across seven states, and 70 to 80 percent of its water goes to irrigation, according to earth.com. Lake Mead and Lake Powell, its two largest reservoirs, fell from about 95 percent full in 2000 to roughly a quarter full by the end of 2022. But groundwater accounts for about half of the basin's total water storage loss since 2004 — a loss the study estimates at around 58 cubic kilometers, or about 15 trillion gallons in all.

The two halves of the basin also differ in how their groundwater responds to surface-water shortages, and the difference reveals something troubling about the Lower Basin. In the Upper Basin, groundwater drought followed surface-water shortage by about five months — a tight, predictable link. Once a surface shortage became severe, the odds of a groundwater drought reached about 95 percent. In the Lower Basin, the order was reversed: groundwater started falling about four months before the reservoirs did, and even a severe surface shortage raised the odds of a groundwater drought only to about 65 percent.

Oyedele and Werth suggest the most likely explanation for that reversed pattern is pumping. In the desert half of the basin, groundwater is the default supply, drawn down regardless of whether the river is running low. The satellites, in other words, may be tracking human extraction as much as natural drought. Within Arizona's water districts, the Phoenix and Pinal districts — which took the biggest cuts in surface-water allocations during the drought — also lost the most groundwater, about 211 billion and 238 billion gallons respectively. Tucson, which imports Colorado River water and stores some of it underground, actually gained about 53 billion gallons.

The unregulated land in the southern part of the lower basin lost more groundwater than all the managed water districts combined, according to earth.com.

Continued groundwater depletion, especially in the Upper Basin, could further reduce groundwater contributions to rivers and intensify the feedback between groundwater loss and surface-water shortages.— Susanna Werth, assistant professor of hydrology and remote sensing, Virginia Tech

That feedback loop is the core danger. Groundwater and surface water are linked: rain and snowmelt recharge aquifers, while groundwater also feeds rivers and lakes through springs. As groundwater falls, it contributes less to rivers — which are already shrinking — creating a cycle that is difficult to reverse. It takes thousands of years to refill deep aquifers.

While surface water gets all the policy (and) management attention, groundwater dominates the losses from the entire basin.— Jay Famiglietti, professor, Arizona State University

Famiglietti, who was an author on a separate study published last year finding that the amount of groundwater pumped from the basin over two decades was enough to fill Lake Mead, told CNN that shrinking groundwater will compound the pressure on Arizona, California and Nevada as those states are already being forced to cut how much water they draw from the Colorado River under a new operating plan published in August.

The new study has real limits. The NASA satellites average over patches about 190 miles across, so they cannot resolve a single valley. The Upper Basin has too few monitoring wells to cross-check the satellite readings — a gap the authors identify as a priority to fix. The study also tracks what changes together, not what causes what, and whether the Upper Basin's recent losses mark a lasting shift or the signature of one exceptionally bad drought will require a longer record to determine. The study does not estimate how much groundwater remains in the basin, only how it has changed.

Groundwater is another essential component. Focusing only on reservoirs could therefore overlook an important part of the region's overall water challenges. Groundwater should not be seen as an 'unlimited reserve.'— Esther Oyedele, geosciences graduate student and lead author, Virginia Tech

The researchers say the next step is determining how much more depletion the basin's aquifers can absorb, work that will require more monitoring wells and finer satellite data. Until then, the study's central message is a straightforward one: managing the Colorado River as though only the surface water matters is no longer adequate.

Why it matters — The Colorado River is the water lifeline for 40 million people and the West's agricultural economy; this study shows that the underground reserves meant to buffer surface-water shortages are themselves being rapidly depleted — and in the mountain region that was supposed to be most resilient.

Reported by cnn.com, news.vt.edu, earth.com, edf.org