The Colorado River Basin Has Lost Enough Groundwater Alone To Fill Lake Mead

“The Colorado River Basin is losing groundwater at an alarming rate,” Karem Abdelmohsen, a postdoctoral researcher at Arizona State University’s School of Sustainability and lead author of a new paper detailing the depletion of the area’s water, told the Los Angeles Times.

“If this trend continues, it could lead to severe water shortages,” he warned, “that impact not only local farmers and residents but also broader agricultural markets and municipal water supplies throughout the southwestern US.”

Groundwater is disappearing in the Colorado River Basin

The findings, which are based on two decades’ worth of satellite data from NASA’s GRACE and GRACE Follow-On missions, land surface models, and in-situ data, are grim news for the 40 million or so Americans who depend on the basin for their water. In the Lower Colorado Basin – an area covering almost all of Arizona, plus a hefty portion of Nevada and a couple chunks of Utah and California, a full 40 percent of the total water supply comes from the groundwater alone; six other states, dozens of tribal nations, and several of the USA’s largest cities rely on the Colorado River for their drinking water.

But despite being so important, the groundwater reserves of the Colorado River basin have also remained unfortunately invisible – both literally and, as a result, metaphorically. 

“The surface water gets all the policy attention,” Jay Famiglietti, a hydrologist at Arizona State University and the study’s senior author, told ABC15 Arizona. “The groundwater is quietly disappearing.”

It’s certainly true that the Colorado River’s surface water is struggling – that’s partly thanks to a historic, decades-long period of drought, and partly because of wild and sustained overuse, all of course tied up with a neat little climate change bow. The river barely makes it out of the Gulf of California these days, while reservoirs designed to act as backup water sources are about two-thirds empty despite two recent wet winters.

But as fast as the river itself is drying up, the groundwater in the area surrounding it is disappearing faster. Since 2003, the region has lost as much as 42.5 cubic kilometers of groundwater, or about 12 trillion US gallons – and it’s drying up about 2.4 times quicker than the surface water.

How much water is that, you ask? Well, you could think of it as enough to fill Lake Mead, the largest reservoir in the US, with quite a bit sloshing over. It’s the equivalent of 15 million Olympic swimming pools. It could fill the entire Grand Canyon to a height of 27 meters (89 feet). It could entirely cover both Los Angeles and Phoenix in 1.8 meters (6 feet) of water at the same time. It is, to put it bluntly, a fricken’ lot.

“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, president of the global water education organization Sustainable Waters, 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,” he told the Los Angeles Times.

Where is all the water going?

The causes of such a dramatic loss of water are manifold – but one stands out as particularly impactful: “Agriculture just uses so much water,” Famiglietti told CNN.

There are millions of acres of farmland in the Colorado River Basin, and all of them are hungry for water. Moreso than your average farms, in fact: one point highlighted by the new paper is that “the top 15 most water-intensive crops account for 71 percent and 55 percent of total crops” in the two groundwater basins of the region.

As these water-intensive crops become more popular, water supplies are crashing. Up to 80 percent of water withdrawals in the area are for agriculture, and nearly all of that is groundwater, with farmers – and, increasingly, Famiglietti told The Guardian, “large-scale industrial farming” –  plundering ever-deeper underground reserves.

All in all, “overpumping” is the biggest cause of the groundwater loss, he said – but “there’s nothing illegal about it, it’s just unprotected.”

Compounding all of this, though, is the elephant in the room: climate change. With warmer temperatures, more variable weather, and more intense droughts, the southwest of the US has been undergoing what scientists describe as “aridification” – basically, it’s getting super dry, even compared to how it was before. 

So, that historically long drought we mentioned earlier? That’s partly normal – but it’s definitely partly global warming, and more than half of the river’s current 20 percent decrease in volume can be attributed to human-induced climate change.

That’s bad enough on its own – but as surface water runs dry, it causes even more intense runs on the groundwater, stressing reserves ever further. That situation is simply unsustainable.

“I think of groundwater as your savings account,” explained Rosemary Carroll, a research professor of hydrology at the Desert Research Institute who was not involved in the new study, in 2023. “It can smooth out the really wet and dry years. But if you start consistently reducing that groundwater year after year, then you can no longer modulate those extremes.”

Planning for a dry future

So, what are we to do about all this? Well, the paper stops short of any specific recommendations – but it’s clear that some serious conversations are going to have to occur around land and water use in the area. 

“Are we going to plan to continue to grow as much food?” Famiglietti suggested, speaking to CNN. “Are we losing food that’s important for the state, that’s important for the country, or is it alfalfa that’s being shipped to Saudi Arabia?”

In some regions of the river basin, the problem is already being considered. In California, for example, a handful of state programs and legislation have aimed to curb overpumping, while cities like Phoenix and Tucson are required by law to similarly manage their groundwater use.

But for the most part, groundwater pumping is basically unregulated. And here’s the real kicker: most of that groundwater isn’t like surface water. Once it’s gone, it’s gone – at least within our lifetimes.

“It takes geologic time” to build up those reserves, Familiglietti told CNN – that is, in the order of thousands of years – “and we as humans have more or less been burning through it in the last over the last century.”

For all of these reasons, Famiglietti hopes the new paper will prompt a renewed focus on the region’s water management, not least because, let’s face it: things aren’t getting better.

“Climate warming is driving this drying of the Colorado River Basin for the long term,” cautioned Richter, “so we really need to come to grips with doing this great rebalancing act.”

“We need to start moving out of this danger zone.”

The study is published in Geophysical Research Letters.