The United States’ two largest reservoirs have dropped to unprecedented lows this year as drought grips the Southwest, but below the ground there’s another water crisis playing out — one that’s much less visible, but no less alarming.

The amount of groundwater in the Colorado River Basin is declining rapidly, and it’s doing so in unexpected ways, according to a new study from scientists at Virginia Tech, published in the journal Water Resources Research.

Lake Mead and Lake Powell are the visible faces of the Southwest’s critical water problem, as the Colorado River — which supplies water to nearly 40 million people — shrinks due to a combination of climate change and human demand.

“However, they represent only part of the region’s water resources,” said Esther Oyedele, a study author and a geosciences graduate student at Virginia Tech. “Groundwater is another essential component.”It is a vital alternative source in the Colorado River Basin, especially when surface water in rivers, lakes and reservoirs dries up.

To build a picture of what’s happening to the region’s groundwater, the scientists compared measurements from NASA satellites with estimates of surface water, soil moisture and snow-water storage between 2004 and 2023.

They found declines across the basin during those two decades, with losses accelerating after 2019. A severe drought between 2020 and 2022 led to particularly significant falls — and the researchers found a surprising twist.

Historically, groundwater has typically declined more in the Lower Colorado River Basin, which encompasses California, Arizona and Nevada. The Upper Basin, which includes Colorado, New Mexico, Utah and Wyoming, has tended to be more resilient as its groundwater benefits from melting snow coursing down the Rocky Mountains.

However, from 2020 to 2022, groundwater in the Upper Basin declined by about 618 cubic feet, nearly twice the rate of the Lower Basin, which fell by an estimated 332 cubic feet.

“The system has hit a turning point: The Upper Basin is now experiencing rapid groundwater losses,” the report authors wrote.

Surface water and groundwater are linked in a delicate dance: Rain and melting snow seep into the ground and recharge aquifers, meanwhile groundwater can also make its way into rivers and lakes, for example through springs.

There are big differences in how these relationships play out between the Upper and Lower Basins, the study found. Snowmelt plays a much more important role in the Upper Basin, which receives more precipitation than the generally hotter, drier Lower Basin. Prolonged drought, higher temperatures and reduced snow appear to be placing the Upper Basin region under additional pressure.

The research follows another study published last year, which found that the amount of groundwater pumped out of the Colorado River Basin over the previous two decades was enough to fill Lake Mead.

It takes thousands of years to refill these aquifers, “and we as humans have more or less been burning through it over the last century,” Jay Famiglietti, an Arizona State University professor who was an author on last year’s study, said at the time.

The messages of this paper and the new study “have been that while surface water gets all the policy (and) management attention, groundwater dominates the losses from the entire basin,” Famiglietti told CNN recently.

Shrinking groundwater is going to be a problem as Arizona, California and Nevada are being forced to make cuts to the water they draw from the Colorado River, Famiglietti said, referring to a new operating plan for the basin published in August.

The study does not give details on the total amount of groundwater remaining in the basin, only how it has changed over time. It also doesn’t quantify the exact impact of climate change on the falling water levels compared to other factors at play, including human water demand and groundwater extraction.

What the study does show is that “groundwater storage has experienced substantial and persistent declines,” Oyedele added.

“Focusing only on reservoirs could therefore overlook an important part of the region’s overall water challenges,” she said, adding that groundwater should not be seen as an “unlimited reserve.”