The Colorado River Basin has lost as much groundwater as the entire volume of Lake Mead

CNN

2 days ago

Deep below the surface of the ground in one of the driest parts of the country, there is a looming problem: The water is running out — but not the kind that fills lakes, streams and reservoirs.
The amount of groundwater that has been pumped out of the Colorado River Basin since 2003 is enough to fill Lake Mead, researchers report in a study published earlier this week. Most of that water was used to irrigate fields of alfalfa and vegetables grown in the desert Southwest.
No one knows exactly how much is left, but the study, published in the journal Geophysical Research Letters, shows an alarming rate of withdrawal of a vital water source for a region that could also see its supply of Colorado River water shrink.
'We're using it faster and faster,' said Jay Famiglietti, an Arizona State University professor and the study's senior author.
In the past two decades, groundwater basins – or large, underground aquifers – lost more than twice the amount of water that was taken out of major surface reservoirs, Famiglietti's team found, like Mead and Lake Powell, which themselves have seen water levels crash.
The Arizona State University research team measured more than two decades of NASA satellite observations and used land modeling to trace how groundwater tables in the Colorado River basin were dwindling. The team focused mostly on Arizona, a state that is particularly vulnerable to future cutbacks on the Colorado River.
Groundwater makes up about 35% of the total water supply for Arizona, said Sarah Porter, director of the Kyl Center for Water Policy at Arizona State University, who was not directly involved in the study.
The study found groundwater tables in the Lower Colorado River basin, and Arizona in particular, have declined significantly in the last decade. The problem is especially pronounced in Arizona's rural areas, many of which don't have groundwater regulations, and little backup supply from rivers. With wells in rural Arizona increasingly running dry, farmers and homeowners now drill thousands of feet into the ground to access water.
Scientists don't know exactly how much groundwater is left in Arizona, Famiglietti added, but the signs are troubling.
'We have seen dry stream beds for decades,' he said. 'That's an indication that the connection between groundwater and rivers has been lost.'
Some land has also begun to cave in, with deep fissures forming in parts of the state as ground water has been pumped out. This is not unique to Arizona, Famiglietti said, with similar signs of disappearing groundwater happening in the agriculture-heavy Central Valley in California.
Porter said the results illuminate the magnitude of the groundwater crisis in the Southwest, which is particularly helpful for state officials and lawmakers.
'There are a lot of people who aren't sure if we have a serious situation with respect to groundwater, because groundwater is hidden,' Porter said. 'The value of the study is that it really adds a lot of information to the picture.'
Groundwater may be hidden, but scientists know with relative certainty that once it is pumped out, it won't be able to recharge within our lifetimes. Much of it was deposited tens or hundreds of thousands of years ago.
'It takes geologic time' to refill these deep aquifers — meaning thousands of years — 'and we as humans have more or less been burning through it in the last over the last century,' Famiglietti said.
Famiglietti warned the groundwater situation could worsen if the state's allocation of Colorado River water is further decreased, a decision that could be made in the next two years.
If Arizona's Colorado River water allocation was cut to zero, 'we could burn through the available groundwater in 50 years,' Famiglietti said. 'We're talking about decades. That's scary. No one wants that to happen.'
But Porter pushed back on that characterization, pointing out that Arizona cities have another stable water supply—the Salt River. Porter added cities like Phoenix and Tucson are storing groundwater and have regulations designed to keep it from running out. Arizona has had a groundwater management law in place since 1980.
'We're not expecting that the whole state would turn to groundwater,' Porter said.
Famiglietti said he hopes the study will prompt discussions over how to more effectively manage groundwater use in the region, especially from agriculture, which uses the lion's share of water. Much of Arizona's crops are exported, either to other states or, as is the case with alfalfa, internationally.
Famiglietti called it the 'absolutely biggest' choice that policymakers will have to decide.
'Agriculture just uses so much water,' Famiglietti said. 'Are we going to plan to continue to grow as much food? 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?'