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17 trillion gallons lost: Southwest groundwater depleting faster than thought

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A study by researchers from the University of California, Irvine suggests that groundwater loss in the Colorado River Basin is not only higher than expected, but that other water sources may be inadequate to fill the gaps if it disappears.

The researchers used data provided via NASA’s Gravity Recovery and Climate Experiment (GRACE) program, which uses satellites to measure fluctuations in the Earth’s gravitational field, to closely estimate the amount of groundwater remaining in the area.

The Colorado River supplies water for seven states, including Arizona and its neighbors. Arizona is mostly within the Lower Basin.

The study shows that the Colorado River Basin lost approximately 64.8 cubic kilometers of freshwater during nine years of drought between 2004 and 2013. About 77 percent of that was groundwater.  A cubic kilometer equals about 264 billion gallons, meaning the amount lost adds up to about 17 trillion gallons.

The study was accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union, which posted the report on July 24.

Stephanie Castle, the study’s lead author and a water resources specialist at UC Irvine, said during the drought, the basin states have paid more attention to surface reservoirs instead of groundwater reserves. She hopes the study will convince water managers to better incorporate groundwater into their planning process.

“With groundwater, we’ve already depleted nearly two underground Lake Meads,” Castle said. “We’re not seeing the same framework to regulate them, so we’re seeing that groundwater is quietly disappearing.”

Castle said it’s too difficult to get a “basin wide” picture from any other means than GRACE, but the satellites can only determine how much the amount of groundwater has changed. She said there is no way to know how much total water is underground.

The GRACE satellites also cannot detect changes in an area much smaller than the Upper and Lower Basins, which means it’s difficult to pinpoint the exact location losing the most groundwater, Castle said. However, she said the Lower Basin is losing groundwater faster than the Upper Basin.

The Arizona Department of Water Resources has taken several steps to achieve sustainable groundwater use, but that goal may not be reached for years. DWR has designated five active management areas, which are the places in the state where groundwater reliance is high.

In most of these areas, the state has plans to achieve “safe yield” by 2025. Safe yield is when an area recharges about the same amount of groundwater it uses over a period of time.

In the Phoenix AMA, groundwater accounted for about 31 percent of the water supply used to meet demand in 2006. The DWR estimates that Phoenix area groundwater use by 2025 would remain near the same percentage of total water, but the gross amount used could increase dramatically.

DWR Director Michael Lacey said groundwater is an important part of the state’s “water portfolio” and that recharging the supply is one of his department’s large responsibilities. The agency is working on implementing a new management plan that would help move groundwater use in the AMAs closer to safe yield.

However, he questioned the ability of the GRACE satellites to accurately measure the amount of groundwater lost.

“I’ve read it and I’m not quite sure of the science behind it,” Lacey said. “It’s a very broad look at things, and how that translates to individual basins and situations within Arizona, I haven’t yet made that link. We have people have people who are specialists and we’re having them look at it.”

Castle said she hasn’t heard many responses from water managers, but she is confident in the study’s findings.

As one of the arid Lower Basin states, Arizona has a history of trying to address the issue of groundwater use.

When it began construction in 1985, one of the Central Arizona Project’s goals was to reduce Arizona’s groundwater dependency by providing greater access to river water. CAP also engages in groundwater recharge efforts.

Pamela Pickard, president of the Central Arizona Water Conservation District board, which oversees CAP, said the utility recharges a significant amount of water every year and that it works closely with the DWR and the implementation of the Active Management Areas has helped it a “great deal in raising the groundwater.”

Both Lacey and Pickard noted that groundwater use is higher during a drought, but during an extreme shortage, the state’s water supply would be secure for several years.

Pickard said if a shortage continued for a significant amount of time, CAP would allocate water away from agriculture users and toward city and industrial customers.

Shortages are likely to happen eventually because of the “structural deficit” surrounding the use of Colorado River water, Pickard said.

“The three Lower Basin states are taking out more water every year than nature’s putting in,” she said. “At some point that’s just not sustainable – obviously. So there are things that we have got to do in order to ensure the health of the river.”

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