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

[Guest AndrewaGelf]

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.
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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.”