Colorado River Basin groundwater levels drop even faster than reservoirs

 

When Lake Mead is full it’s the largest reservoir in the U.S., capable of holding two years’ worth of water from the Colorado River. But the Southwest has been trapped in a 14-year drought, and the states Mead feeds – Nevada, Arizona and California – are thirsty. The reservoir is now only about half full (or half empty, depending on your outlook). This month it hit a record low since it was first topped off in 1937.

Now researchers have discovered another water disappearance that’s equally dramatic, but not nearly as visible as the newly exposed sandstone walls of Lake Mead. Over the last nine years the states drawing on the Colorado River Basin groundwater have pumped enough out from underground to fill Lake Mead nearly twice. That means groundwater has been drawn down even faster than Lake Mead or Lake Powell during those years.

1280pxLake_Mead_Panorama_2.jpg
Lake Mead's white bathtub ring symbolizes drought in the West. But there's been an even greater, invisible draw down of groundwater from beneath the Colorado River Basin in the last nine years. (Photo by Kumar Appaiah, Wikimedia Commons)

It’s well known that when surface water is scarce, cities and farms fill the gap by pumping aquifers. But no one knew that Arizona, California, Colorado, New Mexico, Nevada, Wyoming and Utah have been using that much groundwater until the researchers measured it for the first time, using data from a NASA satellite. “I think the key phrase has been ‘shocking’,” says Stephanie Castle, a water resources researcher at the University of California, Irvine, and the lead author of the study published last week in the journal Geophysical Research Letters. That means a lot coming from a group of scientists studying worldwide groundwater depletion.

So why isn’t more being done to encourage judicious groundwater use? It’s not that managers aren’t interested in it, says Sharon B. Megdal, an expert in groundwater governance and the director of the University of Arizona Water Resources Research Center. Ideas have been thrown around for years, but it’s fundamentally a hard problem to solve and the politics of water don’t make it any easier. This study is also the first to measure groundwater depletion in the Colorado Basin on such a large scale. The good news though is that “right now there’s a lot of pent up demand (within the water-watching community) for developing solutions."


The first barrier to understanding – and caring about – groundwater use is that it’s mostly invisible. With aquifers, there are no photos of marinas stranded well above Lake Powell’s water line, just wells to drill and models to run. That’s why satellite data is so valuable.

Plus, there’s no single entity in charge of accounting for groundwater. That's different from surface water in the Colorado River Basin, since the U.S. Bureau of Reclamation keeps an eye on how much is entering and leaving their reservoirs.  And every state using the river is bound by a 1922 agreement that the northern Colorado River states, where most of water comes from, will share half with the Southwestern states. Since those states know that the water demands of about 40 million people are perilously close to outstripping supply, they’ve begun thinking about ways to stave off water conflict by using less of it or finding new sources.

But states aren’t held to any such agreement about their aquifers, and oversight varies, if there’s any at all. For example, California’s groundwater is the least regulated of any western state. With that state in throes of drought, overpumping has drawn saltwater into coastal wells. That's helped inspire two pieces of legislation that, if passed, could change the free-for-all nature of California's groundwater.

Arizona, where 40 percent of the water comes from underground, has made strides to preserve its groundwater stores, in part by recharging aquifers with leftovers from their Colorado River allotment, instead of sending it downstream to California, and by creating special water management districts. But those districts don’t cover the entire state, and rapid depletion is still a problem.

Since groundwater is a local resource, solutions to its runaway depletion will likely be local too. But not solving the problem, and running out in important agricultural regions, like California’s Central Valley, or cities, would have ripple effects that extend far beyond local wells.

Castle and her colleagues hope their revelations about the region’s groundwater will raise the resource’s profile in the management community and with the public. That’s an important step to slowing the drain on groundwater reserves. “I do personally believe that we can all do more to conserve,” says Megdal. “If people understood the pressure that is being put on our aquifers, they might think more about (their water use).”

Sarah Jane Keller is a High Country News correspondent based in Bozeman, Montana. She tweets @sjanekeller.

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