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Groundwater Disappearing Much Faster Than Lake Mead in Colorado River Basin

Stefanie Spear
Jul. 24, 2014 04:50PM EST Climate

The mineral-stained canyon walls and the plunging water levels at Lake Mead, the nation’s largest reservoir, are the most visible signs of the driest 14-year period in the Colorado River Basin’s historical record.

The Colorado River flows past irrigated fields in Arizona. During a dry decade, most of the water losses in the seven-state basin come from groundwater, according to new research. Click image to enlarge. Photo credit: Carl Ganter/Circle of Blue

But the receding shorelines at the Basin’s major reservoirs—including Lake Mead, which fell to a record-low level this month, and Lake Powell, the second largest reservoir on the Colorado, 290 kilometers (180 miles) upstream from Mead—are an insignificant hydrological change compared to the monumental disruption taking place underground.

Satellite data show that in the last nine years, as a powerful drought held fast and river flows plummeted, the majority of the freshwater losses in the Basin—nearly 80 percent—came from water pumped out of aquifers.

The decrease in groundwater reserves is a volume of water equivalent to one and a half times the amount held in a full Lake Mead, according to a study published today in the journal Geophysical Research Letters.

“We didn’t think it would be this bad,” said Stephanie Castle, a researcher at the University of California, Irvine, and the study’s lead author. “Basin-wide groundwater losses are not well documented. The number was shocking.”

Scales in the Sky

The research team used data from the GRACE satellite mission to calculate the water balance for a Basin that spreads across seven states and reaches into Mexico.

Launched in 2002, GRACE is a pair of satellites that translate fluctuations in the Earth’s gravitational pull into changes in total water storage. Separate data sets are used to pull apart the contributions from snowpack, soil moisture, surface water and groundwater. GRACE data has been used to assess groundwater depletion in California and the Middle East, as well as flood potential in the Missouri River Basin.

Jay Famiglietti, an Earth systems professor at the University of California, Irvine, and a study co-author, calls the instruments “scales in the sky.”

The black line shows monthly trends in groundwater storage in the Colorado River Basin. The blue line is the average trend for the study’s December 2004 to November 2013 time frame. The red lines show the trends before and after February 2010, when the rate of groundwater depletion increased. Click image to enlarge. Image credit: UC Center for Hydrologic Modeling at the University of California, Irvine

For the study, the researchers looked at the period from December 2004 to November 2013. This time frame allowed them to isolate the effects of the drought on groundwater. By 2004, the Basin’s reservoirs were nearing record lows. Today, they are slightly, but not significantly lower, at 51 percent full.

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The steepest rates of water loss occurred in 2013, which followed one of the driest years on record.

The total decline in the Basin’s water reserves was 64.8 cubic kilometers (52.5 million acre-feet), with 50.1 cubic kilometers (40.6 million acre-feet) from groundwater. The capacity of Lake Mead, for comparison, is 32 cubic kilometers (26 million acre-feet). At least 70 percent of the Basin’s water use is for irrigated agriculture.

Chuck Collum, Colorado River manager at the Central Arizona Project, which delivers more than half of Arizona’s Colorado River allocation, said the study was helpful.

“The study is useful in using GRACE to verify at a coarse scale what water managers in the Basin know,” Cullom told Circle of Blue.

To check their conclusions, the researchers compared the satellite data against measurements taken from individual wells. Measurements from nine U.S. Geological Survey wells and 65 Arizona Department of Water Resources wells showed similar trends at the smaller scale.

“That gives us confidence in what GRACE is seeing,” Castle said.

Still, GRACE data has limits. For one, the satellites cannot tell what share of the groundwater decline comes from an increase in pumping versus a decrease in the recharge rate, the amount that flows into an aquifer. In a drought, pumping would be expected to go up and recharge down, Famiglietti said.

GRACE also has spatial constraints. The satellites work best for land areas larger than 200,000 square kilometers (77,220 square miles), roughly the size of Nebraska. The satellites cannot pick out hotspots of groundwater depletion within the states.

“It’s difficult to break GRACE data down farther than we have,” Famiglietti told Circle of Blue. “We need additional data on the ground to tell what is happening.”

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"Faced with global warming, rising sea levels, and the climate-related extremes they intensify, the question is no longer whether some communities will retreat—moving people and assets out of harm's way—but why, where, when, and how they will retreat," the study begins.

The researchers suggest that it is time to rethink retreat, which is often seen as a last resort and a sign of weakness. Instead, it should be seen as the smart option and an opportunity to build new communities.

"We propose a reconceptualization of retreat as a suite of adaptation options that are both strategic and managed," the paper states. "Strategy integrates retreat into long-term development goals and identifies why retreat should occur and, in doing so, influences where and when."

The billions of dollars spent to rebuild the Jersey Shore and to create dunes to protect from future storms after Superstorm Sandy in 2012 may be a waste if sea level rise inundates the entire coastline.

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Rethinking retreat could make it a strategic, efficient, and equitable way to adapt to the climate crisis, the study says.

Dr. Siders pointed out that it has happened before. She noted that in the 1970s, the small town of Soldiers Grove, Wisconsin moved itself out of the flood plain after one too many floods. The community found and reoriented the business district to take advantage of highway traffic and powered it entirely with solar energy, as the New York Times reported.

That's an important lesson now that rising sea levels pose a catastrophic risk around the world. Nearly 75 percent of the world's cities are along shorelines. In the U.S. alone coastline communities make up nearly 40 percent of the population— more than 123 million people, which is why Siders and her research team are so forthright about the urgency and the complexities of their findings, according to Harvard Magazine.

Some of those complexities include, coordinating moves across city, state or even international lines; cultural and social considerations like the importance of burial grounds or ancestral lands; reparations for losses or damage to historic practices; long-term social and psychological consequences; financial incentives that often contradict environmental imperatives; and the critical importance of managing retreat in a way that protects vulnerable and poor populations and that doesn't exacerbate past injustices, as Harvard Magazine reported.

If communities could practice strategic retreats, the study says, doing so would not only reduce the need for people to choose among bad options, but also improve their circumstances.

"It's a lot to think about," said Siders to Harvard Magazine. "And there are going to be hard choices. It will hurt—I mean, we have to get from here to some new future state, and that transition is going to be hard.…But the longer we put off making these decisions, the worse it will get, and the harder the decisions will become."

To help the transition, the paper recommends improved access to climate-hazard maps so communities can make informed choices about risk. And, the maps need to be improved and updated regularly, the paper said as the New York Times reported.


"It's not that everywhere should retreat," said Dr. Siders to the New York Times. "It's that retreat should be an option. It should be a real viable option on the table that some places will need to use."

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