By Kevin Wilcox
A team examining satellite data determines that the region has been drawing down significant amounts of groundwater from underground aquifers, and the current drought is making things worse.
August 5, 2014—The states in the Colorado River Basin have drawn from underground aquifers at unsustainable rates in much of the past nine years, the trend greatly exacerbated by diminished surface water availability during the extreme drought that is now gripping parts of the region. This groundwater depletion will bring new challenges to civil engineers in the region.
Water levels in underground aquifers are notoriously difficult to measure. So recently, a team of researchers from the University of California, Irvine, turned to data from a satellite research program launched jointly by the National Aeronautics and Space Administration and the German Aerospace Center in 2002: the Gravity Recovery and Climate Experiment (GRACE).
As measured by the United States Drought Monitor on July 22, much of the West and parts of the South are dry or in droughts of varying intensities. On the map, yellow indicates abnormally dry, peach indicates a moderate drought, orange indicates a severe drought, red indicates extreme drought, and maroon indicates an exceptional drought. Courtesy of the National Drought Mitigation Center at the University of Nebraska-Lincoln
"GRACE works more like a scale than it does an optical satellite," explains Jay Famiglietti, Ph.D., a professor of earth system science and civil and environmental engineering at UC Irvine. "What it is really doing is monitoring gravitational changes—the gravitational tug that is being exerted, which is a function of mass. And mass changes over time."
As the research satellite passes over areas with greater water mass, it is pulled closer to the earth by a matter of mere microns—and those movements are precisely measured and recorded. Originally designed to monitor ice melt at the polar caps, the satellite has proven accurate at measuring groundwater levels in aquifers as well. "It has far exceeded our expectations of how it would do and what it would reveal," Famiglietti says.
Famiglietti led the research team, which developed a paper on the study, Groundwater Depletion during Drought Threatens Future Water Security of the Colorado River Basin. Stephanie Castle, a water resources specialist at UC Irvine was the lead author of the paper, which was published online recently in the journal Geophysical Research Letters.
The research indicates that the Colorado River Basin lost a staggering 64.8 cubic km of freshwater between December 2004 and November 2013. Of that loss, 50.1 cubic km was groundwater. Groundwater losses were greatest during droughts and abated briefly during a period of heavier precipitation from 2009-2010.
The situation has likely worsened in the past six months as an even greater portion of the region slipped into a drought that has been classified as extreme by the U.S. Drought Monitor and water levels in Lake Mead, in Nevada, have dropped to readings not seen since it was originally filled during the Dust Bowl era. (The U.S. Drought Monitor is produced jointly by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the U.S. Department of Agriculture, and the National Oceanic and Atmospheric Administration.)
Observations in the field corroborate the findings, Famiglietti notes. And this leaves a pressing question: how much groundwater remains? Unfortunately, that question is much more difficult to answer than it appears. Some areas are already running out of groundwater because the aquifer is not as large in some areas as it once was. Additionally, as water levels decline in an aquifer, the water quality becomes poorer and the cost of pumping the water to the surface increases.
The authors note that they saw no prevailing trends in the levels of Lake Mead, or in Lake Powell in Utah and Arizona, during the period of the study. They surmise that this might reflect the large centralized storage of water in the basin and the water management practices in place. As much as 85 percent of surface water in the basin is found in the two lakes, which are both managed by the U.S. Bureau of Reclamation. The storage capacity there represents four years of natural flow of the Colorado River, the authors write.
However, the research team estimated that water in the Colorado River Basin is overallocated by 30 percent, Famiglietti says, and groundwater is bridging the gap between supply and demand.
"A question that I hope the paper raises for discussion is how long can we keep doing this? Can we just keep depleting these groundwater reserves with each successive drought?" Famiglietti asks. He notes that paleoclimate data reveal that the area endured much longer droughts before historical record keeping began. And climate change models project that droughts in the future will be more frequent, longer, and more severe.
"We need to begin working groundwater preservation and groundwater management into the Colorado River Basin discussions. Right now they are missing," Famiglietti says. "Groundwater is the strategic water reserve during drought, but we are hitting that reserve really hard without regard to its availability in the future."
As groundwater is depleted in the basin, civil engineers are likely to be called on to devise innovative solutions. "There will be more work to do in terms of planning for whatever solution strategies that we decide to pursue," Famiglietti says. "That might be more construction of desalination plants, sewage recycling plants, or shoring up existing infrastructure."
Another challenge will involve designing for subsidence and repairing infrastructure already damaged by it as groundwater is depleted. "It's like letting air out of a tire," Famiglietti explains. "When you pump groundwater out, just like a tire deflates, there are many places where the ground actually deflates." Anything that is on the surface—bridges, roads, train tracks, pipeline—can be affected by that subsidence.
Famiglietti says the team is working to develop a complex computer model that would allow them to explore different scenarios of water use and allocation to aid policy makers as they address the issues of water supplies within the Colorado River Basin.
"Drought accelerates the rate of groundwater depletion. Just like a bank account, the lack of water flowing into a river basin forces us to withdraw much more water from underground storage—our water savings account—to meet water supply commitments," Famiglietti says. "Unfortunately, not all the water gets put back in. You can only go back to it so many times before it is gone."