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Satellites Show Shrinking Aquifers in Drought-Stricken Areas

The southern half of the U.S. has been rapidly depleting its groundwater over the last decade, according to new scientific research
Rio Grande



Flickr/U.S. Army Corps of Engineers

In New Mexico, the Rio Grande is trickling through Albuquerque at only a quarter of its normal flow. The parched range and pastureland in the southwest part of the state are all rated in poor condition by the Department of Agriculture.

According to the U.S. Drought Monitor published Thursday, 45 percent of the state is suffering from "exceptional" drought -- the worst in the nation. Similar conditions exist across much of the southern Great Plains.

In times like these, when rivers and irrigation canals run dry and the rain fails to fall, farmers and ranchers turn to groundwater, stored beneath the surface of the land, to wet their crops and pastures.

And as persistent drought has become almost a way of life for much of the agricultural land in the U.S., the straw of agriculture is sipping up more and more of the underground water stash.

While some regions have left drought's clutches for the time being, overall it has been a while since Mother Nature has come around with the rain or snowfall needed for a full refill.

This has left a swath of regions across the southern half of the United States, stretching from West Virginia to California, with significant groundwater depletion since 2003, according to a paper published last week in the journal Science.

A worldwide problem
Scientists and observers of groundwater can see this nationwide change in part because of a satellite program called the NASA Gravity Recovery and Climate Experiment (GRACE), a fascinating effort that is able to measure the changes in water underneath the surface of the land.

In the paper, authors Jay Famiglietti, director of the UC Center for Hydrologic Modeling at the University of California, Irvine, and Matthew Rodell, chief of the Hydrological Sciences Laboratory at the NASA Goddard Space Flight Center, look back at some of the GRACE program's achievements and call for continued investment in the program to improve management of groundwater resources in the United States and worldwide.

"[There is a] very dire situation that we face right now in the United States that, frankly, I don't think many people recognize," Famiglietti said. "I'm talking about the very rapid rates of groundwater depletion."

In Famiglietti's paper, there is a colored map of the United States, all blues, greens, yellows and reds. Across the southern section, concentrated red pixels note where total water loss is greater than gain, from 2003 to 2013.

There are a lot of them.

Although on a local level, some water managers may know their groundwater or aquifer is being depleted, Famiglietti highlighted the unique ability of the GRACE satellite to see what is happening with groundwater nationally and even worldwide.

The researcher is working on a global map of groundwater depletion and said that what he sees in America fits with worldwide trends, where arid and semiarid regions are losing groundwater, while more northern latitudes and the tropics are getting wetter.

Sensing differences in gravitational pull
Unlike most satellites, which collect data by looking at the part of the Earth they are above, GRACE does not see so much as it "feels" the effects of gravity, NASA's Rodell explained. This unique method is what enables it to keep track of water underground.

The mission is actually a pair of twin satellites that continually measure the distance to each other, roughly 100 miles, "to within the size of a red blood cell," he said.

As they travel 285 miles above the Earth, the satellites respond to bumps in the Earth's gravitational field by going faster or slower. The leading GRACE satellite responds first and the other seconds later, causing the distance between them to vary.

The gravitational pull of a mountain range, for example, will cause each satellite first to accelerate, then to slow as it passes over. If that mountain had snow on it, the satellites would respond even more strongly, accelerating more at first, then moving even slower as they passed.

"Water is heavy, and as a result snow, groundwater, soil moisture and surface water affect the orbits of the GRACE satellites enough to be detected using the inter-satellite range [separation] measurements," Rodell said.

That information, along with measurements from the GPS instruments and accelerometers on the satellite that account for movements not related to the Earth's gravitational field, are all combined within a supercomputer to create a new map of the Earth's gravity field each month.

Other things that affect the gravitational pull, like the oceans and the atmosphere, are subtracted out, and the difference between the new map and the average map allows scientists to see where on Earth water levels are above or below normal, from the Ogallala Aquifer to the Greenland ice sheet.

The Drought Monitor, which provides a nationwide picture of drought updated each Thursday, uses the GRACE data, said Mark Svoboda, a climatologist at the University of Nebraska, Lincoln's National Drought Mitigation Center.

GRACE helps the Drought Monitor staff members in three ways, Svoboda said. It gives them national coverage even in areas where they lack on-the-ground data collection; it sees soil moisture, which is relatively poorly monitored yet a key component of drought; and it sees groundwater.

Groundwater "is monitored even less" than soil moisture, Svoboda said.

Seeing what others can't
"GRACE allowed us to go much deeper. It goes through the soil column all the way to groundwater tables," he said.

While the program is very useful, it does have limitations.

Even though the measurement between the satellites is quite fine, because of all the other factors that have to be subtracted out and accounted for, its resolution is only accurate to about 150,000 square kilometers, an area about the size of Illinois.

That is still useful on a river basin level, but Famiglietti would like to see that number cut down to about 50,000 square kilometers, perhaps with the next version of the GRACE satellites, which are scheduled to launch in 2017.

The bigger issue, though, the scientists said, is that the data do not become available until two to six months after they are collected, due to the amount of time it takes to process them.

This means the information is less usable for managers who want to make decisions based on it.

"It is important to get the data out quickly," Famiglietti said. "We could use the data, say, all over the southern part of the United States and use what's been happening over the past three or four months to predict what the drought situation will be like over the end of the summer.

"But we can't do that right now because we don't have the data, it doesn't come out fast enough."

That's in the works, said NASA's Rodell, noting that a researcher at the University of Texas, Srinivas Bettadpur, is working to improve that delay to one to two weeks after the data get collected.

"It's essential for any real-time operational product that it's producing something daily or weekly," Svoboda said. "That should be the goal."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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