Groundwater Dropping Globally

In the June 2 Issue:

Groundwater dropping globally

Satellites find supply falling mostly due to agriculture

By Devin Powell

January 14th, 2012; Vol.181 #1 (p. 5)

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Drying OutA simulation based on data from the GRACE satellites and historical weather records reveals the effects of this year’s drought in Texas (driest conditions shown in dark red). A new analysis of data from the GRACE satellites reveals worldwide changes in groundwater. NASA/National Drought Mitigation Center at the University of Nebraska-Lincoln

SAN FRANCISCO — Groundwater levels have dropped in many places across the globe over the past nine years, a pair of gravity-monitoring satellites finds. This trend raises concerns that farmers are pumping too much water out of the ground in dry regions.

Water has been disappearing beneath southern Argentina, western Australia and stretches of the United States. The decline is especially pronounced in parts of California, India, the Middle East and China, where expanding agriculture has increased water demand.

“Groundwater is being depleted at a rapid clip in virtually of all of the major aquifers in the world's arid and semiarid regions,” says Jay Famiglietti, a hydrologist at the University of California Center for Hydrologic Modeling in Irvine, whose team presented the new trends December 6 at a meeting of the American Geophysical Union.

Famiglietti and his colleagues detect water hidden below the surface using the modern equivalent of a dowsing rod: a pair of car-sized satellites, nicknamed Tom and Jerry, that are especially sensitive to the tug of gravity from below.

As the spacecraft chase each other around the planet like their cat and mouse namesakes, they are pulled apart and pushed together by areas of higher or lower gravity. Mountains and other large concentrations of mass have a big, obvious effect that’s consistent from month to month. But water moves around over time, creating small gravity fluctuations that the satellites’ orbital motions respond to.

It takes a lot of flow to noticeably change the distance between the satellites. After subtracting the contributions of snowpack, rivers, lakes and soil moisture, scientists can detect changes in groundwater greater than a centimeter over an area about the size of Illinois.

This joint mission between NASA and the German Aerospace Center — called the Gravity Recovery and Climate Experiment, or GRACE — has been creating monthly snapshots of global groundwater since 2002. The trends now identified in this data help fill in monitoring gaps and confirm problems in places where official groundwater information is unreliable or nonexistent.

“GRACE is very good for areas of the world where we don’t have good ground observations,” says Marc Bierkens, a hydrologist who studies groundwater at Utrecht University in the Netherlands.

China, for example, has been shown to underestimate groundwater use. The country lacks the nationwide network of monitoring wells found in the United States. GRACE’s measurements suggest that water levels have been dropping six or seven centimeters per year beneath plains in the northeast.

In some areas, short-term climate variability may be to blame. For example, the plains of Patagonia in Argentina and areas across the southeastern United States — areas that have been hit hard by droughts — store less groundwater today than they did in 2002.

But there’s little doubt as to what’s behind the biggest drops: farming. An agricultural boom in northern India has helped to squeeze nearly 18 cubic kilometers of water from the ground every year (SN: 9/12/09, p. 5). That’s enough water to fill more than seven million Olympic swimming pools. And in California’s Central Valley, which supports about one-sixth of the nation’s irrigated land, the ground has been sinking for decades as landowners drill more wells and pull out almost 4 cubic kilometers of water per year (SN: 1/16/10, p. 14).

“People are using groundwater faster than it can be naturally recharged,” says Matthew Rodell, a hydrologist and GRACE team member at NASA’s Goddard Space Flight Center in Greenbelt, Md.

Agricultural pressures are particularly worrisome in places like the Middle East, another hotspot on the new GRACE map. Water pumped out of the Arabian aquifer beneath Saudi Arabia and surrounding countries today fell as rain thousands of years ago. Once this fossil water disappears, there’s little new rainfall to replenish it.

Climate change will only worsen the problem, says Famiglietti. Precipitation patterns are becoming more extreme, increasing the severity of droughts. Wet areas are also becoming wetter and dry areas drier, which may accelerate declines in groundwater in some places over the coming years.

But even as the researchers sound the alarm, they don’t know how loud to crank up the volume. GRACE reveals only changes in groundwater. It doesn’t divulge how much water is left.

“We don’t really know how stressed the world’s largest aquifers are,” says Sasha Richey of the University of California Center for Hydrologic Modeling.

Some reservoirs, like the giant Nubian Aquifer that underlies North Africa, may be large enough to meet demand for centuries. But few reliable estimates exist of the amount of groundwater stored in the world’s aquifers.

Despite the uncertainties, Leonard Konikow, a hydrogeologist at the U.S. Geological Survey in Reston, Va., says that water use has become unsustainable in many places. Better irrigation systems that use less water could help to curb the problem, he says. So could channeling water during especially wet periods into aquifers instead of letting it run off into the ocean.

“There are too many areas in the world where groundwater development far exceeds a sustainable level,” says Konikow. “Something will have to change.”

Back Story – DRY AS TEXAS TOAST

Useful for tracking long-term groundwater trends in areas with poor monitoring, GRACE data can also yield insights about places with well-tracked conditions. For the continental United States, a computer combining GRACE data with meteorological records can simulate the movement of rainwater into and out of the ground. One simulation’s output, shown here, reveals the situation on November 28, which saw supplies in many locations reduced to levels seen only 2 percent of the time since 1948 (wetness percentile).

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Comments 11

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Since Agriculture is necessary for our survival, would it not be more correct to say that poor irrigation methods are causing the problem? Broad brush blaming of agriculture is inappropriate.

James Lucaas

Dec. 23, 2011 at 9:05am
Always thought there was something suspect about those vegetarians. Even Yahweh looked down on farmer Cain. Carnivores of the world unite. Save the world from salad!

Jan Vones

Dec. 27, 2011 at 12:30pm
James Lucas, that is an excellent point! It is not, however, the whole story. People are still to blame.

We humans are moving in mass numbers into areas of the world that really cannot hold us and don't have the water capacity to allow us to live there. People are (in the United States) spreading out into prime farmland, converting it into houses, shopping centers, and industrial complexes. This deprives us of prime farmland, which in turn stresses marginal farmland, wetlands, and other places due to agricultural expansion into those areas. While we do need houses and industry (I'm not sure we need shopping centers with their big box stores), we need to develop these in ways that do not deprive us of prime agricultural land.

I note also that Ohio (where I live) has experienced what appears to be an all-time record high for rainfall statewide in 2011. That is ironic given the concern about the depletion of groundwater.

Robert Woodman

Dec. 27, 2011 at 12:30pm
Poor irrigation, yes, and over population of many areas. There are too many people to feed, and too many people living in desert areas without a natural water supply. People have technical abilities, so they "drill, baby, drill." Thus the effects seen by GRACE.

LindaJ

Dec. 27, 2011 at 12:30pm
The the change that could immediately alleviate this problem if for people to switch to mainly vegan diets. Far less land is required to feed vegans, than is required to feed animals for human consumption. In fact, 15 vegans can be fed on the same amount of land that it takes to feed 1 person eating the standard American diet, centered on animal foods. Less crops grown, less water required.

Jack Roesler

Dec. 27, 2011 at 12:30pm
Beaver Ponds have helped. Grouping cattle together for3 days, and returning them 9 months later helps also.

Wayne Johnson

Dec. 30, 2011 at 9:10am
A further problem, apparently not revealed by GRACE, is the increasing salinity of remaining groundwater reserves in many places. The Vegans are going to have to make do with salt tolerant plant diets!
BillG

Bill Geddes

Dec. 30, 2011 at 9:10am
Jack Roesler, I have actively tried to maintain a vegan diet. It has had deleterious effects on my health. I've switched back to a vegetarian diet.

Robert Woodman

Dec. 30, 2011 at 9:10am
Not only are the Vegans going to have to make do with salt tolerant plant diets, but the cattle that feed the meat-arians will have to do the same. me? ...I'm an omnivorous breathe-arian. ...but seriously, it all points to too many people and not enough resources to support them.

Anthony Kerwin

Jan. 6, 2012 at 9:21am
India Agriculture needs water as it has to feed 1200 million in India, however, on one hand it should be necessary to adopt efficient irrigation methods to reduce losses viz. drip and sprinkler. On other hand there is a need to increase artificial groundwater recharge. For example The Gujarat state in India adopted the rain water harvesting through series of small check dams on all rivulets and tributaries of small and major rivers, this has created additional artificial recharge of 8 to 12 % of the annual rainfall, which has ultimately shorted out the ground water depletion and sea water intrusion problem of the region.

Narendra Gontia

Jan. 9, 2012 at 10:19am
One issues that seems to continuously be overlooked is the impact of stormwater mangement on ground water stores. The program in this country varies in its effectiveness state to state, and the cost to taxpayers in mismanagement is in the billions, even trillions of dollars every year. It escapes notice simply because "damages" are divvied up between a myriad of programs and impacts, so there is no one solid number. However, Mis-managed stormwater programs result in increased flooding, pollutant loads, and decreased ground water stores. Increased impervious surface is the number one impact to watershed health nationwide, but not just surface water. The solution, green infrastructure, is much more cost effective to install, maintain, and create thousands or more jobs nationwide. By abandoning antiquated practices such as pipe and discharge and instead moving towards 100% capture, we can decrease surface water usage while simultaneously replenish groundwater stores. There is a push for this, but its slow in coming about simply because there is not nearly enough education about it. By capturing stormwater, pollutant loads are reduced, storm sewer overflow incidence is reduced, hydrologic regimes more closely mimic natural, downstream flooding is reduced; caputured rainwater can be used for irrigation, as flush water, or can even be cleaned up to potable. It is less expensive to operate, allows decentralization so if one location goes down it doesn't affect the entire grid, is much more cost effective to fix, allows home owners and businesses to be less dependent on outside sources, plus uses less energy and helps to reduce the heat island signiture of a location. Please become informed on this important issue.

lotusdogger

Jan. 19, 2012 at 11:40am

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