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December 27 2011

17:25

Satellite Studies Reveal Groundwater Depletion around the World


Access to freshwater resources has always been a critical need for human and all forms of life on Earth. With a world population estimated at just shy of 7 billion and growing, the UN Food and Agriculture Organization says agricultural production will need to increase 70% by 2050. As agriculture takes up most of human water use, that’s going to put vastly greater demands and strains on our water resources at a time when climate change is changing temperature and precipitation levels and patterns in ways that cannot be predicted at local levels but are likely to make this even more difficult to achieve.

One thing that has been determined is that groundwater levels have dropped in many places around the world in the past nine years, including across key agricultural areas, such as southern Argentina, western Australia and the western US, according to a pair of studies of satellite gravity monitoring data conducted by researchers at the University of California Center for Hydrologic Modeling in Irvine, Science News reports.

The GRACE Project

Groundwater depletion is especially pronounced beneath parts of California, India, the Middle East and China. Besides showing that water is being pumped out of underground groundwater aquifers faster than it’s being replenished, the results raise concerns that farming in particular is the primary cause, according to the Science News report.

“Groundwater is being depleted at a rapid clip in virtually of all of the major aquifers in the world’s arid and semiarid regions,” cautioned UC Center hydrologist Jay Famiglietti, whose team presented the results at a Dec. 6 meeting of the American Geophysical Union.

The Gravity Recovery and Climate Experiment (GRACE), conducted jointly by NASA and the German Aerospace Center, has been taking monthly snapshots of global groundwater used in the two studies since 2002. GRACE data is especially useful in accumulating data across countries where governments do not maintain extensive networks of groundwater monitoring wells. While the US maintains an extensive nationwide network of such wells, countries, such as China, do not.

Nicknamed Tom and Jerry, GRACE’s two satellites are pulled apart and pushed together by variations in the gravitational pull of the areas of the earth they pass over. While mountains and other large concentrations of mass have large, steady impacts on earth’s gravitational pull on the areas where they’re found, water moves over time and creates small fluctuations that the two satellites sense.

Isolating groundwater changes

To isolate the effects of groundwater in particular, researchers have to subtract the effects of snow pack, rivers, lakes and soil moisture, the Science Times article explains. Doing so, they can detect changes in groundwater levels greater than one centimeter (~0.4 inches) over an area about the size of Illinois.

Results of analyzing the data obtained in the two UC Center studies shows that China’s been underestimating groundwater use. GRACE’s measurements indicate that water levels have been dropping 6 or 7 centimeters per year beneath the country’s northeast plains.

Short-term variability in climate is also taking its toll on groundwater levels. having suffered recent droughts, aquifers in Patagonia and the southeastern US now store less groundwater than they did in 2002.

Farming is almost certainly the largest contributing factor, however. Booming agriculture in northern India, takes some 18 cubic kilometers of water out of the ground every year, more than enough to fill 7 million Olympic-size swimming pools, according to Science News.

Farmers in California’s Central Valley, which accounts for nearly 1/6 of irrigated land in the entire country, pump nearly 4 cubic kilometers of water per year out from underground. The valley has been sinking for decades as more wells have been drilled and water pumped out, land subsidence that’s also been occurring and causing increasing concerns, and costly remediation efforts, in Mexico City.

Aquifers in arid and desert areas with fast-growing populations, such as the Middle East, are also being depleted. The “fossil water” that fell millions of years ago and is now stored in the Arabian aquifer beneath Saudi Arabia and neighboring countries is being pumped out faster than it’s being replenished.

Just how much water is there?

Climate change only makes the problem more acute, according to UC Center’s Famiglietti. Precipitation patterns are becoming more extreme, with the severity of droughts increasing. Wet areas are becoming wetters and dry areas drier, Science News reports, and that may accelerate groundwater depletion in some areas.

A big question remains unanswered, however, as hydrologists don’t really know just how large these aquifers are and just how much water is left in them. That’s because GRACE can only show changes in aquifer levels, not their total volume.

Yet while they lack reliable estimates for the total amount of groundwater stored in the world’s aquifers, it’s become clear to hydrologists studying them that water use has become unsustainable in many areas. Better irrigation systems would help reduce water usage, as could channeling water runoff into aquifers during wet periods.

“There are too many areas in the world where groundwater development far exceeds a sustainable level,” US Geological Survey hydrogeologist Leonard Konikow, was quoted as saying. “Something will have to change.”

Image credit: GRACE

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