When James Cordon took over as host of the CBS Late Late Show earlier this year, he quipped: "I hear that California has about one more year left before it runs out of water. When I took this job and moved here, I didn't realize that my children might actually die in one year!"

Although rumors of the end of the world are greatly exaggerated (to paraphrase Mark Twain), there is some cause for concern: Two papers published in the journal Water Resources Research use data from NASA's GRACE satellite pair to show that many of the world's major aquifers are being depleted, some at a rate faster than predicted by previous models.

The conclusion: There is less water than previously thought, and time may very well be running out.

Map shows major aquifers, with most threatened toward red end of spectrum. Image courtesy NASA/JPL-Caltech.

NASA's Global Recovery And Climate Experiment (GRACE), launched in 2002. It had a nominal mission of five years, but was extended at least through 2015. Using the satellite data acquired between January 2003 and December 2013, a study was done by a research team led by the University of California at Irvine. The recent GRACE-based reports produced some alarming results.

To begin, most of the world's fresh water is in the form of ice, primarily in Antarctica and Greenland. Other sources are lakes, rivers, snow and surface (canopy) water. For the purposes of the study, these sources are left out. Some, like surface canopy water, are deliberately excluded. This is because most of the water that is used by humans is in underground storage, in aquifers.

The first paper deals with the Rechargeable Groundwater Stress ratio (which is determined by: Use divided by availability). Through an analysis of the use/availability, RGS can tell us how sustainable an aquifer is.

Ultimately, studies of this nature are very important because, not only are aquifers the major source of the world's drinking and irrigation water, they are especially important in times of drought. Right now, for example, California is experiencing a multi-year drought, and it is pumping from its underground reserves to provide drinking water and to help its agricultural economy (the largest in the U.S.) survive.

So. Where do we stand?

Of the 37 aquifers studied, 21 are being depleted faster than they are being recharged. 13 are significantly stressed, with negligible recharge or high usage rates. Eight of these are overstressed, with negative recharge due to capillary flux (upward loss through the soil layers). Another three are extremely stressed, and two are highly stressed, including California's Central Valley.

The most overstressed is the Arabian Aquifer System, serving some 60 million people, followed by the Murzuk-Djado Basin of North Central Africa and the Indus Basin of India and Pakistan, while those found in California are also extremely stressed.

Image courtesy NASA/JPL-Caltech.

The second paper deals with the Total Groundwater Stress ratio, in which the authors attempt to approximate when the aquifers will be exhausted. However, due to insufficient ground-based data on the depth of these aquifers, total times to depletion remain unknown.

The lacking data comes about because GRACE is able to detect the changes in groundwater, but not the total volume. And drilling to underlying bedrock, in many cases thousands of meters deep, is difficult and expensive. As a result, previous researchers relied heavily on guesswork to establish depth and total volume. Thus, the total amount of water in underground storage is unknown. The authors state that this uncertainty in existing estimates of total groundwater capacity makes predictions of realistic times to depletion unattainable, and they were quick to emphasize the need for a global effort to quantify total aquifer storage. This would include in situ observations of saturated thickness (aquifer depth) and soil properties.

However, and this is key, we do know that we are using more than we are replenishing.


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