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Arjen Hoekstra and Mesfin Mekonnen, PNAS Early Edition (2012)

Drink up. Humankind’s annual water footprint from 1996 to 2005 (top). More than 22% of the water consumed worldwide (bottom) is imported as “virtual water,” the amount needed to produce the commodity elsewhere. (Shades of green denote water-exporting nations; lighter shades and slimmer arrows signify smaller volumes. Other colors denote nations importing virtual water, with red nations importing the most.)

Is Agriculture Sucking Fresh Water Dry?

By: 
Sid Perkins
2012-02-13 15:01
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The average American uses enough water each year to fill an Olympic-sized swimming pool, and global agriculture consumes a whopping 92% of all fresh water used annually. Those are the conclusions of the most comprehensive analysis to date of global water use, which also finds that one-fifth of humankind’s water consumption flows across international borders as “virtual water”—the water needed to produce a commodity, such as meat or electronics, if the ultimate consumers were to make it themselves rather than outsource its growth or manufacture.

The new study “is the most comprehensive and finest-resolution analysis to date,” says Sandra Postel, director of the Global Water Policy Project, which is based in Los Lunas, New Mexico.

Humans consume water in a number of ways: They pump it from rivers and reservoirs, draw it from underground aquifers, and render it unusable by polluting it, says Arjen Hoekstra, a water policy analyst at the University of Twente in Enschede, the Netherlands. Expanding on previous studies by them and others, he and colleague Mesfin Mekonnen analyzed humankind’s water footprint at high geographical resolution for the decade from 1996 to 2005, the most recent such interval for which comprehensive data are available. In the study, the researchers divided Earth’s surface into blocks about 85 square kilometers or smaller and then used data compiled by individual nations to estimate water-consumption patterns for all agricultural and industrial processes and for all household uses taking place in each.

Overall, humans used about 9087 cubic kilometers of fresh water—enough to flood the entire state of California with a little more than 21.4 meters of water—each year during that decade, Hoekstra and Mekonnen report online today in the Proceedings of the National Academy of Sciences. Just three countries—China, India, and the United States—are responsible for almost 38% of that water use, with footprints of 1.207, 1.182, and 1.053 cubic kilometers per year, respectively. Together, they have more than 44% of the world’s population.

Although the United States came in third on the list, it has less than 5% of the world’s population. So it led the world in annual per capita consumption of fresh water—which, including the amount of water needed to produce all of the goods and services the average American consumes, is a whopping 2842 cubic meters each year, more than enough to fill an Olympic-sized pool. Global average per capita consumption is about 1385 cubic meters per year.

Also, unlike most previous studies, the new analysis doesn’t just measure the amount of water pumped from surface and underground sources, it considers the possibility that that water, once withdrawn, can be recycled and reused several times before it flows to the sea—a “useful distinction,” says Chris Hendrickson, a civil engineer at Carnegie Mellon University in Pittsburgh, Pennsylvania, who is not part of the research team.

The study also tracked the flow of "virtual water." For example, a previous analysis found that it takes about 5300 liters of water to grow and process a dollar’s worth of grain—an immense volume of water that’s not apparent when you consider a sack of flour sitting on a store shelf. Many nations are water-poor, and they, in essence, outsource their consumption by importing water-intensive commodities, such as grain or electronics, that are produced elsewhere, Hoekstra says. “This flow of ‘virtual water’ is a large part of global economics,” he adds. In all, about 22% of the water consumed worldwide is “virtual water” imported across international borders.

Agriculture accounted for about 92% of the world’s water footprint, the researchers estimate. The water needed to grow so-called cereal grains such as wheat, rice, and corn accounted for about 27% of global water consumption; meat and dairy products accounted for another 22% and 7%, respectively. “Grain is the currency by which we trade water,” Postel says.

Agriculture’s huge water usage offers hope that humans can reduce overall water consumption, Hoekstra says. Improving the efficiency of irrigation, for example, will allow enhanced use of surface water derived from precipitation and reduce dependence on unsustainable withdrawals of groundwater. Each cubic meter of water that’s drawn from surface sources, which are generally renewable, is a cubic meter that doesn’t have to be pumped from aquifers. Such underground sources of water typically aren’t considered renewable at timescales relevant for humans, he notes.

Another way to shrink our water footprint is to change our eating habits, Postel says. In particular, people can opt to eat less meat or to switch from grain-fed beef—which, again, requires about 5300 liters of water for each dollar’s worth of grain fed to a cow—to grass-fed beef, which typically requires only the rainwater falling on a pasture. “Not all burgers are created equal,” she says.

Studies such as the new analysis, as well as others that detail the actual amount of water needed to produce commodities such as coffee or cotton, will help consumers make more informed choices, Postel suggests. “These sorts of choices, multiplied many times, can make a big difference."

Correction: This article originally stated the amount of water consumed by humans each year, 9.087 cubic kilometers, would flood the state of California to a depth of a little over 2 centimeters. The actual depth would be about 21.4 meters.

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