Science: Is Agriculture Sucking Fresh Water Dry?

In the case of America? No.

They make a big deal out of crops being “water export” but whether those crops are grown, harvested and exported has no real relevance when you talk about rainfall and impoundment water – that will simply flow to the sea or return to the atmosphere as evaporation and most of it does after having been utilized by growing crops anyway.

They claim it takes 5,300 liters of water to produce a dollar’s worth of grain but that is about as misleading as it gets. Does $1’s worth of grain weigh 5.3 metric tons? No, so the water isn’t in there, so where is it? Mostly it’s where it would have been anyway, regardless of whether humans benefitted from its transient use.

The situation is slightly different when discussing “fossil water” (drawing from aquifers at rates beyond replenishment) but only slightly – in this case crop export is simply those with greater fresh water assets sharing with those with less. This movement of subsurface water to surface use does add something to sea level rise as it returns to the hydrological cycle and if we wish to address that the answer is simple – build more dams.

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.

Science Now

One response to “Science: Is Agriculture Sucking Fresh Water Dry?

  1. How is rain water factored in?

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