Most people are familiar with the concept of a "carbon footprint," and some even feel dutifully guilty about their own. "Water footprints" get a little less play. Even though we live on a blue planet (oddly named "Earth" by one of its continentally-biased inhabitants), water is still a precious resource that warrants conservation. After all, over 99 percent of that water is either in salty oceans or frozen glaciers.

A paper published last week in the Proceedings of the National Academy of Sciences delves into an analysis of humanity’s water footprint and the various components that contribute to it. This isn’t the first stab at a global assessment, but it’s a significant improvement on earlier work. And, in one of the more interesting twists, it accounts for the international movement of water use that occurs when a good that requires water is shipped to another country.

Rather than a simple accounting by nation, the researchers broke the globe down into cells five minutes (longitude and latitude) on a side. They also divided water use into three categories: rainwater, surface and groundwater, and water pollution (calculated as the amount of water it takes to dilute pollution down to safe levels). Their analysis accounts for water that returns to freshwater resources, so only water that evaporates or is incorporated into a product counts as "use." They also used more accurate estimates of international trade and water use in livestock.

Where's the water? It's in the beef

They estimated that the total global water footprint is about 74 percent rainwater, 11 percent surface and groundwater, and 15 percent polluted. Agriculture is easily the largest use of water at 92 percent, with industrial and domestic water use about splitting the rest.

China, India, and the United States have the largest national water footprints. About 30 percent of China’s footprint comes from water pollution—in fact, it accounts for fully one quarter of global water pollution. The numbers get more interesting when you calculate per capita water footprints. Here, the US is among the very highest at about 2,800 m3/yr per person (the global average is about 1,400 m3/yr). The authors compare this to the UK, which checks in at close to 1,300 m3/yr (despite having a lot of rain to work with).

Per capita water footprints depend on two things: consumption patterns and water intensity of the good being consumed. An example that helps account for the difference between the UK and US is beef consumption and production. The average American eats about 95 pounds of beef each year—four and a half times the global average. In the UK, the average person eats less than half as much beef as Americans do. On top of this, beef raised in the US uses nearly 50 percent more water.

Bolivia actually ranks higher than the US in per capita water footprint, but that’s mostly due to the water intensity of meat production there. The average Bolivian eats only 27 pounds of beef per year (slightly above the global average), but the water footprint of meat production there is five times the global average.

Shipping water internationally

The researchers say that governments don’t pay enough attention to global trade when evaluating water use. They found that 19 percent of the global water footprint comes from products that are exported. One of the problems with carbon emissions accounting is that developed nations outsource a significant portion of their industrial production. While doing so may improve a nation’s carbon balance sheet, it does nothing to halt rising concentrations in the atmosphere.

Similar things can happen when nations look at whether their water use is sustainable—they may outsource a significant portion of their water intensive production. Conversely, nations may be exacerbating water stress in their own countries by exporting goods from regions with water supply issues.

This is described as "virtual water trade," because it's a trade in water use more than a physical transfer of water. As Prof. Arjen Hoekstra, one of the paper's authors, explained to Ars, "The virtual water balance shows something quite different than the real water balance." While some water is incorporated into exported products, the virtual water trade is much larger. "The virtual water balance of a country shows how much domestic water resources are used for making export products (the virtual water export) and how the country depends on foreign water resources (the virtual water import)," Hoekstra said.

The United States is easily the largest exporter, followed by China, India, and Brazil. The researchers point out that water prices is most places don’t reflect the scarcity of the resource, so neither does international trade. That means there’s no economic brake that slows exports when unsustainable water use becomes problematic.

The US is also the largest importer, but this only offsets 75 percent of exports, leaving the United States with one of the largest virtual water trade deficits. Other major net exporters include Canada, Australia, India, Argentina, and Brazil. The biggest net importers are North African, Middle Eastern, and European countries, as well as Mexico and Japan.

Prof. Hoekstra says the study provides consumers with information about the water footprints of various products, and hopes the global perspective will be useful to policy makers. "Governments can use our results to explore how they can contribute to reducing the water footprint within their territory or within water-scarce countries from where water-intensive commodities are imported," he said.

From here, the researchers are working to provide information that could help companies reduce their water footprint. They also plan to investigate how climatic events affect water use. "Water demands and therefore water footprints are generally bigger in dry years. The ultimate challenge is to understand what size of water footprint can still be sustainable [at the local level]," Hoekstra said.

PNAS, 2012. DOI: 10.1073/pnas.1109936109 (About DOIs).