Web edition: April 7, 2010
Next time you need a spoonful of sugar to help your medicine go down, be thankful for an ample water supply: It takes more than a gallon of water to produce that single heaping tablespoon of sweetness.
A couple of years ago, I wrote about how changing dietary habits in China in recent decades (largely resulting from increased living standards), along with a rising population, will increasingly strain that nation’s water resources (SN: 1/19/08, p. 36). Now, a study published in the March 15 Environmental Science & Technology gives an outline of the United States’ “water footprint” — and it’s a big footprint indeed.
Take, for example, that sugar. It’s jam-packed with “virtual water,” a term that describes the water not actually included in a product but needed to manufacture the item. It takes more than 88 gallons (333 liters) of water to produce a 5-pound (2.3-kilogram) bag of sugar, says Chris Hendrickson, a civil engineer at Carnegie Mellon University in Pittsburgh.
Hendrickson and his colleagues recently looked at water use in all sectors of the U.S. economy. That analysis, he notes, is the first comprehensive look at water use in the United States since the early 1980s, when the U.S. Census Bureau stopped gathering such data from U.S. industries.
In 2002, water use in the United States was around 140 quadrillion gallons, an amount more than 90 percent of the volume the Mississippi River dumps into the Gulf of Mexico in an average year. Generating electrical power was responsible for nearly one-half of that year’s water use, and irrigation used more than one-third. Residential uses of water — such as bathing, cooking, washing the car and watering the lawn — accounted for only 6.4 percent of the total, Hendrickson says.
To some extent, the new study “double counts” water use for power generation because that water isn’t truly consumed and can be reused, says Hubert H.G. Savenije, a hydrologist at Delft University of Technology in the Netherlands. “Agriculture is by far the largest user,” he adds. “The amount of water used in industrial processes is peanuts compared to that present in produce.”
The team’s new analysis bears that out. Take, for example, the manufacturing of semiconductors. That process, which requires about 8.5 gallons of water to produce a dollar’s worth of computer chips, is generally considered to be water-intensive, says Hendrickson. But it takes a whopping 1,400 gallons of water to grow and process a dollar’s worth of grain. You’d have to draw 1,300 gallons from the spigot to grow and process a dollar’s worth of cotton.
Of the top 15 water-using sectors of the U.S. economy (measured by how much water is needed to produce $1 worth of product; see table), 13 are related to food. And one of the other two sectors is cotton farming, an often heavily irrigated crop.
|Economic Sector||Gallons Required|
|Sugar cane and beet farming||830|
|Tree nut farming||500|
|Flour milling and malt manufacturing||470|
|Power generation and supply||450|
|Wet corn milling||380|
|Beet sugar manufacturing||330|
|Vegetable and melon farming||280|
|Other animal food manufacturing||270|
|Sugar cane mills and refining||270|
|Poultry and egg production||250|
|Dog and cat food manufacturing||200|
|Cattle ranching and farming||190|
“This study affirms that if we care about saving water, we need to look at our consumption patterns,” says Sandra Postel, director of the Global Water Policy Project, based in Los Lunas, N.M. Studies she’s familiar with suggest that it takes about 634 gallons of water to produce the beef in a single hamburger, and about 2,900 gallons to produce the cotton in a single pair of jeans.
“Everything around us has water embedded in it,” she notes. “Everything we eat, and everything we buy.”
And often, like Perrier, that water is imported. Like carbon dioxide emissions (SN: 1/16/10, p. 15), water use can be outsourced. About 18 percent of the virtual water in agricultural products and about 24 percent of that in industrial products consumed in the United States between 1997 and 2001 was imported, says Arjen Hoekstra, a water policy analyst at the University of Twente in the Netherlands.
Overall, though, the United States is a net exporter of virtual water, he notes. Each year between 1997 and 2001, on average, the U.S. exported more than 53 cubic kilometers of virtual water, largely due to foreign sales of its agricultural goods.
Blackhurst, M., C. Hendrickson, and J.S. i Vidal. 2010. Direct and Indirect Water Withdrawals for U.S. Industrial Sectors. Environmental Science & Technology 44(March 15):2126. Full text available at [Go to]
Perkins, S. 2002. Crisis on Tap? Pollution and burgeoning populations stress Earth's water resources. Science News 162(July 20):42. Available to subscribers only at [Go to]
Perkins, S. 2008. A Thirst for Meat: Changes in diet, rising population may strain China's water supply. Science News 173(Jan. 19):36. Available to subscribers at [Go to]
More information about the water footprint of the United States and other nations is available at [Go to]