Which costs more, a dollar's worth of sugar or a dollar's worth of paint? That's not a trick question-the sugar costs more, if you count the liters of water that go into making it, according to a new study. Uncovering the water behind the dollars in sectors including cotton farming and movie making could help industries use water more wisely, the study's authors say.
Researchers know little about how much and where water is used. The United States Census Bureau stopped monitoring companies' water consumption in the 1980s, so the most detailed information available is the U. S. Geological Survey's 2000 report on water use in eight broad categories, such as irrigation, public supply, and power generation. But as demand for water increases, it would be useful to know water's relative importance to various industries, says Chris Hendrickson, a civil engineer at CarnegieMellonUniversity in Pittsburgh, Pennsylvania.
To refine the broad categories, Henderson and his colleagues followed the money. They started with the Census Bureau's 2002 economic input-output table, which tracks payments among all 428 industrial sectors, from architecture firms to frozen foods to zoos, and determined how much money each sector spent on water from the public supply. Then they looked at less obvious clues-dollars spent on power, for example, because power plants use massive amounts of water as a coolant. Finally, they used a computer model to quantify the amount of water needed to generate every dollar that flowed from one sector to another. This revealed which sectors used the most water and why.
Not surprisingly, grain and cotton farming top the list, with about 5000 liters of water going into every dollar of the final product. But the model's ability to trace water flow between sectors revealed water where you might not expect to find much. For example, manufacturing $1 of tortillas requires a scant liter of water. But because the process starts with corn or wheat as an ingredient and occurs in factories that run on electricity, that buck's worth of tortilla actually consumes about an additional 500 liters. In a whopping 96% of the sectors analyzed (power generation and agriculture are the two big exceptions), this indirect water consumption outweighed direct water consumption, Hendrickson and colleagues reported  15 March in Environmental Science and Technology. An online version of their model is available here .
Unveiling this "virtual water" achieves, for the first time, a true accounting of how much water a sector uses, says Michael Blackhurst, a graduate student at Carnegie Mellon and the study's lead author. "If you just look at [a sector's] purchases, you might miss very water-intensive activity." Knowing exactly where water is used could also help identify where we can conserve, Hendrickson adds.
"It's a very important piece of work," says Arpad Horvath, a civil engineer at the University of California, Berkeley. The next step is to take this study to the level of individual products and regions, which would show states and counties how much water local industries consume and help them decide whether it makes sense, given their resources, to approve new factories or expand farming. For example, "it makes a huge difference whether [a water-intensive activity] happens in Minnesota or California," Arpad says. "That geographic specificity will have to be included in the next piece of work, ... but it's wonderful that the authors have gotten to this point."