If you think your morning cup of joe only has 12 ounces (35 centiliters) of water in it, you're sorely mistaken—it has closer to 40 gallons (150 liters). Conservation scientists say it's time consumers become aware of the quantity and source of water that goes into growing, manufacturing and shipping food.

Concerns over greenhouse gas emissions have vaulted the term "carbon footprint" into mainstream vernacular. Now, by promoting the concept of a "water footprint" with the goal of including it on product labels, researchers are hoping to draw similar attention to how drastically we're draining our most precious resource. As the use of a footprint to gauge water use gains popularity, however, researchers are struggling to reach a consensus on how best to measure that footprint so the public understands its full impact.

As currently defined, a product's water footprint is an inventory of the total amount of water that goes into its manufacture. For that cup of coffee, for instance, most of the 40 gallons flow either into watering coffee plants or cooling the roasters during processing.

"Most people have no idea how much fresh water they're consuming," says Brad Ridoutt, a water conservation specialist from Australia's Commonwealth Scientific and Industrial Research Organization. According to Ridoutt, food and energy production account for nearly 90 percent of the world's fresh water consumption.

The water footprint is designed to help consumers and businesses understand just how much water is required to make products like a cotton T-shirt or a can of corn. But according to Ridoutt, just counting gallons is not enough, because consumers also value where that water came from. Corn grown in Minnesota, for example, depends on rainwater, which is abundant and not otherwise used by people. But in Arizona corn crops depend on scarce reservoir water also used for drinking, hygiene and other consumer needs. The current definition of the water footprint doesn't address these discrepancies.

In a study published in the February issue of the journal Global Environmental Change, Ridoutt proposed a strategy that takes the original location of the water into account in evaluating the environmental impact of its use in product manufacturing.

To illustrate his ideas, Ridoutt chose two common household food items: an 18-ounce (53-centiliter) jar of Dolmio pasta sauce and a small bag of peanut M&M's. For the pasta sauce, the volume of water needed to grow the tomatoes, sugar, garlic and onions added up to 52 gallons (197 liters). For the M&M's, the total volume going into all the ingredients was a whopping 300 gallons (1,135 liters).

Comparing these conventional water footprint values would lead one to think the bag of M&M's takes a far worse toll on freshwater resources. But that isn't the complete picture, Ridoutt says.

Because tomato plants are typically grown in hot, dry climates, they are watered using irrigation systems that draw from the same locations as human drinking water. On the other hand, the cocoa and peanuts in M&M's are grown in more temperate regions, where the crops absorb rainwater directly from the ground. Taking location into account, Ridoutt says, drastically changes how you think about the water going into your food. According to his calculations, the pasta sauce is about 10 times more likely than the M&M's to contribute to water scarcity.

Ridoutt is not the only one trying to redefine the water footprint. Conservationists around the world are trying to figure out how to best include environmental impact in the footprints so they can be incorporated into food labels. The International Organization for Standardization now has a project underway to tackle this problem using methods similar to Ridoutt's.

Although many researchers support Ridoutt's work, others say we don't yet know enough about global water cycles to accurately measure environmental impact. Organizations such as the Water Footprint Network and the World Wildlife Fund (WWF) still believe that simply reporting the total volume of water is currently the best and clearest way to communicate a water footprint.

"The paper Brad has written has quite a high value, but there is a long way to go," says the WWF's Ashok Chapagain, who has been studying water footprint methods for over five years. Without an agreed-on standard, reporting water footprints simply as volumes is the easiest for consumers and businesses to understand, he says.

Ridoutt, on the other hand, believes his method will turn out to be more useful for consumers, and he hopes that when footprints are applied to food products in the future, they won't be just a sum of all the water they have used. "If you want to communicate something to the public in a simple way," he says, "you have to express it in a way that gives the [environmental] impact."

This article is provided by Scienceline, a project of New York University's Science, Health and Environmental Reporting Program.