A cup of coffee has a hidden cost.

If the full water requirements of a morning roast are calculated - farm irrigation, bean transportation, and the serving of the coffee - one cup requires 140 liters of water.

This notion of a product's "water footprint" is gaining traction. Defined as the total volume of freshwater required to produce a nation's goods and services, the tool tracks domestic water demand and the impact of consumption on water resources across the globe.

As world water availability begins to decline as the result of population growth, overconsumption, and climate change, more water advocates are encouraging governments and consumers to internalize the true cost of water through an account of their water footprint.

The global water footprint is about 7.5 trillion cubic meters per year, not including irrigation losses, according to estimates [PDF] by Dutch researchers and the United Nations Educational Scientific and Cultural Organization (UNESCO). India, with 17 percent of the global population, has the largest water footprint in absolute terms. But its footprint represents only 13 percent of the world total. The United States, in comparison, comprises 4.5 percent of the world population and consumes 9 percent of the world's water.

Agriculture has the greatest impact on a water footprint. Global crop production requires more than 6 trillion cubic meters of water each year, with nearly a quarter of supplies flowing to rice paddies. Livestock production requires the most water resources in the food chain. One hamburger, for instance, needs 2,400 liters of water on average.

During World Water Week, which runs through Sunday, the water footprint concept is benefiting from a spike in attention. This year's Stockholm Water Prize was awarded to professor John Anthony Allan of King's College London for introducing the predecessor to water footprints: the term "virtual water" - the volume of water required to produce a commodity or service.

The conservation group WWF-UK estimated that the 4,645 average liters of water that Britons consume daily leads the country to import 62 percent of its water sources - making it the sixth largest net importer worldwide behind Brazil, Mexico, Japan, China, and Italy, according to a report released Wednesday. "Only 38 percent of the UK's total water use comes from its own rivers, lakes and groundwater reserves," said WWF's Stuart Orr in a press statement. "The rest is taken from...water resources [often] stressed or very likely to become so in the near future."

Plastic manufacturer Borealis and plumbing supplier Uponor revealed a joint plan to include water footprints in the future planning of plastic products on Wednesday. "Understanding our footprint can be a key tool to further guide the development of more water-saving products," said Tarmo Anttlla, Uponor's communication vice president, in a prepared statement.

Roughly one-third of the world population is estimated to be living in areas of water scarcity. Unless water footprints recede, fierce conflicts over water resources are likely unavoidable, experts warn.

"Feeding everyone - including the undernourished and additional 3 billion people expected in 2050 - will require 50 percent more water than is needed today," said Anders Berntell, executive director of Stockholm International Water Institute at the World Water Week opening ceremony. "We are not prepared to deal with the implications this has for our planet."

Ben Block is a staff writer with the Worldwatch Institute. He can be contacted at bblock@worldwatch.org

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