"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."