Some 70 percent of the water consumed worldwide, including both that diverted from rivers and that pumped from underground, is used for irrigation, while some 20 percent is used by industry, and 10 percent for residential purposes. In the increasingly intense competition for water among sectors, agriculture almost always loses. This ratio helps explain why, in the American West, the sale of irrigation water rights by farmers to cities is an almost daily occurrence. In addition to population growth, urbanization and industrialization also expand the demand for water. As developing country villagers, traditionally reliant on the village well, move to urban high-rise apartment buildings with indoor plumbing, their residential water use can easily triple.
Industrialization takes even more water than urbanization.
Rising affluence in itself generates additional demand for water. As people move up the food chain, consuming more beef, pork, poultry, eggs, and dairy products, they use more grain. A US diet rich in livestock products requires 800 kilograms of grain per person a year, whereas diets in India, dominated by a starchy food staple such as rice, typically need only 200 kilograms. Using four times as much grain per person means using four times as much water. Once a localized phenomenon, water scarcity is now crossing national borders via the international grain trade.