Postel estimates that 40 percent of the world's food comes from irrigated cropland — and she points out that historically, most irrigation-based civilizations have failed. Problems associated with irrigated farming include water-logging, salting and silting. The introduction of diesel and electrically powered pumps has added a new wrinkle: aquifer depletion. The report sites examples of falling water tables — and their effects — on agricultural output:

In India, researchers estimate that water is being pumped from the ground at double the rate of aquifer recharge from rainfall. The International Water Management Institute estimates that India's grain harvest could be reduced by up to one fourth as a result of aquifer depletion. The country's population reached 1 billion in August and is expected to add an additional 18 million people a year for the foreseeable future.

Depletion of the Ogallala aquifer in the southern Great Plains of the United States has led to irrigation cutbacks in farming states — Texas, Oklahoma, Kansas and Colorado have been losing irrigated land over the last two decades. Texas has lost irrigated land at roughly one percent a year since 1980.

Water tables are falling in China almost everywhere that the land is flat. Under the North China Plain, the country's breadbasket, water tables are falling by roughly 5 feet (1.5 meters) a year. Where wells have gone dry, farmers have been forced either to drill deeper, if they can afford it, or to abandon irrigated agriculture, converting back to lower-yield rain-fed farming. China's population is also estimated at 1 billion.

Together, China, the United States and India produce about one-half the world's food.

As populations continue to grow and pull more water from rivers, a new phenomenon — rivers running dry — has developed.

China's Yellow River first ran dry in 1972. Since 1985, it has run dry for part of each year. In 1997, it failed to reach the sea during 226 days, or roughly 7 months of the year. India's Ganges River has little water left during the dry season when it reaches the Bay of Bengal, leaving farmers in Bangladesh strapped for water. The same is true of the Nile River. Most of the water is now claimed, but the populations of the three basin countries, Egypt, Sudan and Ethiopia, are expected to nearly double by 2050.

In central Asia, the Amu Darya, one of two rivers that once fed the Aral Sea, is now drained dry by farmers in Turkmenistan and Uzbekistan. As the sea has shrunk to scarcely half its original size, the rising salt concentration has destroyed all fish, eliminating a rich fishery that once landed 100 million pounds of fish a year.

Similarly, the Colorado, the major river in the southwestern United States, rarely ever makes it to the Gulf of California, and the fishery at its mouth has disappeared.

The link between food shortages and water shortages is close. Hydrologists estimate that when the amount of fresh water per person in a country drops below 1,700 cubic meters per year the country is facing water stress. Postel estimates that the number of people living in countries experiencing water stress will increase from 467 million in 1995 to over 3 billion by 2025 as population continues to grow.

Water scarcity leads inevitably to competition between urban and rural residents. Since it takes 1,000 tons of water to produce 1 ton of grain, it's cheaper for a country to divert water to its urban centers and import grain than to have its own farmers use the limited water to grow food. This works fine until a country runs out of money or there's a crash in the world grain market.

With more and more countries looking to the world market for food, spreading water scarcity may translate into world food scarcity sooner rather than later.