Human beings have survived in deserts or arid areas only because they have been able to increase the quantity of water available to meet their needs. An elaborate system of dams, reservoirs, irrigation pipelines, aqueducts, and canals allows residents of the American West, for example—especially California—to ignore the fact that they live in a naturally dry climate. That fact has made California's Central Valley the most productive agricultural region in the world on only 3 percent of U.S. farmland.

Constant irrigation, however, is not a miracle solution as once was thought. According to the United Nations Environment Program (UNEP), 90 percent of the land in Egypt, 68 percent in Pakistan, 50 percent in Iraq, 38 percent in Peru, 30 percent in the United States, and 20 percent each in India, Russia, and Australia is suffering salinization (saltiness) caused by irrigation. Sodium in the soil or irrigation water accumulates at the root level of soils or turns into a sterile, rock-hard crust. An estimated 5 million acres of irrigated land are pulled from production each year because of waterlogging and salinization, the result of poor land management. In addition, irrigated land is often paved over for housing, factories, and roads, especially in the United States and Asia, further reducing the productive use of land for agriculture.