Irrigation | Research & Encyclopedia Articles

Irrigation is the method of supplying water to land to support plant growth. This technology has had a powerful role in the history of civilization. In arid regions sunshine is plentiful and soil is usually fertile, so irrigation supplies the critical factor needed for plant growth. Yields have been high, but not without costs. Historic problems include salinization and water logging; contemporary difficulties include immense costs, spread of water-borne diseases, and degraded aquatic environments.

One geographer described California's Sierra Nevada as the "mother nurse of the San Joaquin Valley." Its heavy winter snowpack provides abundant and extended runoff for the rich valley soils below. Numerous irrigation districts, formed to build diversion and storage dams, supply water through gravity-fed canals. The snow melt is low in nutrients, so salinization problems are minimal. Wealth from the lush fruit orchards has enriched the state.

By contrast, the Colorado River, like the Nile, flows mainly through arid lands. Deeply incised in places, the river is also limited for irrigation by the high salt content of desert tributaries. Still, demand for water exceeds supply. Water crossing the border into Mexico is so saline that the federal government has built a desalinization plant at Yuma, Arizona. Colorado River water is imperative to the Imperial Valley, which specializes in winter produce in the rich, delta soils. To reduce salinization problems, one-fifth of the water used must be drained off into the growing Salton Sea.

Salinization and water logging have long plagued the Tigris, Euphrates, and Indus River flood plains. Once fertile areas of Iraq and Pakistan are covered with salt crystals. Half of the irrigated land in our western states is threatened by salt buildup.

Some of the worst problems are degraded aquatic environments. The Aswan High Dam in Egypt has greatly amplified surface evaporation, reduced nutrients to the land and to fisheries in the delta, and has contributed to the spread of schistosomiasis via water snails in irrigation ditches. Diversion of drainage away from the Aral Sea for cotton irrigation has severely lowered the shoreline, and threatens this water body with ecological disaster.

Spray irrigation in the High Plains is lowering the Ogallala Aquifer's water table, raising pumping costs. Kesterson Marsh in the San Joaquin Valley has become a hazard to wildlife because of selenium poisoning from irrigation drainage. The federal Bureau of Reclamation has invested huge sums in dams and reservoirs in western states. Some question the wisdom of such investments, given the past century of farm surpluses, and argue that water users are not paying the true cost.

Irrigation still offers great potential, but only if used with wisdom and understanding. New technologies may yet contribute to the world's ever-increasing need for food.

Climate; Commercial Fishing; Reclamation

Huffman, R. E. Irrigation Development and Public Water Policy. New York: Ronald Press, 1953.

Powell, J. W. "The Reclamation Idea." In American Environmentalism: Readings in Conservation History. 3rd ed., edited by R. F. Nash. New York: McGraw-Hill, 1990.

Wittfogel, K. A. "The Hydraulic Civilizations." In Man's Role in Changing the Face of the Earth, edited by W. L. Thomas Jr. Chicago: University of Chicago Press, 1956.

Zimmerman, J. D. Irrigation. New York: Wiley, 1966.

U.S. Department of Agriculture. Water: 1955 Yearbook of Agriculture. Washington, DC: U.S. Government Printing Office, 1955.

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