Irrigation (Encyclopedia of Environmental Issues, Revised Edition)
The demands of feeding and clothing the rapidly expanding world population require the production of increasing amounts of food and fiber. One important strategy for achieving the necessary levels of production has been the use of irrigation techniques to supply additional water to arid and semiarid regions where few, if any, crops could otherwise be grown.
Approximately 141.6 million hectares (350 million acres) of land worldwide are irrigated. In the United States more than 10 percent of all crops, encompassing approximately 20.2 million hectares (50 million acres), receive water through irrigation techniques; 80 percent of these are west of the Mississippi River. In certain other countries, including India, Israel, North Korea, and South Korea, more than one-half of food production requires irrigation. From 1950 to 1980, the amount of irrigated cropland doubled worldwide; increases since the 1980’s have been more modest.
An often-cited example of irrigation success is that of the Imperial Valley of Southern California. The valley, more than 12,900 square kilometers (5,000 square miles) in size, was originally considered to be a desert wasteland. The low annual rainfall resulted in a typical desert, with cacti, lizards, and other arid-adapted plants and animals. In 1940, however, engineers completed the construction of the All-American Canal, which carries water 130 kilometers (80 miles) from the Colorado River to the valley....
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Methods (Encyclopedia of Environmental Issues, Revised Edition)
All types of irrigation are expensive, requiring advanced technologies and large investments of capital. In many cases, irrigation systems convey water from sources hundreds of miles distant. In the United States, such vast engineering feats are largely financed by taxpayers. Typically, water from a river is diverted into a main canal and from there into lateral canals that supply each farm. From the lateral canals, various systems are used to supply water to the crop plants in the field.
Flood irrigation supplies water to fields at the surface level. Using the sheet method, land is prepared so that water flows in a shallow sheet from the higher part of the field to the lower part. This method is especially suitable for hay and pasture crops. Row crops are better supplied by furrow irrigation, in which water is diverted into furrows that run between the rows. Both types of flood irrigation cause erosion and loss of nutrients. However, erosion can be reduced in the latter type through the contouring of the furrows.
Sprinkler irrigation systems, though costly to install and operate, are often used in areas where fields are steeply sloped. Sprinklers may be supplied by stationary underground pipes, or a center pivot system may be used, in which water is sprinkled by a raised horizontal pipe that moves slowly around a pivot point. Aside from its expense, another disadvantage of sprinkler irrigation is loss of water by evaporation....
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Negative Impacts (Encyclopedia of Environmental Issues, Revised Edition)
As fresh water evaporates from irrigated fields over time, a residue of salt is left behind. The process, called salinization, results in a gradual decline in productivity and can eventually render fields unsuitable for further agricultural use. Correcting saline soils is not a simple process. In principle, large amounts of water can be used to leach salt away from the soil, but in practice the amount of water required is seldom available, and if it is used, it may waterlog the soil. Also, the leached salt usually pollutes groundwater or streams. One way in which farmers address the problem of salinization is by using genetically selected crops adapted to salinized soils.
As the number of hectares of farmland requiring irrigation increases, so does the demand for water. When water is taken from surface streams and rivers, the normal flow is often severely reduced, changing the ecology downstream and reducing its biodiversity. Also, less water becomes available for other farmers downstream, a situation that often leads to disputes over water rights. In other cases water is pumped from deep wells or aquifers. Drilling wells and pumping water from such sources can be expensive and may lead to additional problems, such as the sinking of land over aquifers. Such land subsidence is a major problem in several parts of the southern and western United States. Subsidence in urban areas can cause huge amounts of damage as water and...
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Further Reading (Encyclopedia of Environmental Issues, Revised Edition)
Albiac, José, and Ariel Dinar, eds. The Management of Water Quality and Irrigation Technologies. Sterling, Va.: Earthscan, 2008.
Graves, William, ed. “Water: The Power, Promise, and Turmoil of North America’s Fresh Water” (special issue). National Geographic, November, 1993.
Meiners, Roger E., and Bruce Yandle, eds. Agricultural Policy and the Environment. Lanham, Md.: Rowman & Littlefield, 2003.
Molden, David, ed. Water for Food, Water for Life: A Comprehensive Assessment of Water Management in Agriculture. Sterling, Va.: Earthscan, 2007.
Wescoat, James L., Jr., and Gilbert F. White. Water for Life: Water Management and Environmental Policy. New York: Cambridge University Press, 2003.
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Background (Encyclopedia of Global Resources)
Irrigation systems were important to many ancient civilizations. They were the basis of life in the ancient civilizations of Egypt, India, China, and Mesopotamia (modern day Iraq). Some irrigation works in the Nile Valley that date back to around 3000 b.c.e. still play an important role in Egyptian agriculture. In the United States, the first irrigation systems were developed by American Indians, and traces of ancient water distribution systems, made up of canals, were still visible at the beginning of the twenty-first century.
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Scope and Land Requirements (Encyclopedia of Global Resources)
In 1977, the Food and Agriculture Organization (FAO) of the United Nations estimated that the total global land area under irrigation was 223 million hectares. By 2000, about 270 million hectares were irrigated worldwide. In the United States, more than 20 million hectares are irrigated for crop production. Some form of irrigation is practiced in every country in the world. Although irrigation results in increased food production, it is extremely water intensive. For example, to grow 1 metric ton of grain (adequate for 50 percent of an average person’s supply for five years and six months) requires as much as 1,700 cubic kilometers of water per person per year. In the United States, 40 percent of total freshwater withdrawals is for irrigation. The value of irrigation is that it greatly increases agricultural productivity. For example, in 1979, the FAO reported that although irrigated agriculture represented only about 13 percent of global arable land (agricultural land that, when properly prepared for agriculture, will produce enough crops to be economically efficient), the value of crop production from irrigated land was 34 percent of the global total production.
For irrigation to be economically viable, the land in consideration must be able to produce enough crops to justify the investment in irrigation works. The land must be arable and irrigable; that is, sufficient water for irrigation must exist. Soil...
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Types of Irrigation Systems (Encyclopedia of Global Resources)
Generally, irrigation systems can be classified as nonpressurized systems (also known as gravity or surface systems) and pressurized systems. Historically, nonpressurized systems, in which water was flooded onto the soil surface via open channels, were the first to be constructed. In fact, nonpressurized systems preceded pressurized ones by thousands of years. Nonpressurized systems include canals, open channels, and pipes that are not flowing full. Pressurized systems include all types of sprinkler systems and low-pressure nozzle systems.
There are five basic methods of implementing irrigation systems: flooding, furrow irrigation, subirrigation, trickle irrigation, and sprinkling. Several subcategories exist within these five basic categories. Flooding systems include wild flooding, controlled flooding, check flooding, and basin flooding applications. In all cases the irrigated area is flooded with water. The degree to which flooding is controlled or administered differentiates the types of flooding. For example, in wild flooding there is not much control or preparation of the irrigated land. In contrast, check flooding is accomplished by admitting water into relatively level plots surrounded by levees. In check flooding the check (area surrounded by levees) is filled with water at a fairly rapid rate and the water is allowed to infiltrate into the soil.
Furrow irrigation is used for row crops—hence the...
(The entire section is 388 words.)
Further Reading (Encyclopedia of Global Resources)
Albiac, Jose, and Ariel Dinar, eds. The Management of Water Quality and Irrigation Technologies. Sterling, Va.: Earthscan, 2009.
Cuenca, Richard H. Irrigation System Design: An Engineering Approach. Englewood Cliffs, N.J.: Prentice Hall, 1989.
Heng, L. K., P. Moutonnet, and M. Smith. Review of World Water Resources by Country. Rome: Food and Agriculture Organization of the United Nations, 2003.
Linsley, Ray K., et al. Water Resources Engineering. 4th ed. New York: McGraw-Hill, 1992.
Morgan, Robert M. Water and the Land: A History of American Irrigation. Fairfax, Va.: The Irrigation Association, 1993.
Postel, Sandra. Pillar of Sand: Can the Irrigation Miracle Last? New York: W. W. Norton, 1999.
Shortle, James S., and Ronald C. Griffin, eds. Irrigated Agriculture and the Environment. Northampton, Mass.: Edward Elgar, 2001.
Zimmerman, Josef D. Irrigation. New York: Wiley, 1966.
U.S. Geological Survey. Irrigation Water Use. http://ga.water.usgs.gov/edu/wuir.html
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Irrigation (American Indians Ready Reference)
Article abstract: Irrigation permitted some tribes of the Southwest, particularly in prehistoric times, to practice effective agriculture in arid lands
Irrigation, the bringing of water to agricultural fields, was practiced widely in pre-Columbian Mexico and Peru, but it was used relatively little by prehistoric North American Indians. Most of eastern North America had adequate rainfall for agriculture, and much of western North America was so dry that agriculture was impractical. As a result, irrigation in pre-Columbian North America was restricted to the Southwest. There, the earliest known irrigation was practiced by people of the Hohokam archaeological tradition, beginning around 100 c.e.
The earliest canals were modest in scope, unlined, and without sophisticated water control features. By 700, they had been expanded to a massive network, including one main canal at least 17 miles long; in addition, control features such as trash gates, head gates, and plunge pools had been added to the system. A few centuries later, the canals were lined to reduce loss from seepage. By 1400, however, Hohokam irrigation had diminished to small-scale ditches with far less engineering sophistication than the earlier systems, and this sort of irrigation was continued by the Pima. Other historic tribes using irrigation include the Pueblo peoples and the Colorado River tribes (Mojave and Yuma), who probably adopted their irrigation practices...
(The entire section is 218 words.)