Irrigation
Irrigation is the process of supplying water to land through human-created systems to enhance agricultural productivity, particularly in arid and semiarid regions. This practice is vital for meeting the food and fiber demands of a growing global population. As of 2020, an estimated 5 billion hectares of land worldwide are under irrigation, producing 40 percent of the world's food from only 20 percent of the Earth's cropland. Various irrigation methods exist, including flood, furrow, sprinkler, and drip irrigation, each with its own advantages and challenges. While irrigation can transform previously barren landscapes into fertile farmland, it also poses significant environmental risks, such as salinization, erosion, and depletion of water resources. As the demand for irrigated land increases, so do concerns about water rights disputes and the sustainability of water sources, particularly in areas relying on aquifers which may suffer from over-extraction. Balancing the benefits of increased agricultural output with the environmental costs remains a critical challenge for irrigated agriculture.
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Subject Terms
Irrigation
DEFINITION: Watering of land through human-created means
Like many other human modifications to natural ecosystems, the use of water for irrigation achieves some remarkable but temporary advantages that are complicated by long-term environmental problems.
The demands of feeding and clothing the rapidly expanding world 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 techniques to supply additional water to arid and semiarid regions where few, if any, crops could otherwise be grown.
The United Nations Food and Agriculture Organization (FAO) calculated that by 2009, approximately 311 million hectares (768.5 million acres) of land worldwide were irrigated. FAO reported in 2020 that this number had increased to 5 billion hectares (12 billion acres). Only about 20 percent of Earth's land is used as irrigated cropland, but 40 percent of the world's food is grown on this land. .
An often-cited example of irrigation success is that of the Imperial Valley in 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. The project converted the Imperial Valley into a fertile, highly productive area where farmers grow fruits and vegetables all year.
Methods
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. In drip irrigation, in contrast, water is delivered by perforated pipes at or near the soil surface. Because water is delivered directly to the plants, much less water is lost to evaporation than is the case with sprinkler irrigation.
Much of the water utilized in irrigation never reaches the plants. It is estimated that irrigation practices are between 40 and 95 percent efficient in delivering the water to the root systems of crop plants. The remaining water is lost to evaporation, supplies weeds, seeps into the ground, or runs off into nearby waterways.
Negative Impacts
As evaporates from irrigated fields over time, a 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 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 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 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 pipes, highways, and buildings are affected. In coastal areas, depletion of aquifers can cause the intrusion of salt water into wells, rendering them unusable. In the United States, the federal government spends millions of dollars each year to repair damage to irrigation facilities.
Like many other human modifications to natural ecosystems, the use of water for irrigation achieves some remarkable but temporary advantages that are complicated by long-term environmental problems. Assessments of the total financial costs and environmental impacts of irrigation are continuously weighed against gains in production.
Bibliography
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.
Mehta, Piyush. "Half of Twenty-First Century Global Irrigation Expansion Has Been in Water-Stressed Regions." Nature Water, vol. 2, 8 Mar. 2024, pp. 254-261, www.nyu.edu/about/news-publications/news/2023/june/new-study-reveals-irrigation-s-mixed-effects-around-the-world.html. Accessed 18 July 2024.
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.
"New Study Reveals Irrigation's Mixed Effects Around the World." New York University, 20 June 2023, www.nyu.edu/about/news-publications/news/2023/june/new-study-reveals-irrigation-s-mixed-effects-around-the-world.html. Accessed 28 July 2024.
Wescoat, James L., Jr., and Gilbert F. White. Water for Life: Water Management and Environmental Policy. New York: Cambridge University Press, 2003.