Introduction to Irrigation
Irrigation, the artificial application of water to soil to produce plant growth. In the broadest sense, irrigation includes watering a lawn or garden. However, the term usually means the supplying of rather large amounts of water to grow crops in arid (dry) regions.
Under extremely arid conditions, irrigation takes the place of rainfall. In regions where rainfall is scant, irrigation may provide an alternative to dry farming. Where rainfall is abundant but uncertain, irrigation gives protection against occasional droughts. Under all circumstances, irrigation is a method of producing crops on a controlled schedule of cultivation and harvest.
Any crop may be grown with irrigation. Lowland rice—the leading source of food for about half of the world's population—is grown on land flooded with water. Sugar beets, alfalfa, and various fruits are typical irrigated crops in the western United States.
The world's rapid population growth has greatly increased the need for food. Irrigation is therefore important as a means of opening new lands to agriculture. Also, higher crop yields are possible on old lands when irrigation is carried on scientifically.
Irrigation is used throughout the world. It is most common in regions having less than 20 inches (500 mm) of rainfall a year. Irrigation also is practiced to some extent in regions where rainfall is 20 to 40 inches (500 to 1,000 mm) a year. Most of the irrigated land in the United States is in the West.
Irrigation Systems
An irrigation system consists of various structures and devices for supplying, conveying, and applying irrigation water. In the United States and Canada, and in many other countries, irrigation systems usually are owned by individual farmers or by ditch companies—private companies or farmers' cooperatives. The largest irrigation systems may be built by government agencies such as the Bureau of Reclamation of the U.S. Department of the Interior. In some countries, irrigation systems are entirely government-owned.
Water SupplyThe main source of irrigation water in some localities is mountain snowfall, supplemented to varying degrees by rainfall. In many places, ground water is an important source.
Surface waters that may be used for irrigation are stored naturally in lakes and ponds. Ground water collects beneath the surface in basins of coarse gravels, and in aquifers (water-bearing rocks). Surface waters are conveyed to farms by rivers and streams, and ground water reaches the surface through springs. These natural sources of supply tend to discharge heavily in the springtime and dry up in midsummer. Therefore, irrigation water is increasingly being stored in artificial surface reservoirs, from which it can be released as needed through canals. Artificial reservoirs are created by various means. The largest are made by dams that flood river valleys.
Water for irrigation can also be obtained from wells. In some places—notably in California, Arizona, and western Texas—water from streams filled by summer storms is diverted and allowed to soak into the ground to refill underground basins.
Conveying the WaterCanals that carry irrigation water usually are called ditches. Main ditches may be hundreds of miles long and carry millions of gallons of water a day. To prevent loss of water by seepage, many ditches are lined with concrete or bentonite, a type of clay.
A branch of a main ditch, leading to a group of farms, is called a lateral. Each farm takes its allotted portion of water from the lateral at a headgate or take out, where sliding wooden or metal gates are used to regulate the flow. Water is led to various parts of the farm through field ditches. Small dams, or flags, of canvas, metal, or wood, easily placed and removed by hand, are used to direct the flow of water onto the fields.
Various mechanical devices are used to measure the amount of water flowing in main ditches and laterals. Measurements are often given in terms of the acre-inch or the acre-foot, defined as the amount of water that will cover one acre to a depth of one inch or one foot, respectively. One acre-inch equals about 100 cubic meters.
Applying the WaterPrincipal methods of applying water to crops are flooding, furrow irrigation, sub-irrigation, sprinkling, and drip irrigation.
Flooding is done by covering the field with a sheet of water until the ground is soaked. This method is used mainly for pasture, grains, and hay. There are four types of flooding:
Ordinary flooding is practiced where water is abundant and inexpensive. The water is allowed to overflow from a ditch onto the fields.
Border-strip flooding requires the division of land into strips separated by low levees, or borders. Water flows slowly toward the lower end of the strip, soaking the ground as it goes.
Check flooding allows water to stand (be checked) for a time on parcels of level land separated by borders, often along contour lines.
Basin flooding is an adaptation of check flooding to orchards. Water is pooled around each tree or group of two to five trees.
Furrow Irrigation is a method of delivering water to crops that are planted or seeded in rows. The water reaches the plant roots from furrows (shallow depressions) between the rows. The surface of the rows remains dry, saving water costs and permitting cultivation soon after irrigation.
Sub-irrigation is the application of water to the root zone from below, leaving the field surface dry. In natural sub-irrigation, water is allowed to seep from deep-cut ditches into the porous subsoil. In artificial sub-irrigation—an expensive method—a system of pipes is laid below ground.
Sprinkling produces a flow of water similar to that of rainfall. Usually, the water is sprayed under pressure from sprinkler heads placed along a pipe. The pipe may be portable or fixed in place. In a center-pivot sprinkler system, a long straight pipe with sprinkler heads is mounted on wheeled supports and connected at one end to a fixed pivot, through which water is supplied to the pipe. The pipe, powered by hydraulic pressure or electric motors, slowly moves around the pivot and sprinkles a large circular area.
Drip (or Trickle) Irrigation is a method of delivering a very slow but steady flow of water to plants individually. The water flows either from small holes or from nozzlelike “emitters” placed along a narrow, flexible plastic tube lying on the ground. The holes or emitters are spaced so that only the ground immediately around each plant is watered.
Irrigation and the Soil
The soil's texture and structure may be damaged by excessive irrigation or the use of heavy tillage machines on wet ground. Soil erosion (washing away of soil) is prevented by careful application of water on sloping ground.
Irrigation water should be free of minerals that injure plants. If the water contains large amounts of soda compounds, for example, Doug Sokell/Visuals Unlimited the soil will become strongly alkaline and crops will not grow. Ground that is too alkaline or saline (containing excessive amounts of soluble salts) may have to be leached. This is a process of dissolving and flushing away harmful minerals by sending large amounts of pure water through the soil.
Irrigation may cause the water table to rise too near the surface. Waterlogged soil has many disadvantages for the farmer. Provisions for drainage, therefore, are essential to a well-planned irrigation system.
Water Law
In the United States, legal rights to the use of irrigation water are called water rights, or water decrees. They are governed by water law—a complicated interweaving of statutes, court decisions, contracts (usually called compacts), and international treaties. Water law may decide how much water a farmer is entitled to apply to a cornfield on a given date, or the law may specify the rights of one state to the waters of a river that has its headwaters in another state.
Rights to surface waters are based on the doctrine of appropriation or the doctrine of riparian (streambank) rights, or a combination of these doctrines. Rights to the use of ground water for irrigation are not as clear-cut as surface rights; various modifications of surface-right doctrines usually are invoked.
Appropriation RightsAll Western states recognize the doctrine of appropriation: beneficial use of the water creates the right. The right is lost when beneficial use ceases.
Riparian Rightsare acquired by ownership of land along a stream or other body of water, in states that recognize this ancient doctrine. Originally the doctrine meant that the landowner was entitled to any or all of the water flowing past or through his property. Hardships and quarrels resulting from the doctrine led the courts to declare that riparian rights must be limited by reasonable use. What constitutes “reasonable use” was left to the courts to decide in particular cases.
History
Primitive humans, observing that natural floods stimulated plant growth, learned to lead river water onto their fields. At an early date, Middle Eastern farmers invented labor-saving devices for lifting irrigation water from rivers. These devices still are used in many parts of the world. The oldest, invented before 2000 B.C., is the shaduf, a counterweighted pole that provides leverage for a filled bucket. Other water-lifting devices from Egypt are Archimedes' screw (invented in the third century B.C., or earlier) and the undershot water wheel (about 200 B.C.).
Along the Nile, irrigation was relatively simple. The river rose gently and predictably in spring and early summer, and the water was diverted by a simple system of dikes and canals to nearby fields. The soaked soil sustained the crops through the hot season. A more elaborate system was required in ancient Mesopotamia. There, floods of the Tigris and Euphrates rivers were violent and unpredictable. Mesopotamian farmers had to impound, store, and convey irrigation water—much as the irrigators of the western United States do today. Archeologists have found evidence of large irrigation structures in Mesopotamia.
Prehistoric Indian civilizations of Peru and Mexico depended on irrigation. In China the Tukiangyien system, constructed about 300 B.C., spreads the waters of the Min River over 500,000 acres (2,000 km2). The Romans built irrigation systems in Algeria and Spain. A notable project of medieval times was the Grand Canal, which irrigated 80,000 acres (300 km2) in Lombardy, northern Italy.
Water law was codified as early as 1750 B.C., when King Hammurabi of Babylon decreed punishments for those who neglected the upkeep of ditches or usurped the water rights of others. The Koran (seventh century A.D.), sacred book of the Muslims, considers observance of water rights a test of good neighborliness.
United StatesAs early as 300 B.C., Hohokam Indians of the American Southwest irrigated patches of corn, squash, and beans. In the 16th century, Spanish missionaries and settlers in California irrigated gardens and vineyards. About 1685, the growing of lowland rice—an irrigated crop—was introduced in South Carolina.
The Mormon settlers of Utah were pioneers of irrigation in the West; they built an irrigation system near the present site of Salt Lake City soon after their arrival in 1847. Since the early 1900's, the federal government has been responsible for the development of a number of large-scale irrigation projects in the western states.
Other CountriesSince World War II many governments have undertaken irrigation projects. A notable example was the construction of the Aswan Dam on the Nile River in Egypt. The Food and Agriculture Organization of the United Nations has helped to teach improved irrigation methods to farmers in many parts of the world.