Irrigation allows farmers to control watering of plants in dry areas with less than average rainfall. (Willem van Aken/Commonwealth Scientific and Industrial Research Organization.) (Willem van Aken/Commonwealth Scientific and Industrial Research Organization.)

Irrigation is the process of bringing a water supply to a dry region, especially with regard to the growing of crops. The practice of diverting water from natural resources to crops has been practiced for at least 7,000 years. The earliest methods, as practiced in places like the areas surrounding the Nile River basin, included Page 2459  |  Top of Articledigging channels to allow water from the river during flood periods to reach cultivated fields along the river's banks. Ancient farmers also built dikes to help retain the water on the flooded land. Other early irrigation techniques included the construction of diversion dams and the use of machinery to lift the water and irrigate land that was higher than the flood plains. Evidence of early irrigation systems has been found in North America, South America, the Middle East, China, and India.

Surface irrigation system techniques include surface flooding, furrow flooding, and dead-level surface flooding. In surface flooding, the whole land area to be irrigated is flooded with water. This technique is good, for instance, for growing rice. Furrow flooding involves planting trees or crops between shallow trench-like channels and flooding the area. In arid regions, dead-level surface irrigation, where fields are leveled to a zero slope, is practiced.

Closed-conduit irrigation includes sprinkler systems, bubbler irrigation, and drip or trickle irrigation. Gardeners, as well as farmers, commonly use these techniques. Sprinkler systems pump water through pipes or hoses to the sprinkler, which can be fixed or mobile. Bubbler and drip systems periodically supply water to the roots of one or more plants. These systems are constructed of tubing or pipes. Drip systems deliver water slowly and are the most conservative users of water resources. They are particularly favored in arid regions, such as the American Southwest, Australia, and the Middle East.

Around the beginning of the nineteenth century, there were about 20 million acres (8 million ha) of the world's land under the use of irrigation. By the start of the twentieth century, the figure had risen to 99 million acres (41 million ha). In 2020, there were about 680 million acres (275 million ha) of irrigated agriculture lands worldwide, about 20 percent of agriculturally productive land. However, irrigated land produces 40 percent of global food. More than 60 percent of the irrigated land is contained within a few countries—China, India, Pakistan, the United States, and the countries of the European Union.

In China, where irrigation has been used since the third century BCE, irrigated land doubled and tripled in some areas after the completion of dam projects undertaken since World War II (1939–1945). The primary irrigation crop in China is rice, but they also irrigate their wheat and cotton fields. One dam, the Tujiang on the Min River, was built around 300 BCE and is still in use. Tujiang Dam is the source of water for 50,000 acres (202,000 ha) of land. The reservoir behind the Three Gorges Dam, the world's largest hydroelectric dam, on the Yangtze River, supplies irrigation water for much of the country's agricultural heartland, including 50 percent of grain production in China.

While the purpose of irrigation is to produce a better crop yield, the need for irrigation varies depending upon seasonal and climatic conditions. Some regions need crop irrigation all year, every year; some only part of the year and only in some years; and others need to irrigate only during seasons of water shortage from rainfall. In Iraq and India, for instance, irrigation is absolutely necessary in order to grow crops, since rain cannot be depended on in those regions. In other areas, irrigation may be used only as a backup in case there is insufficient rainfall during a crop's growing season. This is termed supplemental irrigation.

Salinization is a major problem associated with irrigation because deposits of salts build up in the soil and can reach levels that are harmful to crops. In addition, the salts can make groundwater, which may be in use for drinking, saltier and unsuitable for drinking. It is mostly in arid and semiarid regions where the problem of high salt content deposited from irrigation threatens crops.

Drip irrigation is a technique that can be used in areas where the groundwater level is high and in danger of suffering from a high salt content. Where salinization is a problem to plants, enough water can be added to the irrigation process to leach salts away from plant roots. When the danger of salinization is to the water table, it is necessary to add drainage to the irrigation system away from the water table.

Crops have different salt tolerances and their selection in relation to the salinity of the soil is an advisable practice. Among the common crops that have a high salt tolerance are red beets, spinach, kale, asparagus, sugar beets, barley, cotton, date palms, and some grasses used for animal feed, such as wild rye and wheat grass. Crops that have a low tolerance for salinity include radishes, celery, green beans, and fruits such as pears, apples, oranges, grapefruit, plums, apricots, peaches, strawberries, lemons, and avocados. Other crops with a low tolerance for salinity include a number of clovers that are used for animal grazing.

Areas in the world where farming is threatened by high salinity include the Indus Basin in Pakistan where people also face the problem of a rising water table. The Imperial Valley in California, formerly productive agricultural lands in South America, China, India, Iraq, and many other regions throughout the world are all facing the threat of losing fertile land because of salinization. After the building of the Aswan Dam in Egypt, the Nile River and the surrounding fields that had been irrigated successfully for over 5,000 years became threatened by high salinity in the water.

The main technique used to reclaim land that has developed a high salt content from irrigation is a leaching process. This is based on a careful analysis of the soil and the amount of water that must be applied to reach a level of acceptable salt content. One problem of leaching is that other nutrients needed by the crops, besides the undesirable salts, may also be leached from the soil. Consequently, nutrients often need to be replaced after an area is reclaimed from high salinization.

The planning of irrigation systems is highly specialized and requires the help of agricultural engineers who understand not only the design and construction of irrigation systems but also farm management and mechanization, soil science, crop husbandry, and the economics of farming. The engineer's education in these related fields is important, so that he or she is able to design an irrigation system that is appropriate to the type of farming in the area that is to be irrigated.

Before an irrigation system can be built, a number of important studies must be made. Among them would be a survey of land and water resources, a study of the current uses of the area, a proposal for an irrigation system, cost estimates of the project, and a projection of its economic benefits. A large regional or national project might also include the economic and material resources for the project that are available by the particular entity, the cost of construction and administration of the project, the financing and marketing of the project to individual farmers, and the training of personnel to carry out the project.

Among the specific surveys that must be made before an irrigation system is constructed are soil, water, and topographic surveys. Sometimes critical decisions must be made about the destruction of towns and monuments from antiquity. For instance, large dam projects, such as the Aswan and Three Gorges dams, submerged thousands of significant archaeological sites. (In Egypt, monuments such as Abu Simbel were relocated when the Aswan High Dam was built.) Another important consideration for building a new irrigation project is whether it will change current farm practices, and if so, how to educate farmers to new methods. Foremost at issue is the consideration of how an irrigation system will impact the farmers and farming in the area.

In surface irrigation systems, the area to be covered with water is sloped away from the supply channel so that the water will flow over the entire area with the water moving both across the surface to be irrigated and filtering down to the root bases of the plants in the field. Among the variations of surface irrigation are the techniques of furrow irrigation, border strips, basins, and wild flooding.

Furrow irrigation has the advantage of allowing the crops to be tended shortly after watering periods. The system is useful for crops that are grown in rows that can be separated by furrows (shallow ditches) along the rows. The furrows are usually dug along the line of the slope, but sometimes they run perpendicular to it. The problem with cross cutting the furrows on a slope is that it may collapse during irrigation periods from the force of the water.

The preferred method of supplying water to the furrows is to siphon water from a main source and carry it through plastic or aluminum pipes set in a main ditch at the head of the field. Another ditch at the end of the furrows collects excess water and runs it along to lower lying fields. The best incline for furrow irrigation is a 0 to 5 percent slope. Crops are usually planted on the rise between the furrows, but sometimes trees are planted at the bottom of the furrow. Since there is less water surface Page 2461  |  Top of Articlein this method, evaporation of water is less than in surface flooding.

In pastures where there are crops that grow closely together, border strips may be used. In this system, a main ditch is constructed along the highest end of the slope and banks, called checks, which can be built as much as 70 ft (21 m) apart. Water is then siphoned from the main ditch onto the strips where the crops are grown. Sometimes the banks are replaced with border supply ditches, which allow more control over the release of water. This system is often used in research studies. In hilly areas, contour ditches are built that follow the contour of a hill. They are carefully graded to control the flow of water.

For landscapes, gardens, and the watering of individual trees, the use of basins may be a suitable method of irrigation. The area to be irrigated is surrounded by banks (checks) and then watered from a main source along a high point in the basin. A drain is also placed along the major depression of a basin to allow water to run off. This system is easy to build since it requires very little movement of earth. It is usually built around the natural contours of the area. Where the land is extremely steep, an adaptation of basin irrigation called terracing can be used. Here basins are created in a step fashion along the slope of the hill. At the end of each basin step, a check is built. Basin watering is not generally recommended for flat ground.

While wild flooding is still practiced, it is not recommended by agricultural engineers because the water distribution is uneven and can lead to high saline contents in the soil and to waterlogging. The crop yields are consequently unpredictable.

In areas where the topsoil is of high quality and porous, and there is an underlay of clay soil that absorbs water slowly, conditions exist for natural subirrigation provided that the water table is high. Ditches dug along the fields can be used to monitor the water level and to also replenish the water supply when it is low. Where there is little or no rainfall and the salts in the water build up on the surface of the soil, leaching is carried out. To overcome excess rain in areas where subirrigation systems are in use, water can be removed by pumping or using natural gravity features available in the terrain, that is, slopes and depressions in the ground.

When subirrigation is desired but the conditions are not available naturally, pipes with evenly distributed punctures can be buried underground. A difficulty involved with these systems is that they can be damaged when the soil is being cultivated. These systems also work by the use of natural sloping features in the terrain or by pumping water through the pipes.

Drip irrigation, which is not actually subirrigation, but uses some of the same principles as in subirrigation, delivers water slowly to the root areas of plants. Here, too, pipes are used as the channels for transporting the water and emitters are placed to water plants directly. While it is economical to use because there is little waste of water and evaporation is at a minimum, initial costs of installing drip irrigation systems are higher than other methods. There is also a tendency for emitters to become clogged by the salts in the water. Salts, however, do not build up around the roots of plants in drip irrigation systems.

These systems use a pumping unit, conveyor pipes, and some form of sprinkler mechanism. Of all the irrigation systems, they most resemble natural rainfall. Some systems are fixed and use pipes laid on the ground with risers that have a sprinkling nozzle at the top that rotates 360 degrees. The size of the water droplets, the speed of rotation, and the evaporation rate are considerations in selecting sprinkler systems, since these all have an effect on the soil. An added use of sprinkler systems is that they can in some situations be used for frost protection.

Besides fixed systems, mobile sprinkling systems are in use in the United States and the United Kingdom. Portable systems use a pump at the water source to pump the water into a main line that is laid throughout the field. The sprinkler units are moved from field to field for irrigation of crops. Other mobile sprinkling systems use a device called a rain gun, which has a nozzle with a large diameter.