Irrigation

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 digging 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 and South America, the Middle East, and in 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 southwest area of the United States, Australia, and the Middle East.

There are more than 600 million acres (243 million ha) worldwide, about 17% of agriculturally productive land, that are routinely irrigated. More than 60% of the irrigated land is contained within a few countries—China, India, Pakistan, the United States, and parts of the former USSR. Since becoming independent in 1947, India has developed over 700 irrigation projects, more than doubling the amount of land they irrigate, which exceeded 100 million acres (41 million ha) by the late 1980s.

In China, where irrigation has been used since the third century B.C., irrigated land doubled and tripled in some areas after the completion of dam projects undertaken since World War II. 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 B.C. and is still in use. Tujiang Dam is the source of water for 500,000 acres (202,000 ha) of land.

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 upon in those regions. In other areas, irrigation may be used only as a backup in case there is not sufficient rainfall during a crop's growing season. This is termed supplemental irrigation.