Irrigation

I

Introduction

Irrigation, artificial watering of land to sustain plant growth. Irrigation is practised in all parts of the world where rainfall does not provide enough ground moisture. In dry areas irrigation must be maintained from the time a crop is planted. In areas of irregular rainfall, irrigation is used during dry spells to ensure harvests and to increase crop yields. The procedure has greatly expanded the amount of arable land and the production of food throughout the world. In 1800 about 8.1 million hectares (20 million acres) were under irrigation, a figure that rose to 41 million hectares (99 million acres) in 1900, to 105 million hectares (260 million acres) in 1950, and to more than 222 million hectares (550 million acres) today. Irrigated land represents about 15 per cent of all land under cultivation but often produces over twice the yield of non-irrigated fields. Irrigation can, however, waterlog soil, or increase a soil's salinity (salt level) to the point where crops are damaged or destroyed. This problem is now jeopardizing about one-third of the world's irrigated land.

II

History

Earliest records date the first use of irrigation to Egyptians along the River Nile about 5000 bc. By 2100 bc elaborate systems were in use, one of them a 19-km (12-mi) channel that diverted Nile floodwaters to Lake Moeris. The Sumerians relied heavily on irrigation to water fields in southern Mesopotamia (now southern Iraq) as early as 2400 bc, and the Chinese had irrigation techniques by 2200 bc. The Peruvians also built sophisticated systems before the time of Christ, and at the same time early Native Americans had more than 101,000 hectares (250,000 acres) of irrigated land in the Salt River valley of Arizona.

Among the early devices for lifting water from streams to higher-lying fields was the Egyptian shadoof, which is a bucket set on one end of a counterweighted pole. The Archimedes' screw, used for the same purpose, is a cylinder containing a wide-threaded screw turned by hand. The cylinder was set on an incline with the lower end in the stream, and as the screw was turned it lifted water to a higher level. The Persian wheel, still in use in India today, is a partly submerged vertical wheel with buckets attached to the rim. As the wheel is turned by draught animals rotating a geared horizontal wheel, the buckets are filled and emptied into a trough above that carries the water to crop-sown fields.

A method far less burdensome than lifting water was that of building permanent dams further upstream, whereby water could be raised to the desired level. The water was then allowed to flow by gravity through canals to lower-lying areas, where it was let out over gently sloping fields. This method had been practised on a large scale by early civilizations, using simple earthwork structures. It is essentially the same principle as that of modern irrigation, using masonry dams or such enormous concrete structures as the Grand Coulee Dam in Washington.

III

Methods of Irrigation

The four main methods used today to irrigate fields are flood, furrow, sprinkler, and drip, or trickle, irrigation. Flood irrigation is used for close-grown crops, such as rice, where fields are level and water is abundant. A sheet of water is allowed to advance from ditches and remain on a field for a given period, depending on the crop, the porosity of the soil, and its drainage. Basin flooding is used in orchards, with basins built around trees and filled with water.

Furrow irrigation is employed with crops grown in rows such as cotton and vegetables. Parallel furrows, called corrugations, are used to spread water over fields that are too irregular to flood.

Sprinkler irrigation uses less water and provides better control. Each sprinkler, spaced along a pipe, sprays droplets of water in a continuous circle until the moisture reaches the root level of the crop. Centre-pivot irrigation uses long lines of sprinklers that move around a circular field like the large hand of a clock. This method is used especially for feed crops such as alfalfa, which, when irrigated, furnish several mowings a year.

Drip, or trickle, irrigation delivers small but frequent amounts of moisture to the root area of each plant by means of narrow, plastic tubes. This method, which is used with great success in the United States, Israel, and Australia, ensures a minimum loss of water through evaporation or filtration into the ground.

IV

Countries Using Irrigation

China, with its abundant rice fields, far exceeds other nations with 77 million hectares (184 million acres) of land under irrigation. India is next with 39 million hectares (94 million acres), followed by the United States with 21.4 million hectares (52 million acres), Pakistan with 12.4 million hectares (30 million acres), and Russia with 11.5 million hectares (28 million acres).

India has made impressive gains in the past 30 years, increasing its irrigated land by more than 50 per cent. Besides undertaking close to 700 large irrigation projects on its rivers, India has tapped underground water, employing hundreds of thousands of motor-driven tube wells.