I.

Introduction

Irrigation, artificial watering of land to sustain plant growth. Irrigation is practiced in all parts of the world where rainfall does not provide enough ground moisture. In dry areas, such as the southwestern United States, 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. Irrigation has greatly expanded the amount of arable land and the production of food throughout the world. In 1800 about 8.1 million hectares (about 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 273 million hectares (675 million acres) today. Irrigated land represents about 18 percent of all land under cultivation but often produces over twice the yield of nonirrigated fields. Irrigation, however, can 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 by Egyptians along the Nile River 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. The Chinese had irrigation by 2200 bc. Peruvians also built sophisticated systems before the time of Christ, and early Native Americans at the same time 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 draft animals rotating a geared horizontal wheel, the buckets are filled and emptied into a trough above that carries the water to crop fields.

A method far less burdensome than lifting water was that of building permanent dams farther upstream, whereby water could be raised to a 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 practiced 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 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 and 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 row crops 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. Center-pivot irrigation uses long lines of sprinklers that move around a circular field like the large hand of a clock. It 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 percolation into the ground.

IV.

Countries Using Irrigation

India, with 56 million hectares (138 million acres) of land under irrigation, and China, with 55 million hectares (135 million acres), far exceed the other nations. Other countries with vast areas of irrigated land include the United States with 22 million hectares (55 million acres), Pakistan with 18 million hectares (45 million acres), and Iran with 8 million hectares (19 million acres).

India has made impressive gains in the last 30 years, increasing its irrigated land by more than 50 percent. 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. In the United States about 13 percent of farmland is irrigated. Nearly 90 percent of irrigated land is concentrated in the 17 Western states.

V.

Problems Arising from Irrigation

The chief problem caused by continuous irrigation is that of salt accumulating in the upper layers of the soil and stunting or preventing plant growth. Nearly all irrigation water, whatever its source, contains some salt, which percolates down to the water table and makes it increasingly brackish. Where drainage is bad and the water table approaches root level, the concentrated salt makes plant growth impossible. Good drainage systems, therefore, which keep the water table well below the root level and allow water to flush salts through the topsoil, are now understood to be a crucial aspect of a successful irrigation system. Increased salinity due to poorly-drained soil began to ruin rich lands of the southern Tigris-Euphrates Valley in Mesopotamia as early as 2100 bc. By 1700 bc yields from these lands dwindled to one-fourth of their once abundant harvests, and the great Sumerian cities that depended on them went to ruin. Pakistan's huge irrigation project in the Indus Valley has the same problem, with one-fifth of the land severely affected by 1960. Measures have been taken to lower the water tables by means of tube wells and allowing waters to flush the salts through the topsoil. Some progress has been made.

Since completion of the Aswān High Dam in the 1960s, the Egyptian government has had to spend increasing amounts of money to prevent salt from building up in Nile Valley fields. For previous millennia the Nile waters removed salt from the land with annual floods, a process that now must be done by constructing artificial drainage systems. No less severe is the problem of some farmers in the rich Imperial and Coachella valleys in California, who have suffered crop failures largely due to the increased salt content of the water of the lower Colorado River.

Although the world's irrigated area grew by 3 percent a year in the third quarter of the 20th century, this rate fell in the last quarter, largely because most of the economically feasible opportunities for large-scale irrigation development had already been exploited. Also, as greater demands are now being made on limited water resources, efficient use of available surface and groundwater supplies is becoming crucial. Irrigation, therefore, which used to be a matter of a farmer's watering the land, is becoming highly technical, calling for the gathering of vast amounts of information about overall water resources, the quality of the soil, and the condition of the water table beneath it—a task that often lies beyond the means of private farming. A major thrust of research today is to develop techniques to conserve land that is already under irrigation. See Agriculture; Soil Management; Water: Water Desalinization.