IRRIGATION

the artificial application of water to land. Modern irrigation and the associated practices of drainage, fertilization, mechanization, and mass production for special markets under professional management have produced a revolution in agricultural efficiency. Normally vegetation grows on soil watered by rain. Where rain is so seasonal that it does not meet the requirements of a particular crop or is deficient or practically nonexistent, the drying of the soil to an ever-increasing degree retards, and eventually prevents, vegetable growth. Irrigation can compensate for the vicissitudes of nature by supplying water regularly and in sufficient volume. The use of irrigation for agricultural purposes is an ancient practice. It can be traced to the early Egyptians, who were irrigating fields with water from the Nile River by 5000 BC. Evidence shows that other ancient civilizations, such as those of the Babylonians and the Chinese, also developed largely as a result of irrigation-based agriculture. Over the centuries various kinds of techniques and devices were devised to facilitate field irrigation. A major system of irrigation canals and ditches, for example, was constructed during the reign of the Babylonian king Hammurabi (179250 BC). The one-man scoop for bailing water from rivers for use on fields was eventually replaced by mechanical lifting aids, such as the counter-weighted and animal-powered devices introduced in the Middle East and the portable Archimedes screw invented in Syracuse. Another notable development was the use of large storage reservoirs. With the advent of steam power, the internal combustion engine, and electricity, irrigation became in many parts of the world a fully mechanized operation. An important mechanical development after World War II was the use of portable irrigation systems of lightweight aluminum pipe. Much irrigation water is pumped from ponds, lakes, streams, and wells. The more abundant and cheaper water supplies are found in the larger streams and usually are obtained therefrom by diversion dams without pumping. Often these structures raise the level of the water in the river and sometimes provide storage in reservoirs. From them, concrete-lined canals and related structures convey and distribute the water over the land through gravity flow. There are two main forms of irrigation: the basin system and the perennial system. The former, in which the area to be served is surrounded by embankments and filled with water by means of canals, obviously originated in the fact that a river can be easily made to give a supply once a year during its flood period, whereas enormous expense would be required to make it do so at other times. The modern and considerably more expensive system, which makes use of a perennial water supply, such as a reservoir, may also be fed by canals, but in this case the canal system is enormously developed compared with that of the basin system. Other, less common methods include warping, an old English practice in which silt from flooding is accumulated as new topsoil; subirrigation, in which water is applied laterally and upward from drains or pipes located below the surface of the ground; and sewage irrigation, in which nitrogen-rich runoff from sewage settling tanks is channeled to cultivable areas.