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.