Irrigation
Farmers cannot always depend solely on rainfall to sustain their crops. In fact, many of the world's agricultural regions receive little or no precipitation. In these areas, water must be diverted from streams or lakes or drawn from wells. The result has been the development of irrigation systems. Mesopotamia, considered the cradle of civilization, had one of the earliest water diversion systems. Agriculture began there about ten thousand years ago. Water channels helped divert water from the Tigris and Euphrates Rivers during the dry months and also helped control the annual July floods. The civilization that developed in Egypt owed its existence to the water diversion schemes that delivered water to the fields from the Nile River and its delta. The ancient Egyptians invented the shaduf, a long pole with a bucket suspended at one end and a counterweight mounted at the other. The pole pivoted vertically to lift water in the bucket to a higher level. Shadufs are still in use. About 2000 b.c., the Chinese were diverting water from the Hwang Ho and Yangtze Rivers and building levees to keep their rice paddies flooded. In approximately 130 a.d. Native Americans built canals, terraces, and check dams that are still visible today in the Sonoran Desert of Mexico and Arizona. The Roman Empire drew much of its stability from public works projects that included canals and aqueduct s.
The Romans had several methods of lifting water, including the treadmill-powered chain of buckets, the human-powered scoop wheel, the water-driven tympanum, and the Archimedean screw. Today, mechanical pumps raise water, while channels and sluice gates regulate its level and flow. Portable pumps are available to farmers who wish to irrigate on a short-term basis,as in a drought, and large permanent lift stations operate in larger irrigation schemes.
Irrigation networks have been constructed in many parts of the world, such as in the rice fields of eastern Arkansas, the olive groves of southern Spain, and the wheat fields of Victoria, Australia. One of the most ambitious irrigation efforts has taken place in the Sacramento, San Joaquin, and Imperial valleys of California. An important figure in the agricultural development of California was Harriet W. R. Strong. Left to fend for herself after her husband's suicide in 1883, she acquired a number of patents for irrigation and flood control measures, including, in 1887, the construction of a series of dams so that water backed up from one helped support the dam immediately upstream from it. Irrigation methods also include the use of sprinkler systems. Field-length pipes supported by wheeled legs water the corn fields of the Midwestern United States. Similarly, the potato fields of Idaho and Oregon are watered by mobile pipes that revolve in a circle. Irrigation has its environmental drawbacks, causing leaching in some areas. Water tables have been depleted from over-pumping, as in the cotton fields of west Texas. Water diversion also has fostered regional disputes over water rights, as is the case among the states of the upper and lower Missouri River Valley. In spite of its financial and environmental costs, irrigation has been and will be a vital part of man's existence for the foreseeable future.
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