IRRIGATION, the watering of land by artificial means to increase crop production. This method of agriculture is widely practised in the western or arid portions of the United States, as well as in the drier parts of the Old World. It necessitates the application of many of the principles of hydraulic engineering (q.v.) and in addition requires for success a knowledge of agriculture and related economic matters. The western third of the United States, as well' as the greater part of Mexico on the south and portions of Canada on the north, has an average annual rainfall of less than 20 inches, with the result that relatively few crops can be raised by dependence upon the natural supply of moisture. There is on the whole a far larger extent of dry land than can ever be provided with sufficient water for maturing crops. Thus land values, in a large way, depend upon the ability to obtain water ; many other industries besides agriculture can be developed only in localities where an arti ficial water supply can be had. Large invest ments have been made in irrigation works in the arid West with the result that there has been a steady increase in the area of land made useful. Beginning with a few hundred thou sand acres in 1880, by 1890 the irrigated area had increased to approximately 4,000,000 acres, in 1900 to 8,000,000 acres, and in 1910 to 15,000,000, representing a total investment of approximately over $300,000,000. The hydraulic works for conserving and distributing the scanty water supply have been built to a point where all of the easily available sources of water have been utilized. Future progress must neces sarily be relatively slow and expensive because of dependence upon works of increasing magni tude and cost per acre served. This cost, be ginning originally with $15 or $20 per acre from small canals built by farmers, has increased to an average of about $50 per acre for supplies obtained from the larger and more difficult undertakings such as the Roosevelt Reservoir in Arizona and the Arrowrock Dam in Idaho.
The most notable advances in irrigation de velopment were made possible by the passage of the Reclamation or Newlands Act, cham pioned by Senator Newlands of Nevada and signed by President Roosevelt on 17 June 1902. (See RECLAMATION Laws). The necessity for this act arose from the condition that a great part of the land to be reclaimed belonged to the national government ; also from the fact that investments in the large privately owned irrigation works requiring water storage had not proved profitable. Under the terms of this
act the proceeds from the disposal of public lands are set aside in a fund to be used in sur vey, examination, construction and maintenance of irrigation works for the reclamation of arid and semi-arid lands. Under the terms of this act over $100,000,000 have been expended in construction of reservoirs, canals, hydro-electric plants and related works in the Western States.
See UNITED STATES RECLAMATION SERVICE.
Water Nearly 95 per cent of the lands irrigated in the United States obtain their water supply by gravity from surface streams. A relatively small, but valuable area, is watered from wells by means of pumps driven by steam, gasoline or hydro-electric power. Most of the streams of the arid region have their source in the high snowcapped or forested mountains from which they flow with rapid descent, passing usually through a series of upland valleys or parks and then cut their way through rocky defiles entering upon the lower valleys. In these the streams spread out and usually lose the greater part of their water in broad, sandy channels. The most effective development of the stream therefore is that in which the water is diverted near the upper edge of these lower valleys and carried out in channels so built as to conserve the supply which would otherwise be lost in the sandy channels.
Water is ordinarily diverted from the stream, not by lifting or pumping from the stream as sometimes inferred, but by taking advantage of the slopes of the country. For example, the streams on issuing from the mountains have a rapid fall of from 10 to 50 feet per mile or more. Water will flow with moderate rapidity in a well-built canal having a fall of one foot per mile or even less. Assuming then that the stream enters the valley on a descending grade of 10 feet per mile and the canal is started out alongside the Stream with a fall of one foot per mile, at the end of 10 miles the canal will be 90 feet above the river and must necessarily have swung back away from the river to be upon supporting ground. Thus it results that the canal departs rapidly from the river and, following the contour of the slopes of the foot hills, is in position to discharge water toward the river over or through the lands which lie below the canal.