Page:The New International Encyclopædia 1st ed. v. 10.djvu/896

IRRIGATION. beneath the canal in a position to receive the water it carries.

It frequently happens that a diverting dam or one or more large storage reservoirs must be provided. In a number of instances submerged dams have been built across the valley of a stream whose waters disappear in summer. The practice is to build reservoirs and headworks generally at the highest suitable point consistent with good location for both these structures and the upper portions of the conduits leading from them.

Reservoirs are often increased by adding to the heights of the dam as the demand for water increases. The financial resources of most American irrigation works are likely to be limited at the start, would-be irrigators following rather than preceding the development of water-supply. On this account early works are often temporary in character. Again, both labor and material are apt to be so dear as to render cement masonry construction quite out of the question. Consequently many timber or timber and loose stone dams, have been built, and more ambitious structures of the rock fill type (see ) have been erected. In some cases even flimsy brush dams, loaded with stone and earth, have been thrown across or partly across a stream, with the full knowledge that they would not last more than a year or two at best.

Artesian wells are a most important source of supply for irrigation in many western sections of the United States. In Kansas, Nebraska, and other States very fair supplies may be obtained from comparatively shallow wells by pumping. In California water is sometimes developed by tunneling into hillsides. Where pumps have been used they have generally been simple in construction and of small capacity. Large numbers of windmills are employed to drive small pumps on the Great Plains, mostly for domestic water-supply, but not infrequently for irrigation. Many of these are home-made, resembling the simplest form of paddle water-wheel, or are more elaborate, according to the mechanical ability or ambition of their makers, serviceable and durable (q.v.) are now so cheap and so much more efficient than the home-made affairs that there is little excuse for not having one wherever it can be put to good use. Small storage reservoirs are a necessary adjunct of windmills, since there may be no wind when water is most needed, and since they also make it possible to save the night pumping. Such reservoirs may he constructed by intelligent farmers at comparatively little cost by throwing up earth embankments. In some cases a concrete or asphalt lining may be required to prevent leakage, but oftentimes the earth may be so packed, or puddled, by wetting and ramming it, as to make more expensive lining unnecessary. Where available, motor power is a cheap means of lifting water. Its applications to that end range all the way from a series of buckets mounted on a wheel placed in and driven by the stream to the most modern and efficient combination of pumps driven by turbines. Hydraulic rams are also employed.

In a comparatively few instances steam pumping engines are used, particularly where large quantities of water are to be lifted to a considerable height. This is true in California,

Hawaii, and elsewhere, where large areas of land are irrigated by immense pumping plants. Gasoline engines are used to drive pumps. They require but little more attendance than a windmill, and have the great advantage of not being dependent upon the uncertainties of the wind. In America the quantity of irrigating water raised by pumps of all kinds is small, but in Europe, Africa, and Asia much pumping is done. In Egjpt and in some Asiatic countries, however, most of the pumping is very primitive, the power being applied by men or animals. Pumping is increasing of late with the development of water from wells and with the demand for water to irrigate land that cannot be reached from existing low-level canals. In many instances water can be secured in this way at less expense than by gravity, since it permits the utilization of near-by sources, thus avoiding long and costly canals. The water, too, is more directly under the control of the irrigator.

Assuming that a good supply of water for irrigation is available either by diversion from streams, by storage of storm waters, by pumping from wells, or from any other source, the question to be here discussed is the best means of utilizing it for the production of crops.

. Main canals and conduits are often the most expensive part of irrigation works, owing to their length and the difficulties encountered in their construction. The cheapest and simplest conduit is a ditch, heading in the source of supply, and departing just sufficiently from the natural contour of the country to insure a flow of water. In the early days of irrigation such ditches were little more than single furrows, or channels no larger than might be formed by a plow, leading a short distance from the banks of a stream. To-day there are thousands of miles of irrigation ditches, or canals, large enough for small boats, while in India it is quite common to build combined irrigation and navigation canals, thus affording a ready outlet for the products of the irrigated area and inlets for supplies. In rolling or hilly country canals may have to follow circuitous routes to maintain their level, thus adding greatly to their length. It may be cheaper, or, when a stream or valley is encountered, even necessary, to continue the line of the canal, changing the construction to an elevated flume, or else substituting a pipe or inverted siphon, laid on or in the