WATER-SUPPLY FOR IRRIGATION. for irrigation is derived from (1) natural streams, springs and lakes; (2) wells: and (3) storage of storm waters. Occasionally also the sewage water of towns is used for irrigating purpose-. The simplest and most common method of secur ing water for irrigation is to divert it from streams by means of a dam and a ditch or eanal running along the sides of the valley of the stream, at a less grade than that of the stream When the fall of the valley is great the canal can be readily led off to such a distance that extensive areas may thus be supplied with water. When the fall of the stream is slight, however, the canal may wind along close to the stream for miles before any extensive areas lie 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 sub merged dams have bees built across the valley of a stream whose waters disappear in summer. The practice is to build reservoir, and head work, generally at the highest suitable point consistent with good location for both these structures and the tipper portions Of the con units leading from them.
Reservoirs are often increased by adding to the heights of the dam as the demand for water CI' increases. The financial resources of most American irrigation works are likely to be limited at the start, would-lie irrigators follow ing rather than preceding the development of water-supply. On this account early works are often temporary in diameter. Again, both labor and material are apt to be so dear as to tender 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 DANIS AND RESERVOIR?) have lawn erected. •n some ca-c,- even Ilinnsy brush dams, loaded with stone and earth. have been thrown across or partly across it stream, with the full knowledge Hunt 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 may be obtained from comparatively shallow wells by pumping. In California water is sometimes de veloped 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 Creat Plains, mostly for domestic water-supply, but not infrequently for irrigation. \lany of these arc home-made, resembling the simplest form of paddle water MARA, or are more elaborate, according to the mechanical ability or ambition of their makers. Serviceable and durable windmills (q.v.) are now- so cheap and so notch more effieient than the home-made affairs that there is little excuse for not having one wherever it can he put to good use. Small storage reservoirs are a neces sary 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 be 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 -firth may be so packed. or piddled. by wetting and ramming it, as to make more expensive lin ing unnecessary. Where available. motor power is
a cheap 'Deans 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 eombination of pumps driven by turf Iles. lly draulie rams are also employed.
In a comparatively few instances steam pump in• engines are used, particularly where large quantities of water are to he lifted to a con siderable height. This is true in California, Hawaii, and elsewhere, where large areas of land are irrigated by immense pumping plants. Gaso line engines are used to (hive pumps. They re quire but little more attendance than a windmill, and have the great advantage of not being depen dent. upon the umprtainties of the wind. In America the quantity of irrigating water raised by pumps of all kinds is small, hut in Europe, Africa, and Asia much pumping is done. In Egypt and in some countries, however, most of the pumping is very primitive, the power being applied by men or animals. I' ping 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 exist ing low-level canals. In many instances water earn be secured in this way at less exiamse than by gravity, since it permits the utilization of nearby 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 ir rigation is available either by diversion from streams, by storage of storm waters, by pumpiug from wells, or from any other sonnet., the question to be here discussed is the best means of utilizing it. for the production of crops.
IMF:mons OF APPLYING IRRIGATION ‘VATER. 3lain canals and conduits are often the most expensive part of irrigation works, owing to their length and the difikulties encountered in their construction. The cheapest and simplest conduit is a ditch, heading in the source of sup ply. and departing just suflieiently from the natural contour of the country to insure a flow of water. In the early (lays of irrigation such ditches were little more than single furrows, or channels no larger than might he 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 out let 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 con tinue the line of the canal, changing the con struction to an elevated flume, or else substitut ing a pipe or inverted siphon, laid on or in the ground. Where grades and other conditions per mit canals should he made narrow and deep rather than wide and shallow, in order to lessen the surface exposed to evaporation.
ary deposits and plant growths on their sides and bottoms. Sometimes leakage may be di minished by throwing powdered clay into the water at the head of the canal. The sediment ary matter naturally carried ,by the water will often reduce the leakage in a few months or years.