DUTY OF WATER IN IRRIGATION. The amount of water required to irrigate different crops under varying conditions is a matter of fundamental importance, hut has never been very accurately determined. and there is no very accurate simple means of judging when a soil needs irrigation. The term duty of water is commonly used to express the nmnber Pt acres which a given quan tity of water will adequately irrigate and is best stated in acre-feet or acre-inches, which arc the amounts of \later required to cover au acre 1 foot or 1 hod] deep, respectively (13,560 and 3630 enhie feet. re,peetively L. The water is usually measured to consumers in cubic het fuer SITUnd, or second-feel. A second-toot is a flow of 1 cubic foot of water in a second of time. f/ne second foot will supply an acre-ineh in 1 hour and 30 seconds, on- acre.foot in 12 hours and 6 minutes. "In 21 hour., a stream of 1 second-foot would supply 23.s aere-inehes, and would cover 7.93 acres of land with water 3 inches deep." (King.) A eommon unit of me.isurement of water in the. Western rnited States is the miner's inch, which is the amount of water which will flow through a hole 1 inch square in I second of time tinder a certain pressure or head (which, theoretically, is 6 inches, but which varies in different States). In California 50 miner's inches are eonsidered equal to 1 second•foot in Colorado, 38.4.
The duty of water, reported on the basis of area actually irrigated with a given amount of water. varies so widely in different locali ties and conditions of water - supply that it is of little or no value as a measure of the water actually required in crop production. how ever useful it may be as a rough guide in esti mating the required capaeity of irrigation work,. A more accurate measure is the actual amount of water required to produce the maximum yield of a crop under given soil and climatic conditions, and this, as already intimated, has not received the investigation its importance demands. King gives the following as the minimum amounts of water required to bring the crops named to matu rity under conditions of absolutely no drainage and the smallest possible evaporation, assuming also that at the time of planting, the soil already possesses a sufficient amount of moisture: pressed in acre; per second-foot. To put those figures on a 11101V definite basis, a "Inches per ten days" has been added in King's Irrigation and Drainage. The modified table is reprinted herewith. It should bo noted that it is based on measurements at the head of the Canals, and therefore includes by seepage and evaporation, as it properly should. The
higher duties in southern California, are duo largely to the ea re taken to prevent these losses and to apply the water to the crops with a minimum of waste. Such care is more feasible in that section than elsewhere, on account of the intense cultivation there employed and the high values of the products obtained. Sonic inure recent American figures of duties, ex pressed in acre-feet. are given in The Use of Water in Irrigation, Bulletin No. 86 of the United States Department of Agriculture, Office of Experiment Stations, being reports on inves tigations made under the direction of Elwood Mead. These records showed a range of from While the above figures give much greater duties than are secured in actual practice. and =mot therefore be taken as absolute guides, they will be helpful in estimating, the possible duty of Neater. 'Flip duty of water in ninny eases is determined very largely by the water-supply. If water is scarce the duty will be high, if abundant the duty is likely to he low.
All general :statements as to the duty of water must be received with caution, ow ing to the variation of duty with the local conditions already mentioned as governing the quantity of water requireX in irrigation. Wil son's Manual of Irrigation Engineering gives a table for various countries of the world, ex 2.10 acre-feet for the period from June 16 to Septenilwr III, 1899, at Bozeman, :Mont., to 6.30 acre-feet from April to September, 1899. at Salt Lake City. The rainfall in each case was less than a half inch during the period named. Special measurements at other localities showed a range of from less than 1 to more than 15 acre feet, but the conditions were abnormal. Where the duty was measured at the point of use, in stead of at the head of main canals, it was found that more than as much water was lost in the canals as was available for application on the fields.
The fact should not he lost sight of that there has been in the past and still exists a general tendency to excessive irrigation. This gives not only low duties, but results in over-saturation of tic soil and has rendered large areas of the lower lying lands in irrigated regions unfit for euitiva tiOn by flooding them with seepage water or by causing the rise of alkali. (See Arataii :son-s.) For these reasons thorough drainage, either pat ina] or artificial, is a neees-ary necompaniment of irrigation, as a protection against the harmful results of excessive irrigation.