Page:The New International Encyclopædia 1st ed. v. 10.djvu/900

IRRIGATION. amounts of water required to cover an acre 1 foot or 1 inch deep, respectively (43,560 and 3630 cubic feet, respectively). The water is usually measured to consumers in cubic feet per second, or second-feet. A second-foot is a flow of 1 cubic foot of water in a second of time. One second-foot will supply an acre-inch in 1 hour and 30 seconds, one acre-foot in 12 hours and 6 minutes. “In 24 hours, a stream of 1 second-foot would supply 23.8 acre-inches, and would cover 7.93 acres of land with water 3 inches deep.” (King.) A common unit of measurement of water in the Western United States is the miner's inch, which is the amount of water which will flow through a hole 1 inch square in 1 second of time under a certain pressure or head (which, theoretically, is 6 inches, but which varies in different States). In California 50 miner's inches are considered 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 localities and conditions of water-supply that it is of little or no value as a measure of the water actually required in crop production, however useful it may be as a rough guide in estimating the required capacity of irrigation works. 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 maturity 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:

While the above figures give much greater duties than are secured in actual practice, and cannot therefore be taken as absolute guides, they will be helpful in estimating the possible duty of water. The duty of water in many cases is determined very largely by the water-supply. If water is scarce the duty will be high, if abundant the duty is likely to be low.

All general statements as to the duty of water must be received with caution, owing to the variation of duty with the local conditions already mentioned as governing the quantity of water required in irrigation. Wilson's Manual of Irrigation Engineering gives a table for various countries of the world, expressed in acres per second-foot. To put those figures on a more definite basis, a column headed “Inches per ten days” has been added in King's Irrigation and Drainage. The modified table is reprinted herewith. It should be noted that it is based on measurements at the head of the canals, and therefore includes losses by seepage and evaporation, as it properly should. The higher duties in southern California are due largely to the care 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. Some more recent American figures of duties, expressed 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 investigations made under the direction of Elwood Mead. These records showed a range of from 2.10 acre-feet for the period from June 16 to September 16, 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, instead 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 be lost sight of that there has been in the past and still exists a general