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Show 1354 CONSUMPTIVE U8E OF WATER IN IRRIGATION Let Dv = the non-recoverable deep-soil percolation losses during the crop year recorded, in acre-feet per acre, based on the total area of irrigated land in the valley, that is, on the sum of the gross areas of projects within the valley. Let Ev = the evaporation and transpiration losses from the non-cropped area of the valley per crop year, in acre-feet, divided by the gross area of irrigated land in the valley. By definition as here used: UV=U + DV + EV.......................(6) However, as in the case of a project, it is very difficult to measure Dv and Ev by direct means. This is particularly true of Dm and, hence, other more easily measured quantities are introduced. Let Hv = the sum of the following: (a) The acre-feet of water entering the valley in measurable channels during the crop year; (6) the crop-year draft on soil moisture; (c) the crop-year rainfall; and (d) the water derived from the ground-water divided by the irrigated valley area, as defined under Dv and Ev. Let Rv = the measurable outflow, in acre-feet, divided by the irrigated valley area, as defined for Dv and E^ By equating inflow plus rainfall plus draft on soil moisture and ground-water (Hv) to valley consumptive use (Uv) plus outflow (Bv): B. = (U + J>v + Ev) + Bv - Bv + Uv therefore, UV = HV - R..........................(7) It is very important that consumptive use as here defined, either in its basic sense or as measured on a farm, a project, or in a valley, should not be considered as representing the quantity of water which the farmer, the project manager, or the valley community should be permitted to divert for irrigation purposes. In localities of very low crop-year rainfall, deep loam soils, favorable topography, reservoir storage for water when not needed, and other favorable conditions, it is possible that the quantities, Rf, Rp, and Rm will be reduced to a minimum, thus making Hf, Hp, and Hv approach Uf, Up, and Uv, respectively, as Equations (3), (5), and (7) indicate would be the case. In general, however, such favorable conditions do not exist, and, consequently, there is no simple relation between the quantity of water necessary properly to irrigate a farm, a project, or a valley, and the respective consumptive uses. Common Difficulties Some of the more common difficulties in measuring consumptive uses under the cases considered, together with notes concerning their variability, are briefly given, after which some actual measurements are reported. The factors which cause variability in U at a particular place have been given in Equation (1). It is obvious that all the factors in the right-hand member of Equation (1) vary greatly from place to place. Based on elaborate experiments in Nebraska, Kiesselbach (10)* concluded that "there is no such thing as a definite water requirement which is constant for any kind of crop"; * Reference Is made by number to literature cited at the end of the report. |