OCR Text |
Show 1352 CONSUMPTIVE USE OF WATER IN IRKIGATION It is apparent that most of the factors which influence the magnitude of U are themselves variable. The number of variables may be reduced somewhat under specified conditions; for example, if the soil of a farm is liberally supplied with organic matter and all the essential plant-food elements, so that the yield of a particular crop will be limited by the quantity of heat available and not by lack of moisture or plant food, it is possible to eliminate the last four factors in Equation (1) and write: U=f(Qh, e) In the light of the foregoing statements, it is unreasonable to expect that a precise experimental determination of U will give a definite magnitude that can always be considered rigorously correct. Engineers must use dependable approximations of the magnitude of. U. Knowledge concerning the magnitude of U is dependent on further analysis. Does U indicate consumptive use per acre on a particular farm, an irrigation project, or in an irrigated valley ? Further definitions are clearly necessary. Let Uf = "farm consumptive use"; Uf = "project consumptive use"; and Uv = "valley consumptive use". Definitions of each of these three modifications of consumptive use are considered desirable and are, therefore, proposed herewith. It seems reasonable to assume that the public may expect the individual irrigator to account for all the water which goes on to his farm. With respect to a particular farm, the water applied is disposed of by consumptive use, plus surface run-off, plus losses from the farm by deep percolation. Farm Consumptive Use.-Let Df = deep percolation farm loss, in acre-feet per cropped acre per crop year. This loss usually cannot be measured with precision by direct means. Then, by definition, as used in this report, Uf = U + Df..........................(2) It is important to distinguish between Uf and a related quantity, Hf, which is defined as follows: Let Hf = the sum of: (a) The quantity of water delivered at the farm plus (6) the water taken from moisture stored in the capillary form before planting, or during the non-growing season plus (c) the crop-year rainfall plus (d) the amount absorbed from the ground-water-all in terms of acre-feet per cropped acre per crop year. Let Bf = surface run-off from a farm, in acre-feet per cropped acre per crop year. Then, by equating the water going on to the farm plus rainfall and draft on soil moisture plus the quantity absorbed from the ground-water, (Hf), to farm consumptive use, (Uf) plus farm run-off, (22 <) : Uf = Hf - Bf..........................(3) Equation (3) gives an indirect means of arriving at the farm consumptive use. Project Consumptive Use.-In order to define Up and indicate an indirect means of measuring it, the following quantities are introduced: |