OCR Text |
Show CONSUMPTIVE USE OF WATER IN IEEIGATION 1351 Symbols Used.-The meaning of each of the more important symbols is fully given subsequently. For convenience of reference, the meaning of these symbols is briefly as follows: Let U = consumptive use. Vf = farm consumptive use. Up = project consumptive use. Uv - valley consumptive use. Qk - quantity of available heat, in day-degrees, during crop year. e = evaporation from water surfaces. m = mean seasonal soil moisture content. s = the soil with all its influencing factors, notably potash, phosphorus, lime, humus, and nitrifying power, c = the kind of crop. y = the yield of crop. Df = deep percolation losses from the farm. Hf = the total quantity of water supplied the farm. Bf = surface run-off from the farm. Dp, Hp, and Rv = project deep percolation, project water supply, and project run-off, respectively. Ep = project evaporation losses. Ev = valley evaporation losses. Dv, Hv, and Bv = valley deep percolation, valley water supply, and valley outflow, respectively. Consumptive Use in a Basic Sense.-The consumptive use, U', is here defined as the quantity of water, in acre-feet per cropped acre per year, absorbed by the crop and transpired or used directly in the building of plant tissue, together with that evaporated from the crop-producing land.* The direct source of "consumed" water is the water in the soil in any form that crops can absorb it. The indirect sources of "consumed water" are those parts of the precipitation and of irrigation water which are stored in the soil in such depths and for such time as to permit direct absorption by crop roots, together with such water as may be obtained by the crop roots from the ground-water after such water is raised to the plant root zone by capillary action. The consumptive use as herein defined is, therefore, equal to the total evapo-transpiration plus the water used in the building of plant tissue. It is natural to inquire concerning the factors which determine the magnitude of U. A statement of the major influencing factors is made in the following equation: u= f (Q» «> m> s> c> * The consumptive use, as here defined, is not rigorously a correct application of the term, "consumptive," since the water which passes out of the crop leaves is not truly consumed. It is merely converted by the plant and the energy of the sun from the liquid to the gaseous phase. However, for the purposes of this report, water vapor in the atmosphere is considered beyond direct recovery by practical means, and hence the conversion of liquid water into water vapor is considered herein as consumptive use. t Plant diseases and insect pests may reduce the crop yield, y, without proportionately reducing the consumptive use, U. Likewise, inefficient farm practices may change th* relation between U and y. |