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Show CONSUMPTIVE USE OF WATEB IN IEBIGATION 1353 Let Dp = the non-measurable deep-soil percolation and capillary losses, in acre-feet per project acre per crop year, from any project. Let Hp = the sum of: (a) The area of measurable surface water which flows into the project plus (&) the crop seasonal draft on soil moisture plus (c) the depletion of ground-water plus (d) the crop-season rainfall, also in acre-feet per project acre per crop year. Items (6) and (c) may be positive or negative. Let Bp = the sum of: (a) The surface run^off losses, in acre-feet per project acre per crop year plus (6) such deep-soil percolation losses as contribute directly to a measurable rise in the height of the water-table plus (c) the deep-soil percolation losses as may re-appear in natural or artificial channels at the boundaries of a project. Let Ep = the evaporation losses, in acre-feet per project acre during the crop year, from the non-cropped area within the boundaries of the project, such as roadways, barnyards, towns, etc., together with the transpiration from natural non-crop vegetation within the project boundaries. Then, by definition, By equating inflow to outflow plus change in moisture content: Hp = {U + DP + Ev) + RP = Up + R, and, therefore, UP = HP-RP..........................(5) Equation (5) gives an indirect means of determining the project consumptive use. It is important to note from Equation (4) that the project consumptive use includes two factors which are difficult to determine directly, namely, Dp and Ep. With poorly prepared land, porous soil, small irrigation streams, and great distances between the head ditches, Dp is likely to become excessive, possibly greater than U. It is not probably extravagant to state that, in general, project managers should endeavor to reduce (Dp + EP) to a minimum and thus decrease Up until it closely approaches U. On some high land projects, Dp may be recovered and used on lower neighboring land. On such projects it is less objectionable to have Dp large although excessive deep percolation losses on high land usually decreases the productivity of lower areas. Valley Consumptive Use.-In order to define Uv and indicate an indirect means of measuring it, some further quantities are needed. * Provided a relatively small area of the land within the boundaries of an irrigation project or valley is Irrigated, the common practice of determining project or valley consumptive use by dividing the difference between the inflow above and outflow below the tract in question by the area of the tract is misleading even if the corrections for contribution of rainfall, draft on soil moisture, and absorption from ground-water are properly made. To illustrate: Let W a tbe total water diminution on the tract, in acre feet; Ac = the cropped area, in acres; and, Av = the area within the project boundaries, in acres. From Equation (4), W= UAc + (Dp + EP)Ap...................................(a) Dividing Equation Co) by Ap, ^ ^EP).....................................(6) p P w If the ratio, --, ts small, approaching, say, one-half, then - is not equal to Up, as here defined. Equation 16) shows the importance of measuring Ae with a fair degree of precision. |
