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Part 663 - Water Requirements <br />00683.2 (a) (2) <br />(2) Consumptive use data is essential for estimating water <br />requirements for irrigation as well as for estimating municipal <br />and industrial water needs. From research studies, formulas have <br />been developed relating consumptive use and climatological data. <br />The net irrigation water requirement can be estimated by <br />transposing observed consumptive use data from one area to. <br />another where few or no data, except climatological records, are <br />available by these formulas. Direct transfer of consumptive use <br />data to an area with widely different climatic conditions is not <br />valid. <br />(b) Method of Computation. <br />(1) In areas for which few or no measurements of <br />consumptive use are available, it is necessary to estimate <br />consumptive use from climatological data. The Soil Conservation <br />Service uses the Blaney-Criddle method, with some modifications, <br />for this purpose. The Blaney-Criddle procedure gives <br />sufficiently accurate results when used for the purpose it was <br />originally developed--that is, for estimating seasonal <br />consumptive use. <br />.(2) The basic assumption is that consumptive use (Et) <br />varies directly with the sum of the products of mean monthly air <br />temperature and monthly percentage of daytime hours for an . <br />actively-growing crop with adequate soil moisture. Therefore: <br />Et=KF= TIcf, <br />Where <br />Et = estimated .evap'otranspiration (consumptive use), <br />K = empirical consumptive use co-efficient" (season or <br />growing period), <br />1 <br />F = sum of monthly consumptive use factors, f, <br />f = tp/100 (where "t" is mean monthly air temperature and <br />"p" is mean monthly percentage of annual daytime <br />hours) <br />k = monthly consumptive use coefficient. <br />(3) The seasonal crop coefficients are not constant for <br />consecutive short periods throughout the growing season. <br />Therefore, the Soil Conservation Service has made two <br />modifications to the original procedure tc obtain reasonably <br />accurate estimates of short-period consumptive use. <br />C0683-2 <br />(C0210-VI-COIG, December 1988)