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Show CONSUMPTIVE USE OF WATEB IN IKBIGATION 1367 To determine Qk, Hedke subtracts the minimum growing temperature of each crop from the mean temperature of each month, or fraction thereof, within the growing season. The result thus obtained, multiplied by the number of days in the month, represents the available heat units in day-degrees for a particular crop during the month considered. The sum of the heat units available to each crop for each month gives the available heat units for the crop-year. Hedke found in the Cache la Poudre Valley of Colorado in 1916 that heat was available to alfalfa during the eight months of March to October, whereas, to potatoes it was available only from May to September, inclusive. To find the total heat units available on a project, the heat units available to each of the crops are multiplied by the proportion of the project lands in the respective crops, and the sum of the products thus obtained gives the available heat units for the project. Hedke has applied his proposed principle to several typical valleys and has obtained some gratifying results. The result of his work in the Cache la Poudre Valley and its application to the San Luis Valley, in Colorado, and the Rio Grande Valley, in New Mexico, suggests that Qh is a predominating factor in fixing the magnitude of the consumptive use. Hedke defines consumptive use as the "total amount of water transpired incidental to farming an area"; thus the inclusion of the water evaporated from the crop-producing land is implied, but not specifically stated. He states further that the consumptive use "is here covered by the net depletion from the stream plus one-half the total yearly rainfall". However, his applications do not show that "net depletion" includes the crop-year draft on soil moisture plus the water derived from the ground-water, both of which should be considered as indicated in this Committee's enumeration of the elements of supply, as used in Equation (7). Based on Hemphill's measurements of valley consumptive use for the Cache la Poudre Valley, and using the equation, Uv = K Qx, Hedke has found that K = 0.000423, approximately. Having the magnitude of K, Hedke considers it possible to determine the valley consumptive use for the highest "standard of agriculture" in any valley in which dependable crop distribution and temperature records are available. His analysis of crops and temperatures in the Middle Rio Grande Valley shows a normal available heat, Q*, of 6 200 day-degrees, and applying the value of K, finds, Uv = 0.000423 X 6 200 = 2.6 acre-ft. Considering the Rio Grande Project, he finds that Qh = 6 800, and, hence, Uv = 0.000423 X 6 800 = 2.9 acre-ft. By deducting the "effective precipitation" from each of these values, he arrives at an estimate of the net depletion in the stream system. The application of these estimates without modification is of necessity based on an actual or an assumed high "standard of agriculture" in the Rio Grande Valley and on the Rio Grande Project similar to that of the Cache la Poudre Valley of Colorado. In this connection it is essential to remember that Hedke urges the importance of making careful observations concerning the relative agricultural practices in different valleys for which information is available, concerning |