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Show 1366 CONSUMPTIVE USE OF WATER IN IRRIGATION but also some native grass on land that might have been used for other crops. Based on these studies, Harding concludes that the valley consumptive use during the six months of April to September, inclusive, for 1900 to 1918, including the 0.19 ft. rainfall, is 3 ft., but excluding the draft on soil moisture, if any. The local practice of applying water at frequent intervals in small quantities is considered in part responsible for this high consumptive use. Moreover, the meadow areas are irrigated almost continuously. The irrigation season is about 165 days. A high precision is not claimed for the results, as evidenced in the following comments by Harding: "The records are not as complete as would be desired, and the resulting consumptive use is higher than, would be expected. The area is typical of two cuttings of alfalfa with either a light third cutting or good fall pasturage valley such as occur in the Mountain States." Columbia Biver Tributaries.-Parker (14) estimated the average valley consumptive use, exclusive of rainfall, on 12 tributaries of the Columbia Eiver for the "climatic years" 1909-10, 1910-11, and 1911-12. The estimated irrigated acreages during each of the three years were 99 000, 103 000, and 108 000, respectively. It is noteworthy that the magnitudes of consumptive use obtained, namely, 2.9, 2.7, and 2.8, respectively, agree closely, despite significant differences in the run-off. Methods op Estimating Valley Consumptive Use Hedke's Method.-It is desirable to estimate the consumptive use of water in irrigation, in different valleys, in advance of complete agricultural development. Recognizing the importance and complexity of the consumptive use problem, Charles R Hedke, M. Am. Soc. C. E., (7) has investigated the relation of consumptive use of water to the quantity of heat available to the crop during the growing season, and has found substantially that under favorable agricultural conditions the use of water is directly proportional to the use of heat available. The relative approach of the actual agricultural practice to the best practice is designated by Hedke as the "Standard of Agriculture". The application of the direct relation between water consumption and available heat, proposed by Hedke, to a valley in which the agricultural practices are of a high standard, necessitates the following assumptions: 1.-That the heat consumed by a particular crop, during any day or other time period, is determined by the amount of heat available to the crop above the germinating or minimum growing temperature. 2.-That undeT favorable agricultural practices, each crop consumes water in direct relation to the heat available as defined. 3.-That the soils considered are abundantly supplied with moisture and plant-food so that the yield of a crop will be limited only by the amount of heat available. 4.-That the influence of variations in wind velocity, relative humidity, and vapor pressure on consumptive use of water are relatively small as compared to the influence of available heat. Under these assumptions, Equation (la) may be written, U = / {Qh)> or simply U = K Qh, which is the basic relation assumed by Hedke. |
