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Show 1358 CONSUMPTIVE USE OF WATER IN IRRIGATION the cost of duty-of-water studies. There is, however, a growing need for more thorough duty-of-water studies in which the consumptive use. will be determined for different conditions. In some such studies the consumptive use has been either determined or closely approximated, and of these the following are typical. Widtsoe's Utah Work.-Widtsoe (23, 24) pioneered the measurement of consumptive use in field plots at the Utah Agricultural Experiment Station, beginning in 1902. This work was done on land having a water-table about 75 ft. below the surface; and, hence, it is reasonably safe to conclude that the crops obtained no ground-water and that the crop-season rainfall, the draft on stored capillary soil moisture, and the irrigation water furnished all the water to which the crops had access. There was no run-off from the experimental plots used by Widtsoe, Rf = 0. The deep percolation losses, Df, were not measured. If these losses are assumed to be negligible, then, U = Mf, that is, the sum of the quantities, (a), (b), and (c), of Hf as used in Equation (3). Widtsoe measured these sources of water for 14 crops during the 10-year period, 1902-11, inclusive. Eeaults for the 7 crops on which most of the work was done, are given herein. The crop-season rainfall used by Widtsoe was 0.42 ft. and the seasonal draft on capillary moisture in the upper 8 ft. of soil, varied from 0.10 ft. for corn to 0.83 ft. for alfalfa. Quantities of irrigation water were applied, varying from 0.42 ft. to 5.00 ft., and wide variations in crop yields were obtained. In the -studies by the Committee the yields obtained by Widtsoe were plotted against the total water used, and, as a basis for arriving at the consumptive use, those yields were selected which appear to be most profitable. With nearly every crop, the yield increased rapidly to a certain point with increase of total water used, and then either decreased with further increase in water or increased very slowly, and at this "break in the curve", the value of U was taken. The results are given in Table 1. TABLE 1.-Consumptive Use as Determined on Field Plots by Widtsoe at the Utah Agricultural Experiment Station, in Cache Valley, Utah. Crop. Number of single trials. Yield, in bushels, or tons per acre.* Consumptive use, in acre-feet per acre. Sugar-beets............... Potatoes.................. Alfalfa.................... Corn...................... Wheat.................... Oats...................... 152 124 49 81 142 29 21.8 267.0 4.7 99.2 45.7 80.7 2.5 2.2 3.8 2.5 2.4 2.5 * Yields are given in tons per acre tor sugar-beets and alfalfa and bushels per acre for the remainder of the crops. Widtsoe's work indicates the importance of yield in determining the consumptive use. It is also important to keep in mind the fact that deep percolation losses from the plots on which Widtsoe worked would result in observed magnitudes of U higher than the true ones. It is far more probable that the given values of U are too high rather than too low. |
