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Show 1356 CONSUMPTIVE USE OF WATEB IN IBEIGATION thus supplying water for crop production. Under such conditions, this factor may make it impractical to obtain dependable measurements of the project or valley water supply. In general, the measurement of surface run-off from the farm is neither difficult nor very expensive. Likewise, the measurement of project run-off depends on the particular project. On some projects situated as long strips of side-hill land, it is difficult to measure run-off. The measurement of runoff from valleys, in general, is not so difficult as it is from projects. Some valleys have deep canyon drainage outlets over impervious floors, and in such cases the run-off can be measured without serious difficulty. Some valleys have surface-flow and ground-water flow outlet drainage, but if it is evident that the surface flow greatly predominates, the measurement may permit a close approximation of valley consumptive use. That the dependability of measurement of the consumptive use rests on a complete analysis of all these factors is not likely to be over-emphasized. Moreover, reports of consumptive use, for the farm, for the project, or for the valley, or, indeed, for any other area, should not be given great weight, unless they are accompanied by complete and detailed descriptions of the conditions under which they were made. Doubtless, in some localities, the relative magnitudes of the inaccessible inflow and outflow below the ground surface, or those under other conditions which render measurements impracticable, are so great as to make computations of project or valley consumptive use of little value if not really dangerous. On the contrary, the need for economic utilization of water from all sources and the urgency of being able to predict with a fair degree of accuracy ihe areas that may be adequately served by a given water supply seem fully to justify extraordinary efforts toward increasing the ability to measure consumptive use under specified conditions. Experimental Measurements The literature' on duty of water is voluminous- The Federal Department of Agriculture, through its Bureaus of Public Roads and of Plant Industry; the Department of the Interior, through the Bureau of Reclamation and the Geological Survey; the State Agricultural Experiment Stations; the State Engineers' Offices; and some private agencies are annually contributing substantial amounts of time and energy to a solution of the duty-of-water problem. In Canada, also, very careful study is being made of duty of water by public and private agencies. Some of the experiments thus conducted give a fair basis for determinations of the consumptive use and the farm consumptive use. Many duty-of-water studies have been conducted without measuring the water obtained from capillary soil' moisture, and, indeed, some have failed to report the crop-year rainfall. Consequently, reference is here made only to those experiments which seem most directly to contribute toward a determination of consumptive use both in its basic sense, U, and in its modified meanings, for the farm, the project, and the valley, respectively. Consumptive Use, U, as Determined from Tank or Pot Experiments Hammatt (2) and Stevens (19) have endeavored to determine the consumptive use from the crop yield on a given area and estimates of the evapo- |
