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Show 1350 CONSUMPTIVE USB OF WATEB IN IRRIGATION fully to ascertain what became of the water which they applied to their lands. That they lost some water by surface run-off was obvious; that some water was absorbed by the crops they grew was likewise apparent; but that large quantities of water percolated deeply into the dry soil was to them merely speculation, if, indeed, such losses were suspected at all. However, the gradual rise of water-tables, with resulting enlargement of natural springs and the development of new springs, and the seepage return to flood-water channels, to small creeks, and, finally, to rivers, gave increasing evidence concerning the magnitude of losses of water through deep percolation. Moreover, it was found through experience that much less water need be applied to the farms to produce profitable crops than was formerly believed necessary, and the areas of land successfully irrigated by the water from a given stream were greatly increased without any apparent increase in the water supply. Obviously, such increase in area of irrigated land could not continue without limit, some water being actually consumed by the growing crops. Definitions and Analysis Gross duty of water, head-gate diversions, and net duty are familiar terms concerning water uses, but "consumptive use" is a term of recent origin, and should not be confused with the net duty on land and other terms used in discussions of duty of water. Some engineers have used the term, consumptive use, as restricted to 'Valley consumption", or net depletion of river flow, whereas others have used it to include water used from all sources, or total water consumption, irrespective of whether it was from river water, rainfall, carry-over soil moisture, or absorption from ground-water. It is believed that the use of the definitions given in this report will avoid confusion in the study of consumptive use of water, and comparisons of engineering data will thus be placed more nearly on a common basis. The term, consumptive use of water, was used in Colorado nearly two decades ago by John E. Field, M. Am. Soc. C. E., then State Engineer. It has since been used in Court testimony concerning water supply studies in the Laramie River inter-state water litigation between Wyoming and Colorado ; in the North Platte River Co-Operative Report between the TJ. S. Reclamation Service and the State of Wyoming; in the Rio Grande Basin water supply studies of the U. S. Reclamation Service; and in the Colorado River Compact deliberations in the several Western States. It has heretofore been defined by R. I. Meeker, M. Am. Soc. C. E., of this Committee, as: "The permanent water loss incident to irrigation of large tracts of land. Generally speaking, consumptive use is the difference between the inflow and the outflow below any specific area under consideration, or the difference between the amount of water diverted and the amount returning to a stream. With respect to an entire river system, it is the net amount after deducting the outflow from the inflow." To add convenience and brevity to the following definitions and analysis of consumptive use proposed by the Committee, mathematical symbols are employed and mathematical methods are followed, despite the fact that the equations are of a comparatively simple form. |