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1311 14. D1BCtESlON OF ULTlMATE DEPLEl'lONS. subscribe to the 100 per cent relation with respect to consumptive uses within the basin, nor to the assumption that unit rates of depletion, as that word is defined and commonly used in the Report will remain constant and not decline in the future. studies and investisations of this office suggest that a consumptive use rate of 1.61 acre feet per acre (which is the average derived from 3,832,000 acre feet and the ultiInate 2,378,850 acres) my be a proper rate for estimating water supply reCluirements upon the assumption of providing a full supply at all times. But such a rate seems too high for depletion estimations, consider- 1ngthe fact that at present and under ultimate development a large share of the total irrigated area will remain dependent on unregulated and erratic flows of tributary streams. For the purposes of this Report the depletion rate of 1.61 acre feet per acre, applied to the average number of acres irrigated, can be accepted as liberal under normal conditions. However, under drought condi- tions, when some acreage cannot be irrigated and a larger share can be irrigated only fractionally, and when diversions, applications, contributions to ground water storage and retu= flows, are necessarily below average, the total depletions will be subnormal and hence the rate applicable to the entire acre- age must be less than the normal rate. 15 . STREAM FLOWS The Report discusses the water resources and water supplies of each division of the Colorado River BaSin, and of each major tributary in terms of so-called "virgin" conditions, commonly as drought cycle (1931-1940) averages at stations in the Upper BaSin, and commonly for longer and more nearly normal periods at stations in the Lower Basin. These so-called "virgin" flows are not defined in the Report. They appear to have been calculated from recorded flows at each station, expressed as averages, by adding thereto the estimated "depletions" upstream from the station. stream depletion is defined on page 3 as: "the net decrease in stream flow resulting from upstream diversions and uses, generally amounting to diversions less return flow." The definition is inadeCluate. Evaporation losses from main stem power and replacement reservoirs are entered as depletions, though manifestly they do not result from diversions nor amount to diversions less return flow. Nor is the definition precise, since it ignores the effects of surface and under-ground storage operations. While "virgin flows" and "depletions" are convenient engineering tools, they are not necessarily the same as those terms used in the Colorado River Compact, which defines the rights and obligations of the Upper and Lower Basins in language such as, "ths waters of the Colorado River System" and the ''beneficial consumptive use" thereof. Not only does the Report avoid the language of the Compact, but when discussing its provisions in Chap. III, enti tled "Dividing the Water," between the Upper and Lower Basins, language is used, for example, such as the words, "available" water, "dependable flow," ''water yield," and "excess" over dependable flow, which neIther appear in the Compact nor are defined in the Report. A statement such as appears in the Synopsis (page 6) that: ''The Compact divided the water on the basis of virgin flow," if not contradicted by the Compact itself, is at least open to Cluestion. (21)