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<br />. <br />" <br /> <br />0028't5 <br /> <br />. <br /> <br />. <br /> <br />~ <br /> <br />2 <br /> <br />Although supporting data is unavailable, it appears that this method was used <br />until sometime after 1965 when the averages used were updated to the 1914-1965 <br />period. About 1970, direct estimates of annual consumptive use were used in <br />updating the data and when the "1971-75 Colorado River System Consumptive Uses <br />and Losses Report" was publ ished, the estimates were revised to conform wi th the <br />data in the report. Although the second (1976-1980) Consumptive Uses and Losses <br />report has recently been published, the virgin flow data for the period has not <br />yet been revised. <br /> <br />The basis for the virgin flow determination appears to have been changed a <br />number of times over the years. Most of the record was derived by somewhat <br />arbitrarily adjusting annual values to match long-term averages. <br /> <br />CRSS Natural Flow <br /> <br />In the June 1983 Draft Documentation of CRSS Hydrology Data Base - (Upper Basin) <br />there is, in Appendix A, a memorandum from John E. Redlinger in which he docu- <br />ments the development of the consumptive use figures used for computing natural <br />flows. Essentially, the modified Blaney-Criddle method of consumptive use <br />determination was used for sub-basin areas and the results summed to gage areas. <br />Care was taken to maintain a close relationship to the 1948 EAC study and the <br />1965 Type I study which were used as benchmarks. Temperature and precipitation <br />stations were selected that had full periods of record. Some adjustments for <br />elevation were necessary to more closely approximate the temperature conditions <br />of the irrigated areas. As in the 1948 EAC study and the 1965 Type I study con- <br />sideration was given to those areas of grass, hay and alfalfa whose growing <br />season is shortened due to limited water supply. <br /> <br />Irrigated acreage was determined through consideration of several sources with <br />emphasis on census data, the 1948 EAC report and the 1965 Type I study. Curves <br />were drawn and adjusted to estimate the acreage between census years and reflect <br />the 1948 and 1965 estimates. <br /> <br />Attached is a year-by-year comparison of virgin and natural flows. <br />