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<br />002844 <br /> <br />. <br /> <br />. <br /> <br />f2..e d d. -f}Q VV1 <br />~""''^ ~~'l\\Vl~ <br />OJ- lO-~5 <br /> <br />0" <br /> <br />VIRGIN FLOWS ANO CRSS NATURAL FLOWS <br /> <br />- <br /> <br />, <br /> <br />- <br /> <br />I ntroducti on <br /> <br />The Colorado River Simul ation System (CRSS) has been adopted as the offici al <br />Bureau of Reclamation tool for conducting operation studies for the Colorado <br />River. The CRSS uses as a data base monthly "natural flows" for the period of <br />record 1906-1980 (currently being updated through 1983). These flows have been <br />derived for 19 stations in the Upper Basin. Early Bureau studies of Colorado <br />River Basin water supply used an annual "virgin flow" at Lee Ferry data base. <br />Because of the differences noted in the annual values of these two flow series, <br />considerable confusion has developed. This paper is an attempt to eliminate <br />much of this confusion. It is not, however, an exhaustive an1aysis of the year <br />by year differences in the two data sets. <br /> <br />Both "virgin flow" and "natural flow" mean that flow that would have occurred in <br />the river had there been no man-made developments. The "natural flow" term was <br />developed simply to differentiate from the earlier used "virgin flow" ter- <br />minology. It should be helpful to examine how each of these data sets were <br />developed. <br /> <br />A number of years were examined to determine where-i.e., transbasin diversions, <br />reservoir regulation, consumptive use, etc. - the differences between the virgin <br />flow and the natural flow adjustments occurred. The major difference was found <br />to be in the values of consumptive use or "within basin depletions." The other <br />factors generally agreed quite closely, thus the effort was directed toward <br />understanding the methods used in determining consumptive use. <br /> <br />Virgin Flow <br /> <br />In 1948 the Engineering Advisory Committee (EAC) of the Upper Colorado River <br />Basin Commission prepared a study of the Upper Colorado River hydrology. As a <br />part of this study, Messrs. Bl aney and Criddle prepared a consumptive use study <br />using average climatological data for the 1914-1945 period. The average deple- <br />tions in the Upper Basin were determined to be 1,849,00U acre-feet and the <br />average annual virgin flow for the 1914-1945 period determined to be 15,638,000 <br />acre-feet. <br /> <br />In 1950, the Bureau of Recl amation prepared the report "Colorado River Storage <br />Project and Participating Projects - Upper Colorado River Basin." In Appendix A <br />of that report (see attached sheets) the EAC's 1914-1945 average depletions and <br />average virgin flow was used to develop annual virgin flows for the years 1896 <br />to 1947. To determine the yearly in-basin depletion values, the Bureau first <br />found an average depletion rate by dividing the average EAC depletion - less <br />average exports - by the average irrigated acres: (1849-112) divided by 1351 <br />= 1.287 (see attached report excerpt). Before this factor was applied to the <br />value of irrigated acres for the individual years, it was "corrected" for a <br />"Debler effect" i.e. - in dry years less than average depletion would be <br />expected to occur because of alack of water and in wet years more than average <br />depletions would be expected to occur because of the availability of water. <br />This correction factor was taken to be <br /> <br />t (1+ annual virgin flow ) <br />(1914-1945 mean annual virgin flow) <br /> <br />and as explained in the attachment, the computation for 'virgin flow proceeded on <br />a trial and error basis. <br />