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<br />OJ1828 <br /> <br />The accuracy of actual stream measurements is generally on the order <br />of plus or minus 5 percent. This leads to an error of plus or minus <br />10 percent of the average flow between successive stations under <br />extreme conditions. Monthly historical flows have often been as high <br />as 1,000,000 acre-feet, resulting in a possible error of 100,000 acre- <br />feet. Even under more favorable circumstances resulting in a total <br />error of 5 percent, the error is 50,000 acre-feet. This is the same <br />order of magnitude as the results in Table III. <br /> <br />Thus, use of ungaged values in an operational model of this type is <br />expected to introduce error-induced variations, due to both defi- <br />ciencies in field data and modeling simplifications. Means would <br />be regarded as realistic, while variances would tend to be high. <br />The true situation is obviously more complicated than that illus- <br />trated by the simple example. <br /> <br />Reservoir Operating Criteria <br /> <br />In duplicating the Biennial Report results, several sets of operating <br />criteria were tried. The set adopted was judged adequate for the base <br />run. It is also believed to adequately define overall operations <br />under the variety of future conditions whose results are described in <br />this report. <br /> <br />The operating criteria used involve a combination of minimum specified <br />releases plus operations to meet downstream demands for each individ- <br />ual reservoir. All upstream reservoirs were operated conjunctively <br />using five control zone volumes. [2, Section 2.2] Table IV summarizes <br />pertinent information. In addition, the flood zone level of Table I <br />is used for Lake Mead. I~enever waters rise above this level, other <br />criteria for conjunctive operation are disregarded and releases are <br />made from Lake Mead. This level is taken as constant and applies to <br />any month of the year. The specified releases from Lake Mohave approx- <br />imate evaporation from Lake Havasu for base conditions. Releases from <br />Lake Mead equal the total expected evaporation for both Lakes Mohave <br />and Havasu. Because of program computation procedures, the release <br />from Lake Head will only reach Lake Mohave when the intervening ungaged <br />flows are positive. When these flows are negative, a portion or all of <br />the releases will be used to satisfy this demand. Using Table C-25, a <br />total of 4,969,000 acre-feet would reach Lake Mohave during the <br />30 years. This is equivalent to 54 percent of the specified release <br />of 9,210,000 acre-feet. <br /> <br />Specified releases from Lake Powell were set equal to those from Lake <br />Mead plus an additional 5,000 acre-feet/month. This was done to <br />reduce the imbalance of Lakes Powell and ~lead and to reduce possible <br />shortages during the runs due to low levels and capacity restrictions <br /> <br />18 <br />