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<br />O~'01~ <br />dCl'"U <br /> <br />Adjustments must also be made for salt content changes in a manner <br />similar to that used in Step 3 for flows. <br /> <br />Serious criticism can be made of both approaches. Use of unregu- <br />lated data ignores the very purpose reservoirs are built, namely, <br />regulation. The reservoir becomes a river reach possessing sig- <br />nificant surface area, resulting in large evaporation losses. When <br />monthly data are used, present modified flows may actually exceed <br />the outlet works capacity, even after historic floods are modified <br />for current projects and expected evaporation. 'The approach may be <br />rationalized as adequate if the actual storage changes are small or <br />when reservoirs are always near full capacity so that they can no <br />longer perform their regulation functions. <br /> <br />Validity of the regulated approach requires that future operations <br />be similar to the historic ones. This imposes the restriction that <br />the reservoir be in operation during the entire historic sequence. <br />When upstream reservoirs are added, particularly large ones, the <br />filling period may require operation and release policies which are <br />radically different from normal operations. <br /> <br />In general, the regulated approach is used by the Lower Colorado <br />Region for data below the Lees Ferry station, while the Upper Colo- <br />rado Region uses the unregulated method above Lees Ferry. Noting <br />that present modified data were used: <br /> <br />- as boundary inputs at the four upstream boundaries <br />- to compute ungaged tributary inputs (described later) <br />- as target flows at the downstream boundary <br /> <br />either method applied consistently is adequate for stations within <br />the model. For upstream boundaries, use of present modified data <br />must be deemed a first approximation. <br /> <br />lihile the different approaches used by the two regions do present <br />some problems in interpreting the data, the approaches selected <br />were the ones most reliable at the time. The Lower Colorado Region <br />used the regulated approach since Lake Mead was in operation during <br />the entire period of data. The unregulated approach produces unre- <br />alistic results below Lake Mead. The unregulated method was used <br />by the Upper Colorado Region because Lake Powell was in the process <br />of filling and was in operation for only a short part of the period <br />of data. <br /> <br />Present modified data based on 1941-1970 historical records were <br />used in the studies described by this report. These data are con- <br />tained in Appendix A. Listed results include the volume of water, <br /> <br />6 <br />