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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />historic record (mean flows, standard deviations), and also provide a much <br />greater combination of above and below-normal streamflows. The synthetic <br />record thus allows the water supply system to be tested against a wide <br />range of conditions that may not have been observed in the historic record <br />but are nevertheless possible in the future. Use of this technique also <br />provides a better perspective of historic droughts. For a 40-year historic <br />record, for example, it is difficult to determine the recurrence interval <br />of the worst drought on record due to the limited length of recorded data. <br />It could be a 40-year drought as easily as a lOO-year drought or a 30-year <br />drought because there is no way to tell how long ago the last drought with <br />similar characteristics occurred. Generation of a long-term synthetic <br />record provides a more extensive range of droughts against which the <br />drought in question can be compared and its severity more accurately <br />determined. <br /> <br />Synthetic streamflow histories for the two watersheds which supply the <br />City's present water sources - the Big Thompson River Basin and Upper <br />Colorado River Basin - were developed as part of this study and were used <br />to determine the reliability of the City's water supply and the severity of <br />any water supply shortages that might be encountered. The development of <br />these streamflow histories was completed as Tasks 2 through 4 and is <br />described in Sections 4 through 6 of this report. <br /> <br />2.2 VATER DEMAND <br /> <br />Naturally, the magnitude and variability of the demand for water are <br />important factors to be considered in evaluating the reliability of a given <br />system or in planning to meet future water needs. Since future populations <br />and water demands are subject to many factors which cannot be accurately <br />determined at present, this study examined a range of possible growth rates <br />over the next three decades. The reliability of the City's water supply <br />(including the anticipated gain in future water supplies due to <br />development) was estimated for each planning scenario. The growth rates <br />and water demands used in each scenario are described in Section 7. <br /> <br />2-4 <br />