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<br />001G57 <br /> <br />historical data between two stations, and then adjusts the difference <br />to a natural condition with respect to the reach only. <br /> <br />Actual and In~e~v~ning Flow Stations <br /> <br />Actual flow stations in the Lower Basin represent entire tributaries. <br />These tributaries are not modeled individually (like Upper Basin <br />tributaries) because of their very small size relative to the flow <br />and salt in the Colorado River mainstem. These tributaries include <br />the Paria (physically located in the Upper Basin), Little Colorado, <br />Vi rgi n, and Bi 11 Will i ams Ri vers. Si nce the tri butari es themselves <br />are not modeled (i.e., depletions, inflows, and reservoir regulation <br />are not modeled on the tributary except for some future depletions on <br />the Virgin), the flow and salt rtata for the station representing the <br />tributaries are left in a historical condition rather than being <br />adjusted to a natural condition. In the hydrology data base, monthly <br />flow and salt values for these four actual flow stations are histori- <br />ca lly recorded data. <br /> <br />Intervening flow and salt .values (gains and losses) in the Lower <br />Basin are determined, in effect, by taking the difference between <br />historical flow and salt values of two stations at the ends of a <br />reach, and then adjusting the difference to a natural condition. The <br />actual procedure involves two steps. First, the "adjusted" flow <br />values are computed at the downstream end of the reach by taking the <br />historical flow values at the upstream station and subtracting all <br />diversions within the reach, adding all inflows within the reach, and <br />adding or subtracting any change in reservoir storage within the <br />reach. Diversions include agricultural and M&I consumptive uses, <br />reservoir evaporation, and any exports that occur within the reach. <br />Adjusted salt values are computed similarly. <br /> <br />The second step is to compute the intervening flow and salt values by <br />subtracting the adjusted flow and salt values computed at the <br />downstream end of the reach from the historical flow and salt values <br />of the downstream station. The intervening flow and salt values com- <br />puted become the natural flow and salt values in the hydrology data <br />base .for the intervening flow stations. <br /> <br />Special Considerations and Assum~t)ons <br /> <br />Several of the intervening flow stations in the Lower Basin use <br />average values for portions for their record sets, rather than esti- <br />mating the historically recorded patterns. For the intervening flow <br />stations at the Grand Canyon and at Hoover Dam, only the 1963 to pre- <br />sent (the closure of Glen Canyon Dam to present) data contain the <br />historically recorded data adjusted to natural conditions. The 1906 <br />to 1962 period of the data base contains average monthly val ues <br />calculated from the 1963 to present data. For example, the average <br />of the January flow values of years 1963 to present is used as the <br />January flow value for each year 1906 through 1962. For the inter- <br />vening flow stations downstream of Hoover Dam, the 1906 to 1934 <br /> <br />22 <br />