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<br />. <br /> <br />. <br /> <br />~.s t. \9J3' <br /> <br />o <br /> <br />HydrologIc DetermInation <br />Supplemental Information <br /> <br />. <br />':'1,) <br />~ <br />c.o <br />(Q <br /> <br />Background <br /> <br />At the March 24, 1987 meeting of the Legal and EngIneerIng <br />CommIttees of the Upper Colorado River CommissIon, the Bureau. <br />presented the results of our Investigations of upper basin yield and <br />the status of the hydrologic determination as requested by Mew <br />Mexico. Before deciding on a course of action, other committee <br />members expressed a desire for additional Information. The Bureau <br />was requested to continue our studies and to provide information <br />relating to the effects on yield of I) eliminating the surplus strategy <br />from CRSS runs, 2) utilizing the mInImum power or Inactive pools In <br />the reservoirs, I.e.- drawing down reservoirs to the top of dead <br />storage, and 3) changing the minimum release to the lower basin <br />from 8.23 MAF annually to 7.5 MAF. These Items were investigated <br />individually and In combination as discussed below. <br /> <br />Surplus Strategy <br /> <br />The earlier CRSS runs which were used to determine the <br />amount of storage considered in the mass balance analysis used a <br />surplus strategy of "0.7". The effect of such a strategy Is In wet <br />years to release water early in the year which would not normally <br />be releas.ed until the flood runoff period. Although the annual <br />amount of release may not significantly differ, the reservoirs would <br />be drawn down or kept down early in the year. Even though this is <br />true, the analysis made with the strategy in effect showed the <br />system to be full after the demand build-up period (present to 2040) <br />and "empty" at the end of the drawdown period. For Instance, Lake <br />Powell was at elevation 3700 ("full") at the start of the drawdown <br />period and at elevation 3490 ("empty") at the end of the perIod. The <br />amount of water between these elevations plus similar amounts in <br />the; otr.er:-eser"t'!rs were tt'e:l. used to dp-termine the storage figure <br />used In the mass balance analySis. <br /> <br />A similar CRSS run was made with the surplus strategy <br />turned-off to see If there would be any effect on yield. The principal <br />effect was to hold the reservoIrs higher early In the water year, <br />particularly In wet years. The system stlll filled the same month as <br />before (Lake Powell at elevatIon 3700) and emptied the same month <br />as before (Lake Powell at elevation 3490). There was, however, a <br />slight difference In the amount of water that this represented. <br />Because Lake Powell had generally been held higher durIng the <br />demand build-Up perIod, more sediment had accumulated in the <br />active poo.! and the amount of water between "full elevation" and <br />"empty elevation" actually decreased by 15,000 acre feet. This would <br />