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'. The assumption is that the mainstem reservoir evapora- tion should be borne in proportion to the level of depletions expected on each tributary. This asssumption is consistent with Article V of the 1948 Compact which charges mainstem reservoir evaporation against each Upper Basin state in pro- portion to the level of depletions in each state which existed in the year prior to the year in which the mainstem reservoir evaporation occurred. This further refinement indicates that the conflict-free depletion level on each tributary under the above assumptions is slightly less than the Colorado DNR's middle depletion scenario. This is because the mainstem evaporation charge allocated to each state is slightly more than the margin bet- ween each state's 1948 Compact entitlement and the Colorado DNR middle depletion scenario. These tables also present the compact benchmarks in terms of the virgin flow for each tributary near upper basin state lines. This comparison shows that the projected deple- tion benchmarks are not constrained by the physcial availabi-. lity of tributary flows. In all cases there is more than ample physical supply to meet the projected depletions. The point here is that only a relatively small percentage of this ample physical supply can be depleted without infringing on: 1) another tributary within an Upper Basin state; 2) another Upper Basin state's compact entitlement; 3) the assumed Lower Basin delivery obligation. Finally, the compact benchmarks shown on the following tables should be put into perspective with the vast storage reserves in the Upper Basin. There was almost 25 million acre-feet in storage in Blue Mesa, Morrow Point, Flaming Gorge, and Lake Powell in 1975. ("Consumptive Uses and Losses Report.") This storage does not increase the virgin water supplies in the Upper Basin, but only smoothes out the wide fluctuation of Upper Basin flows, and permits these supplies to be more closely matched with the demand for them in both Upper and Lower Basins. If, however, Upper Basin depletions and Lower Basin deliveries begin to exceed a realistic esti- mate of long-term virgin flows, the Upper Basin storage system will be thrown into disequilibrium and will start to be emptied without the prospect of eventual refilling. This storage system, then, is the Upper Basin's bank account which can be overdrawn if it is not kept in balance. ..It .. The compact benchmarks in the table show when projected depletions on anyone tributary could throw the Upper Basin storage system into disequilibrium. The rate of permanent drawdown will, of course, depend on the extent to which the benchmarks are exceeded, and it may take some time for excessive depletions to exhaust the Upper Basin storage -26- ____.._..__... .,...._ ..__...v~ '