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I .1 J I I I I I 1 I I I I I I I I I I 0,\";0'7[' U'Jv.J~(j ANNUAL OPERATING PLAN FOR WATER YEAR 1991 Three operation studies were developed for the Project to establish an AOP for water year 1991. Each of the studies conformed to the established operating criteria but used different inflow conditions and different water demand conditions. The three inflow conditions were determined from a probability analysis of historic inflows and were labeled reasonable minimum, reasonable maximum, and most probable. Reservoir inflow during water year 1991 has a one-in-ten chance of being less than the reasonable minimum and a one-in-ten chance of being greater than the reasonable maximum. Statistically, inflows in 1991 will have an eight-in-ten chance of falling between the two extremes. The most probable inflow is based on long-term averages and approximates a 50 percent chance of occurrence. The three studies for water year 1991 are summarized numerically in tables 5A, 5B, and 5C and graphically in exhibits 4 through 9. The annual operating plans for Green Mountain Reservoir are also summarized in table 6. The AOP, as developed and reflected in the three studies, provides the flexibility to adjust operations as conditions change during the water year. Forecasts of the April-July reservoir inflow will be made at the beginning of each month for February through June. Projected operating schedules will be adjusted, as required, throughout the water year as changes occur in the forecasted inflows, irrigation demands, maintenance schedules, and power loads. October-Januarv Period The three studies for the October-January period of water year 1991 are similar because winter inflows are assumed to be nearly the same under the three cond it ions of i nfl ow. Therefore, the most probable i nfl ow condition is utilized for the 1991 October-January Project operations that are summarized in the following paragraphs. Operations for this period account for scheduled maintenance on several powerplants and other project features. Colorado-BiQ Thomason Proiect Green Mountain Reservoir Green Mountain Reservoir began the water year with 78,927 acre-feet in storage, which was only 62 percent of the 1960-1989 average. Releases for replacement, bypass of inflow, and natural flow shortage will continue until the end of October. Little depletion by Colorado Springs and Denver is expected unt il next runoff season. Re 1 eases from Green Mountain Reservoir for replacement, bypass of inflow, power, and river and reservoir operations are expected to average 174 ft3/s during the November-January period. These releases will lower storage to about 64,400 acre-feet by January 31. 19