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16 I I I I I I I I I I I 1 I I I I I I I ANNUAL OPERATING PLAN FOR WATER YEAR 1990 Three operation studies were developed for the Project to establish an AOP for water year 1990. Each of the studies conformed to the established operating criteria but used different inflow conditions and different water demand conditions. The three i nfl ow conditi ons were determi ned from a probabil i ty ana lys i s of historic inflows and were labeled reasonable minimum, reasonable maximum, and most probable. Reservoir inflow during water year 1990 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 1990 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 stud i es for water year 1990 are summari zed numeri ca lly 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 wi 11 be adjusted, as required, throughout the water year as changes occur in the forecasted inflows, irrigation demands, maintenance schedules, and power loads. October-January Period The three studies for the October-January period of water year 1990 are similar because winter inflows are assumed to be nearly the same under the three conditions of inflow. Therefore, the most probable inflow condition is utilized for the 1990 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-Big Thompson Project Green Mountain Reservoir Green Mountain Reservoir began the water year with 98,000 acre-feet in storage, which was 76 percent of the 1959-1985 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 until next runoff season. Releases from Green Mountain Reservoir for replacement, bypass of inflow, power, and river and reservoi r operat ions are expected to average 160 ft3/s duri ng the November-January period. These releases will lower storage to about 69,000 acre-feet by January 31.