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UOu585 , Glen Canyon Dam E~pected Releases Water Year 1999. I June 16, 1998 The following description of potential releases for wY 1999 are prepared for use by the TWG in the planning of research, monitoring, and environmental compliance activities for the coming year. Numerical projc;:ctions are the same as in the 1999 AOP currently in preparation. The AOP uses 3 scenarios to illustrate various levels of basin hydrology: probable maximum, most probable, and probable minimum. These correspond to upper decile, mean, and lower decile annual inflow volumes and have 10,50, and 90 percent exceedence probabilities respectively. If this paragraph doesn't: make sense, call me at (801)524-3715. In considering these scenarios, you should remember :that these cases are merely examples of what might happen under various inflow assumptions. The upper and lower decile inflow scenarios tend to provide upper and lower bounds on expected releases, though more extreme inflows could also occur. Actual operations will of course be different and will change as hydrologic conditions change throughout the year. 1)1e value in looking at various potential hydrologic conditions is to discover the likelihood or probability that various things would occur. Probable Maximum This scenario has an aimual release volume of 16.9 MAF and includes 6 months that have releases greater than 115 MAF (Feb - Jul). The reservoir would reach a minimum elevation of about 3668 feet in April and a maximum elevation of about 3697 feet during July. A BHBF would be triggered in February, perhaps earlier if the snowpack were well established early in the winter. : The probable maximwr scenario represents a wet year, one in which the basin has a very large snowpack. Some of the climate analysis done this past year indicated that the Upper Basin has higher than normal wi~ter precipitation during La Nina years and higher than normal spring precipitation during EI:Nino years. Since there is already beginning to be some expectation of a La Nina forming this sUmmer, a heavy winter snowpack is certainly possible. In many ways this situation is much easier to deal with from a reservoir operations point of view than the late spring surprises sometimes fo~d with El Nino anomalies. From the BHBF analysis performed last fall, there is about a 1 in 3 chance of triggering a BHBF when the reservoir is full as at the present, so I the likelihood of a BHBF next year is about 33 percent. Most Probable This scenario has an annual release volume of 10.7 MAF with monthly release volumes ranging between 700,000 and 1,200,000 AF. The reservoir would reach a minimum elevation of about 3674 feet in March and a maximum elevation of about 3694 feet during July. A BHBF would likely not be triggered, :neither on January 1 as the result of a forecasted high runoff nor during the spring as the result pf a monthly release volume exceeding 1.5 MAF. I The most probable sceijario represents an average year. There are many ways to get an average e'u/~ ~ ~ k cl::y~