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.' '" . . Area Manager DRAFT 10 To determine if sufficient water will be available in any given year to meet the authorized purposes of the Navajo Unit, as well as meet the Flow Recommendations, Reclamation compares ___ ___ _ _ ..'... __u _ ~ _ _..:I ____~1 _L 1 ~ ~_._..__ ~_ 1\.T __._=... n _.._._=_ ...........:....+ +\....... ........~.nl ....1;. ''''''''...;nn pJU~l't::\;LJVC lWlUU i:UIU ~Va.llc1UIC; WClL.C;;I .lJl1"'(1ViC1JV 1\..\..:)'-'1 VUU, iC15CL.LJI.:)L UJ,,", WllU.U.U U.I' ........nvu requirements of the various uses in the basin. Detennination of an extreme dry year, which could result in a shortage situation, can be triggered by lack of snowpack or a lack of available water in storage, or a combination of both. Seasonal water supply forecasts are generated montWy from January through July for the San Juan River Basin. Reclamation develops an annual operating plan for Navajo Reservoir based on the Most Probable Forecast. This forecast is the best estimate of stream flow volume that can be produced given current conditions and based on the outcome of similar past situations. There is a 50 percent chance that the stream flow volume will exceed the forecast value and a 50 percent chance that the stream flow volume will be less than the forecast value. Uncertainties are inherent in water supply forecasts due to imperfection in the techniques in developing the forecasts and the unpredictability of weather. By January I, there is enough information available to give a general indication of what the year may be like. Each subsequent month's forecast is generally more accurate than that of the previous month. As the year progresses, there is more certainty about the information affecting stream flow. The Flow Recommendations found that a minimum carryover storage level of 900,000 af (lake elevation of6,018.8 feet) on July 31 was needed to prevent shortages to water users in future years and meet the Flow Recommendations for the San Juan River downstream of Farmington. This was a calibrated value for 1999 depletion levels in the basin using the 1929-1993 period of record and a minimum Navajo Dam release of250 cfs. This minimum carryover value will be updated as water use increases in the basin. The SJRIP Hydrology Committee used this modeling method to make the detennination that 2003 was an "extreme year" since the Most Probable Forecast placed the July 31 level below 6,018.8 feet. The Hydrology Committee is continuing to develop the method for determining extreme years, but the basis will be similar to what was used in 2003. Reclamation developed a computer program to calculate anticipated shortages using the most recent Minimum Probable Forecast, the available water supply in Navajo Reservoir, and the anticipated demands from the various users and uses. This program is updated twice a month as new forecasts become available. Using the Minimum Probable Forecast and the anticipated demands for water, the computer program creates a Navajo Reservoir operational scenario for that year, If the program causes the reservoir level to drop below elevation 5,990 feet (bottom of active storage, delineated by the intake structure for the Navajo Indian Irrigation Project) anytime during the irrigation season (March through early November), a shortage would occur. The program then proportionally allocates that shortage to all users, including the endangered fish, and uses based upon their respective demands for that year. As a result of decreasing (or shorting) the demands of all users and uses, the reservoir level does not drop below elevation 5,990 feet. As the inflow forecasts and actual water levels in Navajo Reservoir change, so does the anticipated shortage amount. Shortages will be shared equally among users. For example, if a 10 percent shortage is agreed to, then the resulting flows through critical habitat would be 450 cfs (500 cfs - 50 cfs). 001407