Glen Canyon Dam and Lake Powell
The unregulated inflow volume into Lake Powell for November was 417,000 acre-feet (77% of average). This was 13,000 acre-feet below what was forecasted at the beginning of the month. Consequently, the elevation of Lake Powell at the end of November was somewhat below what was projected in the November 24-Month Study. The end of November elevation of Lake Powell was 3631.10 feet above sea level. The November 24-Month Study projected the elevation would end November at 3631.56. So the initial condition for Lake Powell elevation for the December 24-Month Study is 0.46 feet below what was projected to occur in the November 24-Month Study. The forecasted unregulated inflow volume to Lake Powell for December is 375,000 acre-feet (86% of average) which has been reduced by 25,000 acre-feet from what was forecasted one month ago.
The scheduled release volume for December was increased from 855,000 acre-feet to 900,000 acre-feet due to a revision of the projected unit maintenance outage schedule in February. It is possible that only 4 of the 8 generating units at Glen Canyon Power Plant will be available during much of February 2010. The projected release volume for February was reduced from 800,000 acre-feet to 700,000 acre-feet so that the volume can be released through the 4 available units in a manner that will allow a load following pattern of generation within the allowances of the Glen Canyon Dam Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). Redistribution of the 100,000 acre-foot reduction in February included an increase to the scheduled release volume for December.
Hourly releases during December will peak during daylight hours into the evening to approximately 18,500 cubic feet per second (cfs) and decrease during early morning hours to approximately 10,500 cfs. Currently, it is projected that the release volume for January will be scheduled to be 955,000 acre-feet. At this volume, it is estimated that the hourly release rates during January would peak during daylight hours to approximately 19,000 cfs and decrease during early morning hours to approximately 11,000 cfs.
As of October 1, 2009, the unregulated inflow to Lake Powell during water year 2010 is projected to have an 80% probability of being within the range between 4.7 maf and 16.5 maf. There is an estimated 10% probability that the water year 2010 unregulated inflow volume will be below 4.7 maf and there is also an estimated 10% probability that the water year 2010 unregulated inflow volume will be greater than 16.5 maf.
Based on a recently updated range of possible inflow volumes for water year 2010 and through implementation of the Interim Guidelines, there is approximately a 36% probability that Equalization will occur in water year 2010. The determination of whether or not Equalization will occur in 2010 will be based on the projected September 30 Lake Powell water surface elevations of the 2010 April 24-Month Study. If Equalization does occur in 2010, the water year release volume is projected to be approximately 10.765 maf. If however, Equalization does not occur in 2010 (64% probability), the water year release volume could be as low as 8.23 maf. Each month these forecasted probabilities will be updated as hydrologic conditions change in the Upper Colorado River Basin.
Upper Colorado River Basin Hydrology
In the Upper Colorado River Basin during water year 2009, the overall precipitation accumulated through September 30, 2009 was approximately 95% of average based on the 30 year average for the period from 1971 through 2000. The final 3 months of water year 2009 all had accumulated precipitation rates that were all below average with 60, 45 and 75% of average occurring in July, August and September respectively. For water year 2010 the dry conditions have continued. Precipitation for October 2009 was 85% of average and for November, precipitation was estimated to be only 40% of average.
The Climate Prediction Center outlook (dated November 19, 2009) for temperature over the next 3 months indicates that temperatures in the southwest have an increased probability of being above average while accumulated precipitation is projected to be near average in the Upper Colorado River Basin.
Upper Colorado River Basin Drought
The Upper Colorado River Basin continues to experience a protracted multi-year drought. Since 1999, inflow to Lake Powell has been below average in every year except water years 2005 and 2008. In the summer of 1999, Lake Powell was close to full with reservoir storage at 23.5 million acre-feet, or 97 percent of capacity. During the next 5 years (2000 through 2004) unregulated inflow to Lake Powell was well below average. This resulted in Lake Powell storage decreasing during this period to 8.0 million acre-feet (33 percent of capacity) which occurred on April 8, 2005. During 2005, 2008 and 2009, drought conditions eased somewhat with net gains in storage to Lake Powell. As of December 6, 2009 the storage in Lake Powell was 14.88 million acre-feet (61.20 percent of capacity) which is still below desired levels while the overall reservoir storage in the Colorado River Basin as of December 6, 2009 is 33.4 million acre-feet (56.14 percent of capacity).
RRFW thanks Rick Clayton of the USBOR for his assistance in providing information for this notification.
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