RRFW Riverwire -- Glen Canyon Dam Update
November 10, 2009
Glen Canyon Dam / Lake Powell
The unregulated inflow volume into Lake Powell for October was 374,000 acre-feet (80% of average). This was 26,000 acre-feet below what was forecasted at the beginning of the month. Consequently, the elevation of Lake Powell at the end of October was somewhat below what was projected in the October 24-month study. The end of October elevation of Lake Powell was 3633.52 feet above sea level. The October 24-month study projected the elevation would end October at 3634.06. So the initial conditions for the November 24-month study are reduced by about 0.5 feet from what was expected in the October 24-month study.
The updated forecast for the unregulated inflow volume to Lake Powell during November is now 430,000 acre-feet (92% of average).
Beginning on November 1, 2009, releases from Glen Canyon Dam will resume normal daily fluctuation consistent with the Glen Canyon Dam Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The scheduled release volume for November is 690,000 acre-feet. Hourly releases during November will peak during daylight hours into the evening to approximately 13,500 cfs and decrease during early morning hours to approximately 7,500 cfs. Currently, it is projected that the release volume for December will be scheduled to be 855,000 acre-feet. At this volume, it is estimated that the hourly releases during December would peak during daylight hours to approximately 17,000 cfs and decrease during early morning hours to approximately 9,000 cfs. These estimated release rates will be updated towards the end of November.
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 the range of probable inflow volumes and through implementation of the Interim Guidelines, there is approximately a 50% probability that Equalization will occur in 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 10.7 maf. If however, Equalization does not occur in 2010 (50% 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 is 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. Precipitation for October 2009 was 90% of average.
The Climate Prediction Center outlook (dated October 15, 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 October 28, 2009 the storage in Lake Powell was 15.38 million acre-feet (63.25 percent of capacity) which is still below desired levels while the overall reservoir storage in the Colorado River Basin as of October 28, 2009 is 34.1 million acre-feet (57.26 percent of capacity).
RRFW thanks Rick Clayton of the USBOR for his assistance in providing information for this notification.
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