The unregulated inflow volume into Lake Powell during September 2009 was 0.265 million acre-feet (maf) which was 56% of average based on the period from 1971-2000. This was below the unregulated inflow volume that was forecasted at the beginning of September, which was 0.400 maf.
As a result, the elevation of Lake Powell at the end of September was about 1.3 feet lower than projected in the September 24-Month Study.
For water year 2009, the annual volume released from Lake Powell was 8.235 maf and the end of water year elevation of Lake Powell was 3635.37 feet above sea level (64.63 feet from full pool). The end of water year elevation for 2009 was 8.5 feet above the end of water year elevation recorded for 2008. The 2009 end of water year storage content was 15.36 maf which was 63.2% of full capacity (24.322 maf). The unregulated inflow to Lake Powell during water year 2009 was 10.63 maf which is 88% of the average based on the historic period from 1971-2000.
During September and October, releases from Glen Canyon Dam have been and will continue to be steady at a targeted release rate of 10,000 cubic feet per second (cfs) pursuant to the ‘February 2008 Finding of No Significant Impact Experimental Releases from Glen Canyon Dam, Arizona 2008 through 2012’ and consistent with the ‘Final Environmental Assessment – Experimental Releases from Glen Canyon Dam, Arizona, 2008 through 2012’. Fluctuations for power system regulation and spinning reserves will occur if necessary during this steady release period. The release volume for October will likely be near 0.615 maf as a continuation of the steady release period
As of October 1, 2009, the unregulated inflow to Lake Powell for 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 55% 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 would be approximately 10.5 maf. If however, Equalization does not occur in 2010 (45% probability), the water year release volume could be 8.23 maf or possibly 9.0 maf depending on the projected September 30 Lake Mead water surface elevation in the April 24-Month Study. 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 80% of average occurring in July, August and September respectively.
The Climate Prediction Center outlook (dated September 17, 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 September 30, 2009 the storage in Lake Powell was 15.46 million acre-feet (63.6 percent of capacity) which is still below desired levels while the overall reservoir storage in the Colorado River Basin as of September 1, 2009 is 34.2 million acre-feet (57.5 percent of capacity).
RRFW thanks Rick Clayton of the USBOR for his assistance in providing information for this notification.