Glen Canyon Dam / Lake Powell –The unregulated inflow into Lake Powell during June was 2.71 million acre-feet (maf) which was 88% of average based on the period from 1971-2000. The most probable (median) inflow forecast for June was 2.3 maf so the actual unregulated inflow for the month was 410,000 acre-feet above what was expected. For this reason, the end of month elevation of Lake Powell for June is about 2 feet higher than previously projected at 3,640.49 feet above sea level. Through the first 3 month of the April through July runoff period, Lake Powell has received 6.419 maf of unregulated inflow. The most probable (median) inflow forecast for July increased to 1.123 maf and combined with the unregulated inflow received thus far, the updated April through July unregulated inflow forecast for Lake Powell is now 7.55 maf (95% of average).
During July 2009, the scheduled release volume for Lake Powell is 800,000 acre-feet which will result in a daily average release of approximately 13,000 cfs. During weekdays, the afternoon peak release from Glen Canyon Dam will be approximately 16,500 cfs with morning lows of approximately 8,500 cfs. During weekends the afternoon peak release will be approximately 16,250 cfs with morning lows of approximately 8,500 cfs. The scheduled release volume for August is also 800,000 cfs and daily fluctuations should be very similar to July.
Beginning on September 1, 2009 and continuing through October 31, 2009 the releases from Glen Canyon Dam will be steady with no fluctuations for power production (excluding system regulation and spinning reserves) for a steady flow experiment pursuant to the 'Final Environmental Assessment and Finding of No Significant Impact Experimental Releases from Glen Canyon Dam, Arizona 2008 through 2012'. The projected release rate currently being targeted is 10,000 cfs which is equivelant to a monthly release volume of approximately 595,000 acre-feet.
The water year release volume for 2009 is 8.23 maf pursuant to the 'Colorado River Interim Guidelines for Lower Basin Shortages and Coordinated Operations for Lakes Powell and Mead'. At the end of August, depending the remaining release volume required to achieve 8.23 maf, the release volume scheduled for September could be moderately adjusted which could impact the steady flow rate targeted for this year.
The July 24-Month Study will likely be published by July 10, 2009 and updated elevation projections through water year 2010 will be posted at the following link: Lake Powell Elevations.
Upper Colorado River Basin Hydrology
The overall precipitation rates during October and November 2008 were well below average at approximately 55% and 80% respectively. In December, however, conditions improved significantly with precipitation measuring approximately 185% of average. Unfortunately this wetter trend did not continue with precipitation in January, February and March measuring 95%, 75% and 65% of average respectively. In April and May conditions appeared returned to a wetter pattern with precipitation measured at 120% and 105% of average, respectively. Precipitation in June was well above average and is currently estimated to be about 185% of average. The overall water year precipitation rate through June 30, 2009 is 105% of average.
The Climate Prediction Center outlook(dated June 30, 2009) for temperature over the next 3 months indicates that temperatures in the southwest have an increased probability of being above average while precipitation also has an increased probability of being above 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 and 2008 drought conditions eased somewhat with net gains in storage to Lake Powell. As of June 30, 2009 the storage in Lake Powell was 16.1 million acre-feet (66 percent of capacity) which is below desired levels while the overall storage in the Colorado River Basin also continues to be below desired levels with the overall system storage as of June 30, 2009 of 35.5 million acre-feet which is 59 percent of capacity.
Updated: June 30, 2009
Rick Clayton
Email comments/inquires to: ResourceMgr@usbr.gov