Join Date: Dec 2004
RRFW Riverwire Glen Canyon Dam Update
RRFW Riverwire - Glen Canyon Dam Update
September 11, 2009
Glen Canyon Dam and Lake Powell
The unregulated inflow volume into Lake Powell during August 2009 was 0.323 million acre-feet (maf) which was 52% of average based on the period from 1971-2000. This was well below the unregulated inflow volume that was forecasted at the beginning of August (0.550 maf). As a result, the elevation of Lake Powell at the end of August was about 2.5 feet lower than projected in the August 24Month Study. The release volume during August from Glen Canyon Dam was 0.802 maf and the elevation of Lake Powell on August 31, 2009 was 3637.40 feet above sea level. The current forecast of the most probable (median) unregulated inflow volume for September through November is 1.350 maf which is 86% of average.
During September thus far through September 10, the unregulated inflow volume to Lake Powell was only 70,500 acre-feet. At this rate, the monthly volume of unregulated inflow would be approximately 211,000 acre-feet which would be well below the forecasted level of 400,000 acre-feet. It is likely that the elevation of Lake Powell at the end of September will be on the order of 1 foot lower than the elevation projected for the end of September in the September 24-month study which was 3636.7 feet above sea level.
During August 2009, releases from Glen Canyon Dam followed a daily fluctuation pattern for power generation that included an afternoon peak to approximately 16,500 cfs with an early morning low release rate of approximately 8,500 cfs. During the last 3 days of August, the fluctuation pattern was tapered by reducing the afternoon peak each day and increasing the early morning low release rate to 10,000 cfs. These modifications were made to provide a transition from the fluctuating release regime to a steady release regime which began on September 1, 2009.
During September and October, releases from Glen Canyon Dam will be held steady at a targeted release rate of 10,000 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 September will be approximately 0.595 maf which will achieve a water year release volume of 8.23 maf for water year 2009 pursuant to the Colorado River Interim Guidelines for Lower Basin Shortages and Coordinated Operations for Lakes Powell and Mead (Interim Guidelines). The release volume for October will likely be near 0.615 maf as a continuation of the steady release period
In water year 2010, the unregulated inflow to Lake Powell is projected to have an 80% probability of being within the range between 5 maf and 18 maf. There is an estimated 10% probability that the 2010 unregulated inflow volume will be below 5 maf and there is also an estimated 10% probability that the 2010 unregulated inflow volume will be greater than 18 maf.
Based on the range of probable inflow volumes and through implementation of the Interim Guidelines, there is approximately a 65% probability that Equalization will occur in 2010. The determination of whether or not Equalization will occur in 2010 will be based on the results of the 2010 April 24 Month Study. If Equalization does occur in 2010, the water year release volume would be approximately 10.4 maf or greater. If however, Equalization does not occur in 2010 (35% probability), the water year release volume would be 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 1, 2009 is approximately 100% of average based on the 30 year average for the period from 1971 through 2000. Early in the water year, October and November 2008, accumulated precipitation was well below average but rebounded in December with the monthly accumulated precipitation estimated to be approximately 185% of average within the Upper Colorado River Basin. Below average precipitation returned during the winter months (January, February and March) followed by above average precipitation during the spring months (April, May and June). Of note, in June the accumulated precipitation was estimated to be 215% of average. Below average accumulated precipitation returned in July and August and projected to persist through the end of water year 2009.
The Climate Prediction Center outlook (dated August 20, 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 1, 2009 the storage in Lake Powell was 15.7 million acre-feet (65 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 34.8 million acre-feet (58.5 percent of capacity).
RRFW thanks Rick Clayton of the USBOR for his assistance in providing information for this notification.
RIVERWIRE is a free service to the community of river lovers from River Runners for Wilderness. To join, send an e-mail address to [email protected] and we'll add it to the RRFW RIVERWIRE e-mail alerts list.
Join RRFW's listserver to stay abreast of and participate in the latest river issues. It's as easy as sending a blank e-mail to [email protected].
Check out RRFW's Rafting Grand Canyon Wiki for free information on Do-It-Yourself Grand Canyon rafting info http://www.rrfw.org/RaftingGrandCanyon/Main_Page.
Check out new items and donate at the RRFW Store! RRFW is a non-profit project of Living Rivers.