Glen Canyon Dam Operations
The monthly release volume for November 2008 is scheduled to be 600,000 acre-feet. During the months of September and October, releases from Glen Canyon Dam were steady at approximately 12,080 cubic feet per second (cfs) as described in the Final Environmental Assessment for Experimental Releases from Glen Canyon Dam, Arizona, 2008 through 2012 (EA).
Beginning on Saturday November 1, 2008 operation of Glen Canyon Dam will begin daily fluctuations for power generation as described in the Operating Criteria for Glen Canyon Dam and the 1996 Record of Decision (ROD) on the Operation of Glen Canyon Dam. Daily average releases during November will be about 10,000 cfs. Monday through Friday releases will peak each afternoon to about 13,000 cfs with early morning releases of approximately 7,000 cfs. Weekend peak flows will be about 12,750 cfs with morning low releases near 7,000 cfs.
The water year release volume from Glen Canyon Dam during water year 2008 was 8.978 million acre feet (maf) with an equalization volume of 748,000 acre-feet. This volume was based on actual and forecasted inflow and reservoir operation conditions for Lake Powell and Lake Mead under the Equalization Tier of the Interim Guidelines. The 2008 water year ending elevation and storage for Lake Powell was 3626.90 feet above sea level and 14.51 maf respectively. This was a 2.58 maf increase in storage year over year. Lake Powell begins water year 2009 at 59.6% of full capacity.
Under the Interim Guidelines, the water year 2009 operational tier is Upper Elevation Balancing. Under the Upper Elevation Balancing Tier, the projected release volume from Glen Canyon Dam for water year 2009 is 8.23 maf. As described in section 6.B.3 of the Interim Guidelines, if the April 2009 24-month study projects Lake Powell’s end of water year 2009 reservoir elevation to be above 3639 feet above see level, the Equalization Tier would govern for the remainder of water year 2009. Under the Equalization Tier, it is possible for the water year release volume to be greater than 8.23 maf.
Actual and forecasted inflows to Lake Powell have been declining over the past several months. The volume of unregulated inflow to Lake Powell in October was 372,290 acre-feet (67% of average). The forecasted unregulated inflow for months of November, December and January is 1.05 million acre-feet (79% of average).
Upper Colorado River Basin Hydrology
Precipitation in the basin above Lake Powell was below average during the summer months and has continued through September. Precipitation during June, July and August 2008 was 70%, 65% and 90% of average respectively with precipitation in September measured at 70% of average. The overall precipitation in the Upper Colorado River Basin for water year 2008 was near average (about 101%). Precipitation in October 2008 was 65% of average. The unregulated inflow to Lake Powell during the April through July 2008 was 8.906 maf (112% of average). The long range outlook for unregulated inflow to Lake Powell for water year 2009 is projected to be 10.58 maf (88% of the 1971-2000 average).
The Upper Colorado River Basin is experiencing a protracted multi-year drought. Since 1999, inflow to Lake Powell has been below average in every year except water year 2005 and 2008. In the summer of 1999, Lake Powell was essentially full with reservoir storage at 23.5 million acre-feet, or 97 percent of capacity. Inflow to Lake Powell in 1999 was 109 percent of average. The manifestation of drought conditions in the Upper Colorado River Basin began in the fall months of 1999. A five year period of extreme drought occurred in water years 2000, 2001, 2002, 2003, and 2004 with unregulated inflow to Lake Powell only 62, 59, 25, 51, and 49 percent of average, respectively. Lake Powell storage decreased through this five-year period, with reservoir storage reaching a low of 8.0 million acre-feet (33 percent of capacity) on April 8, 2005.
Drought conditions eased in water year 2005 in the Upper Colorado River Basin. Precipitation was above average in 2005 and unregulated inflow to Lake Powell was 105 percent of average. Lake Powell increased by 2.77 million acre-feet (31 feet in elevation) during water year 2005. But as is often the case, one favorable year does not necessarily end a protracted drought. In 2006, there was a return to drier conditions in the Colorado River Basin. Unregulated inflow to Lake Powell in water year 2006 was only 71 percent of average.
Water year 2007 was another year of below average inflow with unregulated inflow into Lake Powell at 68 percent of average. Over the past 9 years (2000 through 2008, inclusive), inflow to Lake Powell has been below average in all but two years (2005 and 2008). Drought conditions have eased in water year 2008 with above average inflows to the main stem Colorado River reservoirs (with the exception of Flaming Gorge and Fontenelle Reservoirs). Reservoir storage in the Colorado River Basin, however, is still below desired levels with the overall Colorado River system storage at the beginning of water year 2009 of 34.0 maf which is 59.3% of capacity.
This release was posted courtesy Rick Clayton, Hydraulic Engineer, Upper Colorado Region US Bureau of Reclamation.