Glen Canyon Dam Operations
The monthly release volume in August 2008 is scheduled to be 890,000 acre-feet. Weekday releases will average about 14,700 cfs with afternoon peak releases to about 18,000 cfs and early morning lows to about 10,000 cfs. Weekend releases will average about 14,000 cfs with afternoon peak releases to about 17,750 cfs and early morning lows to about 10,000 cfs.
The release volume in September is being managed to achieve the objectives of the Equalization Tier of the Interim Guidelines. For water year 2008 the Equalization Tier calls for the annual release volume for Glen Canyon Dam to be managed as practicably as possible to achieve a Lake Mead end of water year elevation of 1105 feet above sea level. In the August 24-month study, the release volume for September that achieves this objective is projected to be 717 KAF which is an average release rate of 12,050 cfs. A two month steady flow experiment will be conducted in September and October 2008. Once this steady flow experiment begins, releases during September will be steady and will not be adjusted. If necessary, Reclamation may make a final adjustment to the September release volume at the end of August 2008 to better achieve the objectives of the Equalization Tier. The October 2008 release volume will be scheduled to maintain consistent steady flows during the steady flow experiment.
The annual release volume for water year 2008 that is projected in the August 24-month study is 8.972 million acre-feet (maf) and the volume of equalization water (volume in excess of 8.23 maf) is 742 thousand acre-feet (kaf). The projected end of year elevation at Lake Powell is 3630.41 feet above sea level which is about 32 feet above the elevation at the beginning of water year 2008 (October 1, 2007). During water year 2008 the live storage in Lake Powell will likely have increased by nearly 3 maf and will likely end water year 2008 at 61% of full capacity. The April through July unregulated inflow to Lake Powell in water year 2008 was 8.84 maf which is 111% of 30-year average (1971 2000).
Upper Colorado River Basin Hydrology
Precipitation in the basin above Lake Powell was above average in July (150% of average). The overall precipitation in the Upper Colorado River Basin for water year 2008 so far has been 105% of average.
The unregulated inflow to Lake Powell during the April through July period was 8.84 maf (111% of average). Unregulated inflow to Lake Powell over the next 3 months (August through October) is projected to be above average (106%). The long range outlook for water year 2009 projects that the most probable unregulated inflow to Lake Powell will be 91% of the 30-year average (1971-2000) however there is a wide range of uncertainty associated with these long range outlooks.
Upper Colorado River Basin Drought
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 (above Lake Mead) projected to be about 58% of capacity at the end of water year 2008.
Reservoir storage in Lake Powell and Lake Mead has decreased during the past 8 years but is projected to increase by the end of water year 2008. Current reservoir storage in Lake Powell is 63 percent of capacity. Storage in Lake Mead is 49 percent of capacity.
This release courtesy Rick Clayton, Hydraulic Engineer, Upper Colorado Region US Bureau of Reclamation
Glen Canyon Dam Operations