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RRFW Riverwire Glen Canyon Dam Update
RRFW Riverwire Glen Canyon Dam Update
December 15, 2008
Glen Canyon Dam Operations
The monthly release volume for December 2008 and January 2009 is scheduled to be 800,000 acre-feet for each month. Daily average releases during December and January will be about 13,000 cubic feet per second (cfs). Monday through Friday releases will peak each afternoon to about 17,000 cfs with early morning releases of approximately 9,000 cfs. Weekend peak flows will be about 16,750 cfs with morning low releases near 9,000 cfs.
Unregulated inflow to Lake Powell for the first 2 month of water year 2009 was 785,000 acre-feet (71% of average). The forecasted unregulated inflow for December, January and February is 1,050,000 acre-feet (83% of average). Next month, the Colorado Basin River Forecast Center will issue the first water supply forecast for Lake Powell(April through July 2009 Unregulated Inflow Volume) for 2009. As of December 9, 2008 the snowpack conditions above Lake Powell are well below average at 61% of average.
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 million acre feet (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 the Equalization Level for 2009 (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.
Upper Colorado River Basin Hydrology
The overall precipitation in the Colorado River Basin during water year 2008 was near average (101% of average). During the summer months, however, precipitation was persistently below average with June, July and August 2008 at 70%, 65% and 90% of average respectively. Precipitation during the fall has continued to be below average with September, October and November 2008 at 70%, 55% and 80% of average respectively. The 3-month climate outlook from the Climate Prediction Center for the Upper Colorado River Basin predicts near average precipitation and near average temperatures for December, January and February.
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 10.59 maf (88% of the 1971-2000 average).
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 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 as of December 1, 2008 of 33.4 maf which is 59.5% of capacity.
This release was posted courtesy Rick Clayton, Hydraulic Engineer, Upper Colorado Region US Bureau of Reclamation.
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