RRFW Riverwire - Glen Canyon Dam Update
July 9, 2012
Glen Canyon Dam / Lake Powell
For the Water Supply period April through July, 2012 will most likely be the third driest year on record since the closure of Glen Canyon Dam in 1963. Only 1977 and 2002 will have had lower April-July unregulated inflow volumes to Lake Powell than what is most likely to occur in 2012.
The monthly unregulated inflow volume to Lake Powell for June was 353 thousand acre-feet (kaf) (13% of average). This was very nearly equal to the forecasted unregulated inflow volume at the beginning of June which was 350 kaf. The release volume from Glen Canyon Dam in June was 709 kaf which was 1,000 acre-feet above what was scheduled for release during the month. The end of June elevation and storage of Lake Powell was 3633.90 feet (66.10 feet from full pool) and 15.29 million acre-feet (maf) (62.88% of full capacity).
The Water Supply Forecast for Lake Powell (April through July Unregulated Inflow Volume) for July remained unchanged from June at 2.01 maf (28% of average).
Current Dam Operations
In August 2011, pursuant to the Interim Guidelines, the Operating Tier for Glen Canyon Dam was established to be the Equalization Tier. Under the Equalization Tier when conditions dry out as they have this year, the minimum annual release from Lake Powell can generally be as low as 8.23 maf. However, water year 2011 was a very wet Equalization year and not all of the Equalization release volume for 2011 could be achieved by September 30, 2011. As a result, 1.233 maf of the 2011 Equalization release volume was actually released after the end of water year 2011. This increased the minimum release volume for water year 2012 under Equalization to 9.463 maf. Under the dry hydrologic conditions currently projected for Lake Powell, the water year 2012 release volume is projected to be at this minimum Equalization level of 9.463 maf. As hydrologic conditions for Lake Powell and Lake Mead change throughout the year, Reclamation will adjust operations of Glen Canyon Dam to release the appropriate annual volume during 2012 to achieve Equalization objectives as practicably as possible by September 30, 2012.
Releases from Glen Canyon Dam are now averaging about 14,500 cubic feet per second (cfs) with fluctuations for power generation throughout the day that peak near 18,000 cfs in the afternoons and with early morning low level releases are about 10,000 cfs and this operation is consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The release volume for July is scheduled to be 889 kaf and this volume is elevated slightly in order to target a release volume in August of 800 kaf. In August, fluctuations are projected to peak near 17,000 cfs during the afternoons with early morning low releases near 9,000 cfs. In September and October, as part of the 2008 FONSI, releases from Glen Canyon Dam will be steady for a steady flow experiment. The targeted release rate for September and October of 2012 is 8,000 cfs.
In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 megawatts (MW) of system regulation. These instantaneous release adjustments stabilize the electrical generation and transmission system and translate to a range of about 1100 cfs above or below the hourly scheduled release rate. Typically, fluctuations for system regulation are very short lived and balance out over the hour and do not have noticeable impacts on downstream river flow conditions.
Releases from Glen Canyon Dam can also fluctuate beyond scheduled fluctuations for power generation when called upon as a partner that shares reserve requirements within the electrical generator community (i.e. balancing area). There are many generators that supply electricity to the transmission system within the balancing area. At times, a participating generator may experience operating conditions such that it cannot make its scheduled delivery of electricity to the system (i.e. unscheduled outage). To provide system reliability, all participating electricity generators within the balancing area maintain a specified level of generation capacity (i.e. reserves) that can be called upon when an unscheduled outage occurs. Glen Canyon Dam typically maintains 113 MW of reserves for this purpose.
Reserve agreements allow the controllers of the balancing area to call upon Glen Canyon Dam to produce up to an additional 113 MW of electricity beyond what is originally scheduled for a given hour. Reserve calls can be maintained for a maximum of 2 hours after which time the generation rate should be returned to the original schedule. The 113 MW reserve requirement for Glen Canyon Dam translates to approximately 2,800 cfs of flow in the river. When the balancing area controllers call for reserve generation from Glen Canyon Dam, releases from the dam can exceed scheduled levels and have a noticeable impact on the river downstream from Glen Canyon Dam. But these calls for reserves are fairly infrequent and typically are for much less than the required level of 113 MW.
Current Inflow Forecasts and Model Projections
Over the next three months (July, August and September) the forecasted unregulated inflow volume to Lake Powell is projected to be 100 kaf (9% of average), 150 kaf (30% of average) and 200 kaf (49% of average), respectively. These percent of averages are all based on the historic period from 1981 through 2010. The most probable (i.e. 50% likely to be exceeded) unregulated inflow volume for WY2012 is projected to be 5.0 maf (46% of average). Comparing this projected water year unregulated inflow volume to the driest year on record (2002) in which the unregulated inflow volume was only 2.64 maf (24% of average), water year 2012 will likely be very dry, yet not nearly as dry as conditions were in 2002. The currently projected water year unregulated inflow volume of 5.0 maf would rank as the 3rd driest year on record since the closure of Glen Canyon Dam (1963).
The annual release volume from Glen Canyon Dam will likely be 9.463 maf and the elevation of Lake Powell at the end of WY2012 is projected to be 3622.6 feet above sea level which is 77.4 feet from the full pool elevation of 3700 feet. This elevation corresponds to a live storage volume of 14.04 maf (58 % of full capacity). These projections are based on conditions in the July 24-Months Study.
The June 2012 24-Month Study has been published and is available here. The July 2012 24-Month Study will be published by July 13, 2012 and a link to this study will be provided at this website. Updated elevation projections for Lake Powell through water year 2012 based on the most recently published 24-Month Study are maintained at: Lake Powell Projected Elevations.
Upper Colorado River Basin Hydrology
Since water year 2005, hydrologic conditions in the Upper Colorado River Basin have been near average with significant variability from year to year. The unregulated inflow to Lake Powell, which is a good measure of the hydrologic condition in the Colorado River Basin, has averaged a water year volume of 10.98 maf (101% of average (period 1981-2010)) during the period from 2005 through 2011. The hydrologic variability during this period has been from a low water year unregulated inflow volume of 8.62 maf (80% of average) in water year 2006 to a high water year unregulated inflow volume of 15.97 maf (147% of average) which occurred in water year 2011.
Overall reservoir storage in the Colorado River Basin has increased by over 8 maf since the beginning of water year 2005 and this is a significant improvement over the drought conditions during water years 2000 through 2004. On October 1, 2004, the beginning of water year 2005, the total reservoir storage in the Colorado River Basin was 29.84 maf (50.2% of capacity). On October 1, 2011, the beginning of water year 2012, the total reservoir storage in the Colorado River Basin was 38.66 maf (64.8% of capacity). As of July 8, 2012 the total reservoir storage in the Colorado River Basin was 35.80 maf (60.0% of capacity).
RRFW thanks Rick Clayton of the USBOR for this information.
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