RRFW Riverwire - Glen Canyon Dam Update
December 15, 2012
Recent Reservoir Operations
The unregulated inflow volume to Lake Powell in November was 246 thousand acre-feet (kaf) (52% of average). The release volume from Glen Canyon Dam in November was 730 kaf. The end of November elevation and storage of Lake Powell were 3615.1 feet (85 feet from full pool) and 13.25 maf (54% of full capacity). The reservoir elevation will continue to decline through the fall and winter months.
From November 18-23, 2012, the Department of Interior conducted the first High Flow Experiment under a multi-year High Flow Protocol announced earlier this year by Secretary Salazar. Under this Protocol, high flow releases are linked to sediment input and other resource conditions below Glen Canyon Dam. Beginning on the evening of November 18th, releases from Glen Canyon Dam began ramping up to full power plant capacity (approximately 28,000 cfs). At midday on November 19th, bypass tubes at Glen Canyon Dam were opened and releases continued to increase up to full power plant and bypass capacity (approximately 43,000 cfs) by the evening of November 19th. Releases were maintained at peak release for 24 hours and then began ramping back down. Releases returned to normal operations in the evening of November 23rd. The entire experiment, including ramping lasted 5 days, with 24 hours at peak release. November releases from Glen Canyon Dam prior to and after the High Flow Experiment fluctuated between 7,000cfs and 9,000cfs. The elevation of Lake Powell decreased approximately 2.75 feet during the 5-day experiment. Approximately 77,800 acre-feet was bypassed during the experiment. The total annual release from Glen Canyon Dam in water year 2013 will not change as a result of the High Flow Experiment.
Current and Planned Reservoir Operations
The operating tier for 2013 is the Upper Elevation Balancing Tier, as establish in August 2012 and pursuant to the Interim Guidelines. However, if hydrologic conditions and projections become wetter, it is possible that beginning in April, the Equalization tier will govern the operations of Lake Powell for the remainder of the water year. Based on analysis of a range of inflow scenarios, however, the current probability of realizing an inflow volume that would trigger Equalization in 2013 is less than 5 percent. 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 2013 to achieve the governing operating tier objectives as practicably as possible by September 30, 2013.
Releases from Glen Canyon Dam in December are currently averaging approximately 13,000 cfs with daily fluctuations between 8,250cfs and 16,250cfs and consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The scheduled release volume for December 2012 is 800 kaf.
In January, the release volume will likely be about 800 kaf, with fluctuations throughout the day from about 8,500 cfs in the early morning to about 16,500 cfs in the evening. In February, the release volume will likely be about 600 kaf with daily fluctuations for hydropower between approximately 8,000 cfs and 14,000 cfs.
In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 MW of system regulation. These instantaneous release adjustments stabilize the electrical generation and transmission system and translate to a range of about 1,100 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). Reserves provide system reliability in the event of an unscheduled outage. Glen Canyon Dam typically maintains 43 MW of reserves (approximately 1,100 cfs). Reserve calls can be maintained for a maximum of 2 hours after which time the generation rate should be returned to the original schedule. If reserves from Glen Canyon Dam are called upon, releases from the dam can exceed scheduled levels and can have a noticeable impact on the river downstream from Glen Canyon Dam. Calls for reserves are fairly infrequent and typically are for much less than 43 MW.
Current Inflow Forecasts and Model Projections
The hydrologic forecast for Lake Powell for water year 2013 projects that the most probable (median) unregulated inflow volume will be 5.76 maf (53% of average based on the period 1981-2010). Based on this hydrologic outlook, the December 24-Month study projects the annual release volume for water year 2013 will be 8.23 maf and the end of water year reservoir elevation and storage for Lake Powell will be 3594.8 and 11.26 maf (48% capacity), respectively.
If hydrologic conditions and projections become significantly wetter, it is possible that beginning in April, the Equalization tier will govern the operations of Lake Powell for the remainder of the water year and the release volume for 2013 could be greater than 8.23 maf. However, based on analysis of a range of inflow scenarios, the current probability of realizing an inflow volume that would trigger Equalization in 2013 is less than 5 percent.
Upper Colorado River Basin Hydrology
Since water year 2005, the Upper Colorado River Basin has experienced significant year to year hydrologic variability. The unregulated inflow to Lake Powell, which is a good measure of hydrologic conditions in the Colorado River Basin, has averaged a water year volume of 10.22 maf (94% of average (period 1981-2010)) during the period from 2005 through 2012. The hydrologic variability during this period has been from a low water year unregulated inflow volume of 4.91 maf (45% of average) in water year 2012 to a high water year unregulated inflow volume of 15.97 maf (147% of average) in water year 2011. Based on observed inflows and current forecasts, water year 2013 unregulated inflow is expected to be 5.76 maf (53% of average).
Overall reservoir storage in the Colorado River Basin has increased by over 4 maf since the beginning of water year 2005 and this is an improvement over the persistent drought conditions during water years 2000 through 2004. From the beginning of water year 2005 to the beginning of water year 2013, the total reservoir storage in the Colorado River Basin increased from 29.8 maf (50% of capacity) to 33.9 maf (57 % of capacity). However, this period experienced year to year increases and decreases in total Colorado Basin storage in response to wet and dry hydrology.
RRFW thanks Katrina Grantz, US Bureau of Reclamation, for this update
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