RRFW Riverwire Glen Canyon Dam Update
August 19, 2013
The unregulated inflow volume to Lake Powell in July was 143 thousand acre-feet (kaf) (13% of average). The release volume from Glen Canyon Dam in July was 848 kaf. The end of July elevation and storage of Lake Powell were 3594.2 feet (106 feet from full pool) and 11.20 million acre-feet (maf) (46% of full capacity), respectively. The reservoir elevation peaked in mid-June at 3601.2 ft and is now declining. The elevation will continue to decline through the fall and winter until spring runoff in 2014.
The operating tier for 2013 is the Upper Elevation Balancing Tier, as established in August 2012 and pursuant to the Interim Guidelines. Since the April 2013 projected end of water year elevation at Lake Powell was below the 2013 Equalization Elevation of 3,646.0 feet and the projected end of water year elevation at Lake Mead was above elevation 1,075.0 feet, Section 6.B.1 and 6.B.4 of the Interim Guidelines provide for an annual release volume of 8.23 maf from Lake Powell during water year 2013. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible an 8.23 maf annual release volume by September 30, 2013.
Releases from Glen Canyon Dam in August are currently averaging approximately 13,000 cfs with daily fluctuations between approximately 9,000 cfs at nighttime and approximately 17,000 cfs during the daytime and consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The scheduled release volume for August 2013 is 800 kaf.
In September, the release volume will likely be about 600 kaf, with daily fluctuations for hydropower between approximately 7,000 cfs in the nighttime and approximately 13,000 cfs in the daytime. In October, the release volume will likely be about 480 kaf with daily fluctuations between approximately 5,000 cfs and 10,000 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 1,200 cfs above or below the hourly scheduled release rate. Under system normal conditions, fluctuations for regulation are typically short lived and generally balance out over the hour with minimal or no 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 1,200 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.
Inflow Forecasts and Model Projections
The hydrologic forecast for Lake Powell, issued by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume for water year 2013 will be 4.33 maf (40% of average based on the period 1981-2010). The water year 2013 forecast decreased from last month, due to significantly below average inflows in July. Based on the current forecast, the August 24-Month study projects Lake Powell elevation will decline approximately 8 feet through August and September and end the water year at 3585.7 feet with 10.4 maf in storage (43% capacity). The annual release volume from Lake Powell during water year 2013 is scheduled to be 8.23 maf. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible an 8.23 maf annual release volume by September 30, 2013.
The hydrologic forecast for Lake Powell for water year 2014 projects that the most probable (median) unregulated inflow volume will be 8.32 maf (77% of average based on the period 1981-2010). At this early point in the season, there is significant uncertainty regarding next year's water supply. The forecast ranges from a minimum probable (90% exccedence) of 5.0 maf (46% of average) to a maximum probable (10% exceedence) of 15.5 maf (143% of average). There is a 10% chance that inflows could be higher than the maximum probable and a 10% chance they could be lower than the minimum probable.
Consistent with Section 6.C.1 of the Interim Guidelines, if the August 24-Month study projects the January 1, 2014, Lake Powell elevation to be less than 3,575.0 feet and at or above 3,525.0 feet and the Lake Mead elevation to be at or above 1,025.0 feet, the operational tier for Lake Powell in water year 2014 will be the Mid-Elevation Release Tier and the water year release volume from Lake Powell would be 7.48 maf. The August 2013 24-Month study projects that, with an 8.23 maf annual release pattern in water year 2014, the January 1, 2014, Lake Powell elevation would be 3,573.69 feet and the Lake Mead elevation would be 1,107.39 feet. Therefore, consistent with Section 6.C.1 of the Interim Guidelines, the Lake Powell operational tier for water year 2014 is the Mid-Elevation Release Tier with an annual release volume of 7.48 maf. This determination will be documented in the 2014 AOP, which is currently in the final stages of development.
Upper Colorado River Basin Hydrology
Since 2005 the Upper Colorado River Basin has experienced significant year to year hydrologic variability. During the period 2005 through 2012, the unregulated inflow to Lake Powell, which is a good measure of hydrologic conditions in the Colorado River Basin, averaged a water year volume of
10.22 maf (94% of average (period 1981-2010)). The unregulated inflow has ranged from a low of 4.91 maf (45% of average) in water year 2012 to a high of 15.97 maf (147% of average) in water year 2011. This has been an improvement over the persistent drought conditions of 2000 to 2004, which averaged a water year unregulated inflow of 5.73 maf. However, based on observed inflows and current forecasts, water year 2013 unregulated inflow is expected to be 4.33 maf (40% of average), which would be a second significantly below-average year in a row. If this occurs, the period 2000-2013 would be the driest 14-year period on record with an average annual unregulated inflow of 8.20 maf per year. (For comparison, the standard 1981-2010 period average is 10.83 maf).
At the beginning of water year 2013, total system storage in the Colorado River Basin was 33.9 maf (57 % of capacity), which was an increase of about 4 maf since water year 2005 which began at 29.8 maf (50% of capacity). Since 2005, however, total Colorado Basin storage has experienced year to year increases and decreases in response to wet and dry hydrology. In addition, conditions in both 2012 and 2013 have been significantly drier than average and based on observed inflows and current forecasts, the current projected end of water year 2013 total Colorado Basin reservoir storage is approximately 29.0 maf (49% of capacity).
This update courtesy of Katrina Grantz, Bureau of Reclamation
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