RRFW Riverwire - Glen Canyon Dam Update
March 17, 2013
Glen Canyon Dam/Lake Powell
The unregulated inflow volume to Lake Powell in February was 262 thousand acre-feet (kaf) (67% of average). The release volume from Glen Canyon Dam in February was 600 kaf. The end of February elevation and storage of Lake Powell were 3601.5 feet (98.5 feet from full pool) and 11.89 maf (49% of full capacity). The reservoir elevation will continue to decline until snowmelt and spring runoff begin to fill the reservoir in late spring.
Releases from Glen Canyon Dam in March are currently averaging approximately 10,000 cubic feet per second (cfs) with daily fluctuations consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). Releases in March are currently fluctuating between a nighttime low of approximately 7,000cfs and a peak in the mid-morning and again in the evening of approximately 13,000cfs. The scheduled release volume for March 2013 is 600 kaf.
In April, the release volume will likely be about 550 kaf, with fluctuations for hydropower of approximately 6,500 cfs in the nighttime and double peaking up to 11,500 cfs during the mid-morning and evening hours. In May, the release volume will likely be about 600 kaf with double-peaking fluctuations between approximately 7,000 cfs and 13,000 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 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.
The operating tier for 2013 is the Upper Elevation Balancing Tier, as establish in August 2012 and pursuant to the Interim Guidelines. However, if the April 24-Month study projects the September 30 Lake Powell elevation to be greater than the 2013 Equalization elevation of 3,646.0 feet, the Equalization Tier will govern operations of Lake Powell for the remainder of the water year and the water year release volume from Lake Powell may be greater than 8.23 maf. Based on analysis of a range of inflow scenarios, an adjustment to Equalization is not anticipated in 2013. 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.
Inflow Forecasts and Model Projections
The hydrologic forecast for Lake Powell for the April to July water supply season projects that the most probable (median) unregulated inflow volume will be 3.4 maf (47% of average based on the period 1981-2010). The forecast has continued to decrease throughout the winter season. Based on the current forecast, the March 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 3589.5 and 10.78 maf (44% capacity), respectively.
Consistent with Section 6.B.3 of the Interim Guidelines, if hydrologic conditions and projections become dramatically 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, given current conditions and observed inflows, the Powell unregulated inflow forecast would need to increase by over 7 maf over the next several weeks to trigger an adjustment to equalization this year. Based on analysis of a range of inflow scenarios and forecasts, an adjustment to equalization is not anticipated in 2013.
Upper Colorado River Basin Hydrology
Since 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 5.31 maf (49% of average).
Overall reservoir storage in the Colorado River Basin has increased by approximately 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, during this time, total Colorado Basin storage experienced year to year increases and decreases in response to wet and dry hydrology. Given observed inflows and current forecasts, the projected end of water year 2013 total reservoir storage is approximately 29.7 maf (50% of capacity).
Update courtesy of Katrina Grantz, US Bureau of Reclamation
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