RRFW Riverwire - Glen Canyon Dam Update
October 21, 2013
Releases from Glen Canyon Dam in October are currently averaging approximately 8,000 cubic Feet per second (cfs) with daily fluctuations between approximately 5,000 cfs at nighttime and approximately 10,000 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 October 2013 is 480 thousand acre feet (kaf).
The anticipated release volume for November is 500 kaf with fluctuations for power generation throughout the day consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). However, the release volume may be adjusted in the event of a High Flow Experiment. Under the High-Flow Protocol, high flow releases are linked to sediment input and other resource conditions below Glen Canyon Dam. Preliminary analysis appears favorable for a high flow experimental release to occur during the period of November 8 to 18, 2013. During the High Flow Experiment, total releases from Glen Canyon Dam at full bypass may reach approximately 37,200 cfs. The total experiment, including ramping, could last up to about five and a half days. In the event of a high flow experiment, releases from Glen Canyon Dam prior to and after the high flow experiment are anticipated to fluctuate between 5,000cfs and 8,000cfs.
In December, the release volume will likely be about 600 kaf, with fluctuations throughout the day for hydropower generation.
In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 Mega Watts (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.
The operating tier for water year 2014 is the Mid-Elevation Release Tier with an annual release volume of 7.48 million acre feet (maf), as established in August 2013 and pursuant to the Interim Guidelines, Section 6.C.1. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible a 7.48 maf annual release volume by September 30, 2014.
The unregulated inflow volume to Lake Powell in September was 857 kaf (210% of average). The release volume from Glen Canyon Dam in September was 600 kaf. The end of September elevation and storage of Lake Powell were 3591.3 feet (108.7 feet from full pool) and 10.93 maf (45% of full capacity), respectively. Due to above average runoff from monsoonal activity in September, Lake Powell elevation increased by about 2 feet over an 11-day period in September. The reservoir elevation is now declining and will continue to decline through the fall and winter until spring runoff in 2014.
The water year 2013 unregulated inflow volume was 5.12 maf (47% of average), placing 2013 as the fourth driest on record since the closure of Glen Canyon Dam in 1963. Water years 2002, 1977, and 2012 were drier, receiving 2.64 maf, 3.53 maf, and 4.91 maf, respectively. In terms of reservoir elevation and storage, Lake Powell reached its peak for water year 2013 on June 18 at 3,601.2 ft (98.8 feet from full pool) which is 35.7 feet lower than last year’s peak elevation of 3636.9 ft. The end of water year 2013 elevation and storage of Lake Powell were 3591.3 feet (108.7 feet from full pool) and 10.93maf (45% of capacity), respectively. This is 3.0 maf less than 2012 end of water year storage which was 13.93 maf (57% of capacity).
Releases for Water Year 2013 totaled 8.232 maf. Pursuant to the Interim Guidelines, Lake Powell operated under the Upper Elevation Balancing Tier in 2013. Throughout water year 2013, Reclamation adjusted operations of Glen Canyon Dam to release the appropriate annual volume during 2013 to achieve Upper Elevation Balancing Tier objectives as practicably as possible by September 30, 2012.
Inflow Forecasts and Model Projections
The hydrologic forecast for water year 2014 for Lake Powell, issued by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 9.65 maf (78% of average based on the period 1981-2010). The water year 2013 forecast increased by 1.24 maf since last month, primarily due to much higher than expected monsoonal precipitation and runoff in September. At this early point in the season, there is significant uncertainty regarding next year’s water supply. The forecast ranges from a minimum probable of 6.5 maf (60% of average) to a maximum probable of 16.5 maf (162% 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.
Based on the current forecast, the October 24-Month study projects Lake Powell elevation will peak near approximately 3,604 ft next summer and end the water year near 3,598 feet with approximately 11.6 maf in storage (48% capacity). Note that projections of elevation and storage have significant uncertainty at this point in the season, primarily due to uncertainty regarding next season’s snowpack and resulting inflow to Lake Powell. Under the minimum probable inflow scenario, the projected summer peak is
3,586 ft and end of water year storage is 9.3 maf (38% capacity). Under the maximum probable inflow scenario the projected summer peak is 3,661 ft and end of water year storage is 18.4 maf (76% capacity). There is a 10% chance that inflows will be higher, resulting in higher elevation and storage, and 10% chance that inflows will be lower, resulting in lower elevation and storage. The annual release volume from Lake Powell during water year 2014 is projected to be 7.48 maf under all inflow scenarios.
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 was determined in the August 2013 24-Month study tier determination run which projected that, with an 8.23 maf annual release pattern in water year 2014, the January 1, 2014, Lake Powell elevation would be below 3,575.0 feet and the Lake Mead elevation would be above 1,025.0 feet. This determination will be documented in the 2014 AOP, which is currently in the final stages of development.
Upper Colorado River Basin Hydrology
The Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 14-year period 2000 to 2013, however, the unregulated inflow to Lake Powell, which is a good measure of hydrologic conditions in the Colorado River Basin, was above average in only 3 out of the past 14 years. The period 2000-2014 is the lowest 14-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.25 maf, or 76% of the 30-year average (1981-2010). (For comparison, the 1981-2010 average is 10.83maf.) The unregulated inflow during the 2000-2013 period has ranged from a low of 2.64 maf (24% of average) in water year 2002 to a high of 15.97 maf (147% of average) in water year 2011. One wet year can significantly increase total system reservoir storage, just as persistent dry years can draw down the system storage.
At the beginning of water year 2014, total system storage in the Colorado River Basin was 29.9 maf (50% of 59.6 maf total system capacity). This is about 4 maf less than the total storage at the beginning of water year 2013 which began at 34.0 maf (57% of capacity). Since the beginning of water year 2000, total Colorado Basin storage has experienced year to year increases and decreases in response to wet and dry hydrology, ranging from a high of 94% of capacity at the beginning of 2000 to a low of 50% of capacity at the beginning of water year 2014. Based on current forecasts, the current projected end of water year 2014 total Colorado Basin reservoir storage is approximately 29.6 maf (50% of capacity).
This update courtesy of Katrina Grantz, Bureau of Reclamation
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