RRFW Riverwire Glen Canyon Dam Update, June 14, 2014
The unregulated inflow volume to Lake Powell in May was 2,082 thousand acre-feet (kaf) (89% of average). The release volume from Glen Canyon Dam in May was 493 kaf. The end of May elevation and storage of Lake Powell were 3,589.4 feet (111 feet from full pool) and 10.76 million acre-feet (maf) (44% of full capacity), respectively. The reservoir elevation is increasing as spring runoff begins to enter the reservoir. Basin-wide snowpack peaked at 111% of median on April 7th and runoff is well underway with less than 10% of the measurable snowpack remaining.
In June, the release volume will be approximately 600 kaf, with fluctuations between about 7,000 cubic feet per second (CFS) in the nighttime to about 13,000 cfs in the daytime and consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). In July, the release volume will likely be approximately 800 kaf with daily fluctuations between about 10,000 cfs and 18,000 cfs. The anticipated release volume for August is about 800 kaf with fluctuations between approximately 9,000 cfs and 17,000 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.
The operating tier for water year 2014 is the Mid-Elevation Release Tier with an annual release volume of 7.48 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 by September 30, 2014.
Releases from Glen Canyon Dam can also fluctuate beyond scheduled releases when called upon to respond to unscheduled power outages or power system emergencies. Depending on the severity of the system emergency, the response from Glen Canyon Dam can be significant, within the full range of the operating capacity of the power plant for as long as is necessary to maintain balance in the transmission system. Glen Canyon Dam typically maintains 41MW (approximately 1,200 cfs) of generation capacity in reserve in order to respond to a system emergency even when generation rates are already high. System emergencies occur fairly infrequently and typically require small responses from Glen Canyon Dam. However, these responses can have a noticeable impact on the river downstream of Glen Canyon Dam.
Inflow Forecasts and Model Projections
The forecast for the 2014 April to July water supply season for Lake Powell, issued on June 2, 2014 by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 7.55 maf (105% of average based on the period 1981-2010). The April-July most probable forecast remained the same as last month. The seasonal snowpack peaked on April 7, 2014 at 111% of median and nearly all of the measurable snowpack has now melted. The April-July forecast ranges from a minimum probable of 6.60 maf (92% of average) to a maximum probable of 8.45 maf (118% of average). (For reference, the 30-year April-July average is 7.16 maf.) There is a 10 percent chance that inflows could be higher than the maximum probable and a 10 percent chance they could be lower than the minimum probable.
Based on the current forecast, the June 24-Month study projects Lake Powell elevation will peak near approximately 3,614 feet near the end of July and end the water year near 3,610 feet with approximately 12.7 maf in storage (52% capacity). Note that projections of elevation and storage have uncertainty, primarily due to uncertainty regarding the spring runoff and resulting inflow to Lake Powell. Under the minimum probable inflow scenario, last updated in April, the projected summer peak is 3,599 ft and end of water year storage is 11.0 maf (45% capacity). Under the maximum probable inflow scenario, updated in April, the projected summer peak is 3,632 ft and end of water year storage is 14.9 maf (61% capacity). There is a 10 percent chance that inflows will be higher, resulting in higher elevation and storage, and 10 percent 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 and documented in the 2014 Annual Operating Plan signed by Secretary Jewell in December 2013.
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 2000-2013 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 total water year average is 10.83 maf.) 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. Under the current forecast, total water year 2014 unregulated inflows to Lake Powell are expected to range between a minimum probable of 9.8 maf (91% of average) and a maximum probable of 11.7 maf (108% of average) with a most probable projection of 10.78 maf (100% of average).
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. One wet year can significantly increase total system reservoir storage, just as persistent dry years can draw down the system storage. Based on current inflow forecasts, the current projected end of water year 2014 total Colorado Basin reservoir storage is approximately 30.4 maf (51% of capacity). The actual end of water year storage may vary significantly from this projection, primarily due to uncertainty regarding this season’s runoff. Based on April minimum and maximum probable inflow forecasts and modeling the range is approximately 28.4 maf (48%) to 33.2 maf (56%), respectively.
This update courtesy of Katrina Grantz, Bureau of Reclamation
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