RRFW Riverwire – Glen Canyon Dam Update
Aug 24, 2017
The April to July 2017 unregulated inflow to Lake Powell was 8.17 million acre-feet (maf) (114% of average). The unregulated inflow in July was 1.073 maf (117% of average). The release volume from Glen Canyon Dam in July was 0.850 maf. The end of July elevation and storage of Lake Powell were 3,634.69 feet (65 feet from full pool) and 15.38 maf (63% of full capacity), respectively. The reservoir elevation peaked at 3,635.80 feet on July 11th and is now in its seasonal decline through the fall and winter months.
In August, the release volume will be approximately 900 thousand acre-feet (kaf), with fluctuations anticipated between about 10,000 cubic feet per second (cfs) in the nighttime to about 18,000 cfs in the daytime and consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The anticipated release volume for September is 663 kaf with daily fluctuations between approximately 8,500 cfs and 14,500 cfs. The expected release for October is 630 kaf with daily fluctuations between approximately 7,300 cfs and 13,000 cfs.
The operating tier for water year 2017 was established in August 2016 as the Upper Elevation Balancing Tier. The April 2017 24-Month Study established that Lake Powell operations will be governed by balancing for the remainder of water year 2017. Under balancing, the contents of Lake Powell and Lake Mead will be balanced by the end of the water year, but not more than 9.0 maf and not less than 8.23 maf shall be released from Lake Powell. Based on the most probable inflow forecast, this August 24-Month Study projects a balancing release of 9.0 maf in water year 2017. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible the appropriate total annual release volume by September 30, 2017.
The operating tier for water year 2018, established this August 2017, is the Upper Elevation Balancing Tier, with an initial water year release volume of 8.23 maf and the potential for an April 2018 adjustment to equalization or balancing releases. Based on the current forecast, an April adjustment to balancing releases is projected to occur and Lake Powell is currently projected to release 9.0 maf in water year 2018. This projection will be updated each month throughout the water year.
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 1,200 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 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 currently maintains 27 MW (approximately 800 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 water year 2018 unregulated inflow to Lake Powell, issued on August 1, 2017, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume next year will be 9.76 maf (90% of average). There is significant uncertainty regarding next season’s snow pack development and resulting runoff into Lake Powell. The forecast ranges from a minimum probable of 6.0 maf (55%) to a maximum probable of 17.5 maf (162%). There is a 10% chance that inflows could be higher than the current maximum probable forecast and a 10% chance that inflows could be lower than the minimum probable forecast.
Based on the current forecast, the August 24-Month Study projects Lake Powell elevation will end water year 2017 near 3,631 feet with approximately 15.0 maf in storage (62% capacity) and water year 2018 near 3,635 feet with approximately 15.4 maf in storage (63% capacity). Note that projections of elevation and storage for water year 2018 have significant uncertainty at this point in the season. Projections of elevation and storage using the minimum and maximum probable inflow forecast are 3,607 feet (12.4 maf, 51% capacity) and 3,654 feet (17.7 maf, 73% capacity), respectively. Under these scenarios, 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 2018 is projected to be 9.0 maf under the minimum and most probable inflow scenarios and 13.8 maf under the maximum probable inflow scenario. There is a chance that inflows could be higher or lower, potentially resulting in releases greater than 13.8 maf or as low as 8.23 maf in water year 2018.
Upper Colorado River Basin Hydrology
The Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 17-year period 2000 to 2016, 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 17 years. The period 2000-2016 is the lowest 17-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.57 maf, or 79% 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 2000-2016 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. In water year 2016 unregulated inflow volume to Lake Powell was 9.62 maf (89% of average). Under the current most probable forecast, the total water year 2017 unregulated inflow to Lake Powell is projected to be 12.21 maf (113% of average).
At the beginning of water year 2017, total system storage in the Colorado River Basin was 30.2 maf (51% of 59.6 maf total system capacity). This is nearly the same as the total storage at the beginning of water years 2015 and 2016 which began at 30.1 maf and 30.3 maf, respectively, both of which were 51% 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 2005. 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 total Colorado Basin reservoir storage for water year 2018 is approximately 32.85 maf (55% of total system capacity). The actual end of water year 2018 system storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and reservoir inflow. Based on the August minimum and maximum probable inflow forecasts and modeling, the range of end of water year 2018 total system capacity is approximately 29.4 maf (49%) to 35.6 maf (60%), respectively.
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