RRFW Riverwire – Glen Canyon Dam Update
May 31, 2017
The unregulated inflow volume to Lake Powell in April was 1,607 thousand acre-feet (kaf) (152 percent of average). The release volume from Glen Canyon Dam in April was 621 kaf. The end of April elevation and storage of Lake Powell were 3,604 ft (96 feet from full pool) and 12.15 maf (50% of full capacity), respectively. The reservoir reached a seasonal low elevation on March 15th near elevation 3593.85 feet. Since that time the reservoir has been increasing as the first of the spring runoff is now entering the reservoir.
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 May 24-Month Study projects a balancing release of 9.0 maf in water year 2017; the actual release in water year 2017, however, will depend on hydrology in the remainder of water year and will range from 8.23 to 9.0 maf. The projected release from Lake Powell in water year 2017 will be updated each month throughout the remainder of the water year. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible the appropriate total annual release volume by September 30, 2017.
In June, the release volume will be approximately 750,000 acre feet, with fluctuations anticipated between about 9,000 cfs in the nighttime to about 15,000 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 July is 850 kaf with daily fluctuations between approximately 9,500 cfs and 17,500 cfs. The expected release for August is 900 kaf with daily fluctuations between approximately 11,000 cfs and 19,000 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 1,200 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 April to July 2017 water supply forecast for unregulated inflow to Lake Powell, issued on May 3, 2017, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 8.8 maf (123 percent of average based on the period 1981-2010). The forecast decreased by 500 kaf since last month. At this point in the season, there is still uncertainty regarding this year’s water supply and the total inflow to Lake Powell. The spring runoff forecast ranges from a minimum probable of 7.2 maf (101 percent of average) to a maximum probable of 11.0 maf (154 percent of average). There is 10% chance that inflows could be higher than the maximum probable and a 10% chance they could be lower than the minimum probable.
As determined in the August 2016 24-Month Study, and documented in the 2017 Annual Operating Plan, Lake Powell’s operations in water year 2017 will be governed by the Upper Elevation Balancing Tier. Starting with an 8.23 million acre-feet (maf) release from Lake Powell in water year 2017, the April 2017 24-Month Study projected the end of water year elevation at Lake Powell to be above 3,575 feet and the end of water year elevation at Lake Mead to be below 1,075 feet. Therefore, in accordance with Section 6.B.4 of the Interim Guidelines, Lake Powell operations shifted to balancing releases 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 May most probable inflow forecast, the annual release volume from Lake Powell during water year 2017 is projected to be 9.0 maf. Under the minimum probable inflow scenario, the water year release is projected to be 9.0 maf. Under the maximum probable inflow scenario, the release is projected to be 9.0 maf. There is 10% chance that inflows will be lower than the current minimum probable forecast, potentially resulting in lower releases. If inflows are less than the minimum probable forecast, the water year 2017 annual release could be as low as 8.23 maf. If inflows are greater than the current forecasted maximum probable inflow, the annual release will be 9.0 maf. The projected release from Lake Powell in water year 2017 will be updated each month throughout the remainder of the water year.
Based on the current forecast, the May 24-Month Study projects Lake Powell elevation will end water year 2017 near 3,638 feet with approximately 15.72 maf in storage (65% capacity). Projections of elevation and storage still have significant uncertainty at this point in the season, primarily due to uncertainty regarding spring runoff and the resulting inflow to Lake Powell. Under the minimum probable inflow scenario, updated in April, the projected end of water year elevation and storage are 3,625 feet and 14.25 maf (59% capacity), respectively. Under the maximum probable inflow scenario, updated in April, the projected end of water year elevation and storage are 3661 feet and 18.62 maf (77% capacity), respectively. Modeling of projected reservoir operations based on the minimum and maximum scenarios will be updated again in August.
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), which, though still below average, was significantly higher than inflows observed in 2012 and 2013 (45% and 47% of average, respectively). Under the current most probable forecast, the total water year 2017 unregulated inflow to Lake Powell is projected to be 12.93 maf (119% 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 2017 is approximately 33.8 maf (57% of total system capacity). The actual end of water year 2017 system storage may vary from this projection, primarily due to uncertainty regarding the season’s snowpack and resulting runoff and reservoir inflow. Based on the April minimum and maximum probable inflow forecasts and modeling, the range of end of water year 2017 total system capacity is approximately 32.47 maf (54%) to 37.0 maf (62%), respectively.
This update courtesy of Paul Davidson, Bureau of Reclamation
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