RRFW Riverwire Glen Canyon Dam Update
May 22, 2016
Glen Canyon Dam Lake Powell
In May, the release volume will be approximately 700 thousand acre feet (kaf), with fluctuations anticipated between about 8,000 cubic feet per second (cfs) in the nighttime to about 14,000 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 June is 800 kaf with daily fluctuations between approximately 9,000 cfs and 17,000 cfs. The expected release for July is 950 kaf with daily fluctuations between approximately 11,500 cfs and 19,500 cfs. The anticipated release volume for August 2016 is 900,000 kaf.
The operating tier for water year 2016 was established in August 2015 as the Upper Elevation Balancing Tier. The April 2016 24-Month Study established that Lake Powell operations will be governed by balancing for the remainder of water year 2016. 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 million acre feet (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 2016; the actual release in water year 2016, 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 2016 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, 2016.
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 typically maintains 30 mw (approximately 880 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.
The unregulated inflow volume to Lake Powell in April was 814 kaf (77 percent of average). The release volume from Glen Canyon Dam in April was 665 kaf. The end of April elevation and storage of Lake Powell were 3,592 ft (108 feet from full pool) and 11.01 maf (45% of full capacity), respectively. The reservoir reached a seasonal low elevation on April 15th near elevation 3591.14 feet. Since that time the reservoir has been increasing as the first of the spring runoff is now entering the reservoir.
Inflow Forecasts and Model Projections
The April to July 2016 water supply forecast for unregulated inflow to Lake Powell, issued on May 3, 2016, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 5.5 maf (77 percent of average based on the period 1981-2010). The forecast increased by 200 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 3.85 maf (54 percent of average) to a maximum probable of 7.65 maf (107 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 2015 24-Month Study, and documented in the 2016 Annual Operating Plan, Lake Powell’s operations in water year 2016 will be governed by the Upper Elevation Balancing Tier. Because the April 2016 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 elevation 1,075.0 feet, Lake Powell operations shifted to balancing (Section 6.B.4 of the 2007 Interim Guidelines) for the remainder of water year 2016. 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 2016 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 2016 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 2016 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 2016 near 3,603 ft with approximately 12.06 maf in storage (50% 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 3585 ft and 10.35 maf (43% capacity), respectively. Under the maximum probable inflow scenario, updated in April, the projected end of water year elevation and storage are 3622 ft and 14.01 maf (58% 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 16-year period 2000 to 2015, 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 16 years. The period 2000-2015 is the lowest 16-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.51 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 2000-2015 period has ranged from a low of 2.64 maf (24 percent of average) in water year 2002 to a high of 15.97 maf (147 percent of average) in water year 2011. The water year 2015 unregulated inflow volume to Lake Powell was 10.174 maf (94 percent of average), which, though still below average, was significantly higher than inflows observed in 2012 and 2013 (45% and 47 percent of average, respectively). Under the current most probable forecast, total water year 2016 unregulated inflows to Lake Powell is projected to be 8.71 maf (80 percent of average), and ranges from a minimum probable inflow of 6.86 maf (63%) and maximum probable inflow of 11.13 maf (103%).
At the beginning of water year 2016, total system storage in the Colorado River Basin was 30.3 maf (51% of 59.6 maf total system capacity). This is nearly the same as the total storage at the beginning of water year 2015 which began at 30.1 maf (50% 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 2016 total Colorado Basin reservoir storage is approximately 29.3 maf (49% of capacity). The actual end of water year storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and resulting reservoir inflow. Based on the April minimum and maximum probable inflow forecasts and modeling the range is approximately 27.6 maf (46%) to 31.4 maf (53%), respectively.
This update courtesy of Paul Davidson, Bureau of Reclamation
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