RRFW Riverwire Glen Canyon Dam Update
February 28, 2017
The unregulated inflow to Lake Powell in January was 359 thousand acre feet (kaf) (99% of average). The release volume from Glen Canyon Dam in January was 880 kaf. The end of January elevation and storage of Lake Powell were 3,596 feet (104 feet from full pool) and 11.4 million acre feet (maf) (47% of full capacity), respectively. The reservoir is declining and will continue to decline until spring runoff begins to enter the reservoir. The current snowpack above Lake Powell is 159% of median.
In February 2017, the release volume will be approximately 715 thousand acre-feet (kaf), with fluctuations anticipated between approximately 9,000 cubic feet per second (cfs) and 15,000 cfs and consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The anticipated release volume for March is approximately 720 kaf with daily fluctuations between approximately 8,000 cfs and 14,000 cfs. The expected release for April is 645 kaf with daily fluctuations between approximately 8,000 cfs and 14,000 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 up to 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.
The operating tier for water year 2017, established in August 2016, is the Upper Elevation Balancing Tier, with an initial water year release volume of 8.23 maf and the potential for an April 2017 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 2017. This projection will be updated each month throughout the water year.
Inflow Forecasts and Model Projections
The April to July 2017 water supply forecast for unregulated inflow to Lake Powell, issued on February 2, 2017, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 9.6 maf (134% of average based on the period 1981- 2010). The projected water year 2017 inflow is 13.2 maf (122%). At this early point in the season, there is still significant uncertainty regarding this years water supply. The April-July forecast ranges from a minimum probable of 6.6 maf (92%) to a maximum probable of 13.5 maf (189%). 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 February 24-Month Study projects Lake Powell elevation will end water year 2017 near 3,640 feet with approximately 16.0 maf in storage (66% capacity). Note that projections of elevation and storage for water year 2017 have significant uncertainty at this point in the season. Projections of elevation and storage using the minimum and maximum probable inflow forecast, updated in January, are 3,586 feet (10.4 maf, 43% capacity) and 3,640 feet (16.0 maf, 66% capacity), respectively. One thing to point out here, the February most probable has increased to the January maximum probable in just one month. 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, potentially in lower elevation and storage. The annual release volume from Lake Powell during water year 2017 is projected to be 9.0 maf under the minimum, most, and maximum probable inflow scenarios. There is a chance that inflows could be higher or lower, potentially resulting in releases greater than 9.0 maf or as low as 8.23 maf in water year 2017. Modeling of projected reservoir operations based on the minimum and maximum scenarios will be updated again in April.
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 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 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 13.2 maf (122% of average).
At the beginning of water year 2017, total system storage in the Colorado River Basin was 30.2 maf (50% 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 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 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 29.8 maf (50% of total system capacity). The actual end of water year 2017 system storage may vary from this projection, primarily due to uncertainty regarding the seasons snowpack and resulting runoff and reservoir inflow. Based on the January minimum and maximum probable inflow forecasts and modeling, the range of end of water year 2017 total system capacity is approximately 27.3 maf (46%) to 33.5 maf (56%), respectively.
This update courtesy of Paul Davidson, Bureau of Reclamation
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