RRFW Riverwire – Glen Canyon Dam Update
January 18, 2018
The unregulated inflow to Lake Powell in December was 299 thousand acre feet (kaf )(82% of average). The release volume from Glen Canyon Dam in December was 740 kaf. The end of December elevation and storage of Lake Powell were 3,623 feet (77 feet from full pool) and 13.7 million acre feet (maf) (56% 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 63% of average.
In January 2018, the release volume will be approximately 860 thousand acre-feet (kaf), with fluctuations anticipated between approximately 9,100 cubic feet per second (cfs) and 16,850 cfs and consistent with the Glen Canyon Dam, Record of Decision (dated December 2016) (https://www.usbr.gov/uc/water/crsp/studies/GCOC.pdf). The anticipated release volume for February is approximately 730 kaf with daily fluctuations between approximately 10,500 cfs and 13,900 cfs. The expected release for March is 800 kaf with daily fluctuations between approximately 7,200 cfs and 16,000 cfs.
The operating tier for water year 2018 was established in August 2017 as 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 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. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible the appropriate total annual release volume by September 30, 2018.
In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 Mega Watts (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.
Inflow Forecasts and Model Projections
The April to July 2018 water supply forecast for unregulated inflow to Lake Powell, issued on January 3, 2018, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 3.9 maf (54% of average based on the period 1981-2010). The projected water year 2018 inflow is 6.75 maf (62%). At this early point in the season, there is still significant uncertainty regarding this year’s water supply. The April-July forecast ranges from a minimum probable of 2.11 maf (29%) to a maximum probable of 9.8 maf (90%). 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 January 24-Month Study projects Lake Powell elevation will end water year 2018 near 3,607 feet with approximately 12.4 maf in storage (51% 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, updated in January, are 3,591 feet (10.9 maf, 45% capacity) and 3,628 feet (14.7 maf, 60% 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, potentially 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, 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 2018. The minimum and maximum probable 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 18-year period 2000 to 2017, 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 4 out of the past 18 years. The period 2000-2017 is the lowest 18-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.76 maf, or 81% 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 2000-2017 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 2017 unregulated inflow volume to Lake Powell was 11.9 maf (110% of average), the fourth year to be above average. Under the current most probable forecast, the total water year 2018 unregulated inflow to Lake Powell is projected to be 6.75 maf (62% of average).
At the beginning of water year 2018, total system storage in the Colorado River Basin was 32.9 maf (55% of 59.6 maf total system capacity). This is an increase of 2.7 maf over the total storage at the beginning of water year 2017 when total system storage was 30.2 maf (51%). 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 29.9 maf (50% 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 January minimum and maximum probable inflow forecasts and modeling, the range of end of water year 2018 total system capacity is approximately 28.0 maf (47%) to 32.7 maf (55%), respectively.
This release courtesy Bureau of Reclamation
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