RRFW Riverwire Glen Canyon Dam Update
November 14, 2019
Current Status
The unregulated inflow to Lake Powell in October was 265 thousand acre-feet (kaf) (52 percent of average). The release volume from Glen Canyon Dam in October was 625 kaf. The end of October elevation and storage of Lake Powell were 3,613 feet (87 feet from full pool) 13 maf (53 percent of full capacity).
Current Operations
In November, the release volume will be approximately 625 kaf, with fluctuations anticipated between about 7,000 cubic feet per second (cfs) in the nighttime to about 12,640 cfs in the daytime, and consistent with the Glen Canyon Dam, Record of Decision (dated December 2016). The anticipated release volume for December is 750 kaf with daily fluctuations between approximately 7,825 cfs and 14,575 cfs. The expected release for January is 760 kaf with daily fluctuations between approximately 8,850 cfs and 16,450 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 28 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 2020 (September 2019 through October 2020) was established in August 2019 as the Upper Elevation Balancing Tier, consistent with Section 6.B of the Interim Guidelines. Under this Tier the initial annual water year release volume is 8.23 maf, and the April 2020 24-Month Study projects 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. Lake Powell operations will shift to balancing releases for the remainder of water year 2020. If the April 2020 24-Month Study projects the end of water year elevation to be above the 2020 Equalization elevation of 3,657 feet Lake Powell operations will shift to Equalization releases for the remainder of water year 2020. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible the appropriate total annual release volume by September 30, 2020.
To view the most current reservoir elevation projections, click
https://www.usbr.gov/uc/water/crsp/studies/images/PowellElevations.png
To view the most current monthly release projections, click
https://www.usbr.gov/uc/water/crsp/studies/images/PowellRelease.png
To view the 2019 progression of snowpack above Lake Powell, click
To view the current inflow forecast relative to past inflows, click
https://www.usbr.gov/uc/water/crsp/studies/images/PowellForecast.png
Inflow Forecasts and Model Projections
The forecast for water year 2020 unregulated inflow to Lake Powell, issued on November 1, 2019, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume this year will be 8.77 million arce feet (maf) (81 percent of average). There is significant uncertainty regarding this season’s snow pack development and resulting runoff into Lake Powell. Reclamation updates the minimum and maximum probable forecasts four times a year: January, April, August and October. The October forecast ranges from a minimum probable of 6.7 maf (62 percent of average) to a maximum probable of 18 maf (166 percent of average). There is a 10 percent chance that inflows could be higher than the current maximum probable forecast and a 10 percent chance that inflows could be lower than the minimum probable forecast.
Based on the current forecast, the November 24-Month Study projects Lake Powell elevation will end water year 2020 near 3,617.91 feet with approximately 13.5 maf in storage (56 percent of capacity). Note that projections of elevation and storage for water year 2020 have significant uncertainty at this point in the season. Projections of end of water year 2020 elevation and storage using the minimum and maximum probable inflow forecast from October 2019 are 3,595.36 feet (11.31 maf, 47 percent of capacity) and 3,657 feet (18.10 maf, 74 percent of 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 2020 is projected to be 8.23 maf under the October most probable scenario, and 9.0 maf under the minimum probable and 11.89 maf under the maximum probable inflow scenarios.
Upper Colorado River Basin Hydrology
Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 20-year period 2000 to 2019, 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 19 years. The period 2000-2019 is the lowest 20-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.76 maf, or 81 percent 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 2000-2019 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. In water year 2018 unregulated inflow volume to Lake Powell was 4.6 maf (43 percent of average), the third driest year on record above 2002 and 1977. Under the current most probable forecast, the total water year 2020 unregulated inflow to Lake Powell is projected to be 8.77 maf (81 percent of average).
At the beginning of water year 2020, total system storage in the Colorado River Basin was 31.64 maf (53 percent of 59.6 maf total system capacity). This is an increase of 3.64 maf over the total storage at the beginning of water year 2019 when total system storage was 28 maf (47 percent 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 percent of capacity at the beginning of 2000 to the now current level of 53 percent of capacity at the beginning of water year 2020. Based on current inflow forecasts, the current projected end of water year total Colorado Basin reservoir storage for water year 2020 is approximately 31.2 maf (52 percent of total system capacity). The actual end of water year 2020 system storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and reservoir inflow.
This release courtesy the US Bureau of Reclamation
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