RRFW Riverwire Glen Canyon Dam Update
December 23, 2015
The unregulated inflow to Lake Powell in November was 445 thousand acre feet (kaf) (94% of average). The release volume from Glen Canyon Dam in November was 604 kaf. The end of November elevation and storage of Lake Powell were 3,605feet (95 feet from full pool) and 12.3 million acre feet (maf) (50% of full capacity), respectively. The water year 2015 unregulated inflow to Lake Powell was 10.17 maf (94% of average). The water year 2015 release from Lake Powell was 9.0 maf. The reservoir elevation peaked at 3,614 feet on July 14, 2015 and is now in its seasonal decline through the fall and wintermonths.
In December 2015, the release volume will be approximately 900 thousand acre-feet (kaf), with fluctuations anticipated between approximately 11,000 cubic feet per second (cfs) and 19,000 cfs and consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The anticipated release volume for January is approximately 900 kaf with daily fluctuations between approximately 11,000 cfs and 19,000 cfs. The expected release for February is 700 kaf with daily fluctuations between approximately 9,000 cfs and 15,000cfs.
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 systemand translate to a range of up to about 1,200 cfs above or below the hourly scheduled releaserate. 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 downstreamriver flowconditions.
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 CanyonDam.
The operating tier for water year 2016, established in August 2015, is the Upper Elevation Balancing Tier, with an initial water year release volume of 8.23 maf and the potential for an April 2016 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 2016. This projection will be updated each month throughout the wateryear.
Inflow Forecasts and Model Projections
The forecast for water year 2016 unregulated inflow to Lake Powell, issued on December 1, 2015, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume next year will be 8.72 maf (81% of average). This is a slightincrease (180 kaf) from the November forecast for water year 2016. There is still significant uncertainty regarding next season’s snow pack development and resulting runoff into Lake Powell. The forecast ranges from a minimum probable of 6.2 maf (57%) to a maximum probable of 16.0 maf (148%). 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 probableforecast.
Based on the current forecast, the December 24-Month Study projects Lake Powell elevation will end water year 2016 near 3,602 feet with approximately 11.9 maf in storage (49%capacity). Note that projections of elevation and storage for water year 2016 have significant uncertainty at this point in the season. Projections of elevation and storage using the minimum and maximum probable inflow forecast, updated in October, are 3,581 feet (10.0 maf, 43% capacity) and 3,639 feet (15.9 maf, 65% 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 2016 is projected to be 9.0 maf under the minimum and most probable inflow scenarios and 11.4 maf under the maximum probable inflow scenario. There is a chance that inflows could be higher or lower, potentially resulting in releases greater than 11.4 maf or as low as 8.23 maf in water year 2016. The minimum and maximum probable scenarios will be updated again inJanuary.
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 of8.51 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% of average) in water year 2002 to a high of 15.97 maf (147% of average) in water year 2011. The water year 2015 unregulated inflow volume to Lake Powell was 10.17 maf (94% 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, total water year 2016 unregulated inflows to Lake Powell is projected tobe 8.72 maf (81% ofaverage).
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 years 2014 and 2015 which began at 29.9 maf and 30.0 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 2016 is approximately 29.2 maf (49% of total system capacity).The actual end of water year 2016 system storage may vary from this projection, primarily due to uncertainty regarding next season’s snowpack and resulting runoff and reservoir inflow. Based on theOctober minimum and maximum probable inflow forecasts and modeling, the range of end of wateryear 2016 total system capacity is approximately 26.8 maf (45%) to 36.4 maf (61%),respectively.
This update courtesy of Katrina Grantz, Bureau of Reclamation
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