The unregulated inflow volume to Lake Powell in November was 418 thousand acre-feet (kaf) (88% of average). The release volume from Glen Canyon Dam in November was 776 kaf. The end of November elevation and storage of Lake Powell were 3,601.9 feet (98 feet from full pool) and 11.93 million acre-feet (maf) (49% of full capacity), respectively. The reservoir elevation is now declining and is expected to continue to decline until spring 2015.
In December, the release volume will be approximately 866 kaf, with fluctuations anticipated between about 9,500 cubic feet per second (cfs) in the nighttime to about 17,500 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 January is 860 kaf.
The operating tier for water year 2015 was established in August 2014 as the Upper Elevation Balancing Tier. In the Upper Elevation Balancing Tier the initial water year release volume is 8.23 maf; however, there is the possibility for an April adjustment to equalization or balancing operations to govern for the remainder of the water year. Under the most probable inflow scenario, an April adjustment to balancing releases is projected to occur and Lake Powell is currently projected to release 9.0 maf in water year 2015. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible the appropriate total annual release volume by September 30, 2015.
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 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 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 forecast for water year 2015 unregulated inflow to Lake Powell, issued on December 1, 2014, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 9.79 maf (90% of average based on the period 1981-2010). This is about a 200 kaf increase from the forecast issued last month. At this early point in the season, there is significant uncertainty regarding next year’s water supply. The forecast ranges from a minimum probable of 7.4 maf (68% of average) to a maximum probable of 18.6 maf (172% 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 2014 24-Month Study, Lake Powell’s operations in water year 2015 will be governed by the Upper Elevation Balancing Tier. In this tier, the initial water year release volume is 8.23 maf, however, there is the potential for an April adjustment to equalization or balancing releases in April 2015. An April adjustment to balancing releases is projected to occur and Lake Powell is currently projected to release 9.0 maf in water year 2015. This determination will be documented in the 2015 Annual Operating Plan, which is currently in the final stages of development.
Based on the current forecast, the December 24-Month Study projects Lake Powell elevation will end water year 2015 near 3,606 feet with approximately 12.35 maf in storage (51% capacity). Note that projections of elevation and storage have significant uncertainty at this early point in the season, primarily due to uncertainty regarding next season’s snowpack and the resulting inflow to Lake Powell. Under the minimum probable inflow scenario, which was updated in October, the projected end of water year elevation and storage are 3589 feet and 10.71 maf (44% capacity), respectively. Under the maximum probable inflow scenario, which was updated in October, the projected end of water year elevation and storage are 3649 feet and 17.09 maf (70% capacity), respectively.
The annual release volume from Lake Powell during water year 2015 is projected to be 9.0 maf under the minimum and most probable inflow scenarios and 12.1 maf under the maximum probable inflow scenario. There is a 10% chance that inflows will be higher, potentially resulting in higher releases; and 10% chance that inflows will be lower, potentially resulting in lower releases. If inflows are less than the current forecasted minimum probable inflow, the water year 2015 annual release could be as low as 8.23 maf. If inflows are greater than the current forecasted maximum probable inflow, the annual release could be greater than 12.1 maf.
Upper Colorado River Basin Hydrology
The Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 15-year period 2000 to 2014, 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 15 years. The period 2000-2014 is the lowest 15-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.39 maf, or 78% 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 2000-2014 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 2014 unregulated inflow volume to Lake Powell was 10.381 maf (96% of average), which 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 2015 unregulated inflows to Lake Powell is projected to be 9.79 maf (90% of average).
At the beginning of water year 2015, total system storage in the Colorado River Basin was 30.0 maf (50% of 59.6 maf total system capacity). This is nearly the same as the total storage at the beginning of water year 2014 which began at 29.9 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 2015 total Colorado Basin reservoir storage is approximately 29.8 maf (50% of capacity). The actual end of water year storage may vary from this projection, primarily due to uncertainty regarding next season’s snowpack and resulting runoff. Based on October minimum and maximum probable inflow forecasts and modeling the range is approximately 27.4 maf (46%) to 38.1 maf (64%), respectively.
This update courtesy of Katrina Grantz, Bureau of Reclamation