Glen Canyon Dam Update Jan 2011

Glen Canyon Dam - Lake Powell

During December 2010 the unregulated inflow to Lake Powell was 417 thousand acre feet (kaf) (96% of average). This was 57 kaf above the volume forecasted by the Colorado Basin River Forecast Center (CBRFC) on December 1, 2010 which was 360 kaf (83% of average). The elevation of Lake Powell at the end of the day on December 31, 2010 was 3626.54 feet above sea level (73.46 feet from full pool) which corresponds to a live storage content of 14.44 maf (59.5% of capacity).

During the last half of December, precipitation within the Upper Colorado River Basin was well above average and the snowpack conditions have increased significantly. On December 17, 2010 the snowpack above Lake Powell was estimated to be 102% of average. By December 31, 2010 the snowpack conditions above Lake Powell had increased to an estimated 151% of average. Precipitation above Lake Powell for the first 3 months of water year 2011 has been well above average at nearly 150% of average.

Based on these conditions and projected climate conditions over the next several months, the CBRFC has issued the Final Water Supply Forecast (April through July 2011 forecasted unregulated inflow volume) for Lake Powell that is well above average at 9.5 million acre feet (maf) which is 120% of average. This forecast translates into an increase to the expected inflow to Lake Powell for water year 2011 that is more than 3 million acre-feet more than what was projected one month ago.

Operation of Glen Canyon Dam during January 2011 has been modified based on this new forecast. On Sunday January 9, 2011, releases from Glen Canyon Dam were increased to an average daily release volume of approximately 34,000 acre-feet which translates to an average daily release of 17,100 cfs. Releases are scheduled to peak for power generation during the afternoon hours for the remainder of January to 20,500 cfs. Releases during the early morning hours are approximately 12,500 cfs.

The release volume for February is projected to be 981,000 kaf which the estimated capacity of Glen Canyon Power plant under the scheduled maintenance unit outage plan with an allowance of capacity to provide spinning reserves and regulation. It is anticipated that fluctuations for power generation will be minimal in February and the estimated release rate will likely be approximately 17,600 cfs.

In addition to the daily fluctuation pattern for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate somewhat to provide approximately 40 megawatts of system regulation. These instantaneous releases adjustments maintain stable conditions within the electrical generation and transmission system and result in momentary release fluctuations within a range that is about 1100 cfs above or below the targeted release rate for a given hour of the day. These momentary fluctuations for regulation are very short lived and typically balance out over the hour. Spinning and non-spinning reserve generation is also maintained at Glen Canyon Dam. When an unanticipated electrical outage event occur within the electrical transmission system, this reserve generation at Glen Canyon Dam can be called upon up to a limit of 98 megawatts (approximately 2,600 cfs of release) for a duration of up to 2 hours. Under normal circumstances, calls for reserve generation occur fairly infrequently and are for much less than the limit of 98 megawatts.

In August of 2010, the August 2010 24-Month Study Model was used to project the January 1, 2010 elevation of Lake Powell and Lake Mead under the most probable inflow scenario. Pursuant to the Interim Guidelines and based on this August projection, the operational tier for water year 2011 was determined to be the Upper Elevation Balancing Tier. Under the Upper Elevation Balancing Tier, there is a possibility that the annual release volume from Lake Powell could be 8.23 maf. There is also a possibility under this tier that Equalization or Balancing could occur in 2011 which would result in an annual release volume greater than 8.23 maf.

The possibility of Equalization or Balancing in 2011 is dependent on the end of water year 2011 reservoir conditions projected in the April 2011 24-Month Study under the most probable inflow scenario and with 8.23 maf projected for release from Lake Powell during water year 2011. For this reason it will not be known for certain whether Equalization or Balancing will occur in water year 2011 until April 2011. 24-Month Studies prior to April 2011 can project that Equalization or Balancing are likely to occur, but these projections are subject to change with changes in the forecasted hydrology of the Colorado River Basin. It is possible that a relatively small change in forecasted hydrology can have a large impact on the projected annual release volume.

The January 2011 24-Month Study with the most probable inflow scenario for water year 2011 projects that Equalization is likely to occur in 2011. For this reason, the projected most probable annual release volume for water year 2011 in the January 24-Month Study is 11.367 maf. Given the current range of uncertainty of the forecasted hydrology for water year 2011, it is possible that Balancing could also occur in water year 2011 which would result if the annual release being 9.0 maf. Each month the 24-Month Study is updated to reflect the most probable inflow scenario which is based on the most recent forecast from the Colorado River Basin Forecast Center (CBRFC). Analysis of the probable range of inflows that could occur during water year 2011 indicates that the probability of realizing an inflow volume that would trigger Equalization in 2011 is currently about 76%. This probability will be updated again during the first part of February 2011.

The unregulated inflow forecast for Lake Powell over the next 3 months is as follows: January-380 kaf (94% of average); February-380 kaf (90% of average); March-670 kaf (101% of average). The outlook for water year 2011, incorporating this new forecast and the January Final Water Supply Forecast, the most probable unregulated inflow volume to Lake Powell during water year 2011 is now 13.19 maf (110% of average). It is possible that the unregulated volume of inflow to Lake Powell in water year 2011 will be greater than or less than the most probable projection. The probable range of unregulated inflow volumes to Lake Powell during water year 2011 is currently projected to be as dry as 8.9 maf (74% of average) to as wet as 18.6 maf (154% of average).

Upper Colorado River Basin Hydrology

In the Upper Colorado River Basin during water year 2010, the overall precipitation accumulated through September 30, 2010 was approximately 90% of average based on the 30 year average for the period from 1971 through 2000. For Water Year 2011 thus far, the estimated monthly precipitation within the Upper Colorado River Basin (above Lake Powell) as a percentage of average has been: (October - 135%, November - 95%, December -230%)

The Climate Prediction Center outlook (dated December 16, 2010) for temperature over the next 3 months indicates that temperatures in the Upper Colorado River Basin are expected to be near average while precipitation over the next 3 months is also projected to be near average.

Upper Colorado River Basin Drought

The Upper Colorado River Basin continues to experience a protracted multi-year drought. Since 1999, inflow to Lake Powell has been below average in every year except water years 2005 and 2008. In the summer of 1999, Lake Powell was close to full with reservoir storage at 23.5 million acre-feet, or 97 percent of capacity. During the next 5 years (2000 through 2004) unregulated inflow to Lake Powell was well below average. This resulted in Lake Powell storage decreasing during this period to 8.0 million acre-feet (33 percent of capacity) which occurred on April 8, 2005. During 2005, 2008 and 2009, drought conditions eased somewhat with net gains in storage to Lake Powell. As of January 10, 2011 the storage in Lake Powell was approximately 14.26 million acre-feet (58.6 % of capacity) which is below desired levels. The overall reservoir storage in the Colorado River Basin as of January 10, 2011 is approximately 32.29 million acre-feet (54.3 % of capacity).

RRFW thanks Rick Clayton of the USBOR for this update.