Glen Canyon Dam Update November 2010

Glen Canyon Dam / Lake Powell

During November 2010 through the first 25 days of the month, the unregulated inflow to Lake Powell has been tracking towards a monthly volume of 452 kaf (83% of average). This volume is slightly greater thant the volume forecasted for November at the beginning of the month which was 425 kaf (78% of average). Although the inflow to Lake Powell during November will likely be above forecasted levels, the water surface elevation elevation of Lake Powell at the end of November will likely be lower than forecasted. The November 24-Month Study projected that Lake Powell would end November at an elevation of 3630.85 feet above sea level. Based on current conditions and projected conditions to the end of November, the water surface elevation is likely to be about 3630.2 feet above sea level which is 68.8 feet below full pool and translates to a storage level of 14.88 maf which is 61.2% of the full capacity of 24.32 maf.

The release volume scheduled for December is 845 kaf which is equivelent to an average daily release rate of approximately 13,750 cfs. Daily fluctuations will likely peak near 16,000 cfs during the morning and afternoon and evening hours. Daily low releases will occur during the early morning hours (i.e. midnight to about 6:00 am) and will be about 8,500 cfs. The projected release volume for January is currently 865 kaf which is equivelent to an average daily release rate of approximately 14,050 cfs. The daily peak and low release rate in January will likely also range from 16,000 cfs to 8,500 cfs respectively.

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 selected 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 November 2010 24-Month Study with the most probable inflow and an 8.23 maf release does project that Balancing is likely to occur in 2011.   For this reason, the projected most probable annual release volume for water year 2011 in the November 24-Month Study is 9.00 maf. Given the current range of uncertainty of the forecasted hydrology for water year 2011, it is possible that Equalization could occur in water year 2011 which would result if the annual release being greater than about 10.7 maf.  Analysis of the probable range of inflows that could occur during water year 2011 indicate that the probability of Equalization occurring in 2011 is currently about 48%. 

The current unregulated inflow forecast for Lake Powell projects the most probable unregulated inflow volumes for the next 3 months as follows: November-425 kaf (78% of average; December-400 kaf (86% of average); January-350 kaf (86% of average). The outlook for water year 2011, incorporating this new forecast, projectes the most probable unregulated inflow volume to Lake Powell during water year 2011 to be 9.64 maf (80% 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 4.5 maf (37% of average) to as wet as 15.8 maf (131% of average).

The November 2010 24-Month Study has been published and is available here. Updated elevation projections for Lake Powell through water year 2011 and 2012 based on the most recently published 24-Month Study are maintained at: Lake Powell Projected Elevations.

 Upper Colorado River Basin Hydrology

In the Upper Colorado River Basin during water year 2010, the overall precipitation accumulated through September 30, 2009 was approximately 90% of average based on the 30 year average for the period from 1971 through 2000. For October 2010, the first month of water year 2011, precipitation in the Upper Colorado River Basin was approximately 135% of average basin wide.

The Climate Prediction Center outlook (dated November 18, 2010) for temperature over the next 3 months indicates that temperatures in the Upper Colorado River Basin are expected to be above average while precipitation over the next 3 months is 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 November 25, 2010 the storage in Lake Powell was approximately 14.97 million acre-feet (61.5 % of capacity) which is below desired levels. The overall reservoir storage in the Colorado River Basin as of November 7, 2010 is approximately 32.44 million acre-feet (54.5 % of capacity).

RRFW thanks Rick Clayton, BOR for this update.