Glen Canyon Dam / Lake Powell
During November 2011 the unregulated inflow volume to Lake Powell was 570 thousand acre feet (kaf ) - 108% of average. This was very close to the forecast volume issued by the Colorado Basin River Forecast Center on November 1, 2011. The forecast volume was 600 kaf (110% of average). Releases from Glen Canyon Dam during November were 1099 kaf and the elevation of Lake Powell decreased by 4.59 ending November at an elevation of 3645.69 feet above sea level. Releases from Glen Canyon Dam are currently averaging approximately 20,300 cfs and are near steady. This release rate is likely to continue to near the end of December and then will likely be reduced to a daily average release of approximately 16,300 cfs with daily fluctuations for power generation.
Current Dam Operations
Releases from Glen Canyon Dam for the Steady Flow Experiment (see Experimental Releases from Glen Canyon Dam, Arizona 2008 through 2012, Final Environmental Assessment and Finding of No Significant Impact) ended at midnight on October 31, 2011. The steady flow target for 2011 was 15,500 cfs which was the maximum sustainable release rate from Glen Canyon Dam during September and October. Since then, releases from Glen Canyon Dam have been maximized at powerplant capacity which varies depending on unit efficiency and availability. Current releases are approximately 20,300 cfs.
Since March 2011, releases from Glen Canyon Dam have been maximized through the powerplant in order to achieve the Equalization objectives of the Interim Guidelines. The Equalization objective for water year 2011 was to release sufficient volume from Glen Canyon Dam during the water year such that the elevation of Lake Powell would be 3643 feet above sea level on September 30, 2011.
Inflows to Lake Powell during the spring and summer of 2011 were well above average and the 7 months of maximum powerplant capacity releases from Glen Canyon Dam were not sufficient to achieve an elevation of 3643 feet on September 30, 2011. The elevation of Lake Powell on September 30, 2011 was 3653.01 feet above sea level and this translates to a volume of 1.233 maf that was in storage in Lake Powell on September 30, 2011 that would have otherwise been released for Equalization during water year 2011 if the powerplant had the capacity to make these higher releases.
Releases through the powerplant during the first 3 months of water year 2012 (October, November and December, 2011) have continued at powerplant capacity in order to make up for the Equalization releases that were not made in water year 2011 due to the limitations of the powerplant. It is currently projected that the additional release volume will be completed by the end of December 2011 at which time releases from Glen Canyon Dam will be reduced. It is estimated that this release reduction will likely occur on or about December 28, 2011. The instantaneous release rate from Glen Canyon Dam may fluctuate somewhat to provide 40 MW of system regulation.
These instantaneous release adjustments maintain stable conditions within the electrical generation and transmission system and result in momentary release fluctuations within a range of about 1100 cfs above or below the targeted hourly release rate. The momentary fluctuations for regulation are very short lived and typically balance out over the hour.
Spinning and non-spinning reserve generation may also be maintained at Glen Canyon Dam. In order for Colorado River Storage Project (CRSP) powerplants to participate in the electrical generation and transmissions system, these powerplants must maintain a level of generation capacity available in reserve to assist the local control area for when unanticipated generation outages occur. The current CRSP powerplant reserve requirement is 109 MW (equivalent to approximately 2,675 cfs of release from Glen Canyon Dam). When an electrical outage occurs within the control area, CRSP powerplants can be called upon to provide up to 109 MW of additional generation for up to 2 hours. Under normal circumstances, calls for reserves are infrequent and for much less than the required 109 MW. Because Glen Canyon Powerplant is the largest facility of the CRSP powerplants, typically most of the CRSP reserve requirement is maintained at Glen Canyon Dam but at times this reserve requirement is maintained at other plants within the CRSP system.
Current Inflow Forecasts and Model Projections
Over the next three months (December, January and February) the forecasted unregulated inflow to Lake Powell is projected to be above average with monthly percent of average forecasts of 115%, 111% and 107%, respectively. Based on these updated forecast values and the Hydrologic Outlook for water year 2012 (provided in October, 2011) the current projection for the most probable unregulated inflow volume to Lake Powell for water year 2012 is 11.4 maf (95% of average). At this time of year, there is a high level of uncertainty associated with this projection. Based on this projection, the December 2011 24-Month Study projects the water year 2012 most probable release volume will be 11.9 maf under the Equalization Tier of the Interim Guidelines. The December 2011 24-Month Study also projects the elevation condition for Lake Powell and Lake Mead at the end of water year 2012 to be 3645.0 feet and 1139.4 feet, respectively.
Upper Colorado River Basin Hydrology
Since water year 2005, hydrologic conditions in the Upper Colorado River Basin have been slightly below average with significant variability from year to year. The unregulated inflow to Lake Powell, which is a good measure of the hydrologic condition in the Colorado River Basin, has averaged 11.15 maf per year during the period from 2005 through 2011. This is slightly below the official average of 12.04 maf per year. The hydrologic variability during this period has been from a low water year unregulated inflow of 8.40 maf (70% of average) in water year 2006 to a high of over 16.77 maf (139% of average) which occurred in water year 2011.
Overall reservoir storage in the Colorado River Basin has increased by nearly 10 maf since the beginning of water year 2005 and this is a significant improvement over the drought conditions during water years 2000 through 2004. On October 1, 2004, the beginning of water year 2005, the total reservoir storage in the Colorado River Basin was 29.84 maf (50.2% of capacity). As of December 6, 2011, the total reservoir storage in the Colorado River Basin was 38.41 maf (64.6% of capacity).
RRFW thanks Rick Clayton, US Bureau of Reclamation