Glen Canyon Dam / Lake Powell
The unregulated inflow to Lake Powell for April 2011 was 983 thousand acre feet (kaf) (100% of average) which was 117 kaf below the level forecasted for April by the Colorado Basin River Forecast Center which was 1100 kaf (112% of average). On April 9, 2011, the elevation of Lake Powell reached 3609.7 feet above sea level which will likely be the lowest level that occurs during water year 2011. As of May 2, 2011 the elevation had increased to 3612.1 feet above sea level. Inflows are averaging about 25,000 cfs while releases are averaging about 14,400 cfs so the elevation is increasing at over 1 inch per day. It is projected that the elevation of Lake Powell could increase by more than 40 feet to a peak elevation of approximately 3655 feet above sea level by late July or early August which would be approximately 45 feet from the full pool elevation of 3700 feet.
Current Dam Operations
The release volume currently scheduled for May is 1103 kaf. Releases in May, from May 1 through May 13 will be near steady at 14,400 cfs. On May 14, 2011 it is projected that Units 3 and 4 will be returned to service. When this occurs, releases from Glen Canyon Dam will be increased such that daily peak releases will be about 22,000 cfs and off peak releases will be about 16,000 cfs.
The projected release volume for June is currently projected to be 1398 kaf to 1458 kaf. Flows in June will likely be steady, with releases between 23,500 cfs and 24,500 cfs during June in all hours. This will be the estimated power plant capacity during that month based on unit performance.
In addition to daily operations that may or may not include daily fluctuation patterns for load following power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate somewhat to provide approximately 40 megawatts 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 is also maintained at Glen Canyon Dam. In order for Glen Canyon Dam (and other Colorado River Storage Project dams) to participate in the electrical generation and transmissions system, Glen Canyon Dam must provide a level of reserve generation to assist the local control area to maintain electrical supply when unanticipated generation unit outages occur within the control area. Glen Canyon is required to maintain 99 megawatts (approximately 2,650 cfs of release) of capacity in reserve for these unanticipated outages. When an electrical outage occurs, Glen Canyon Dam can be called upon to provide up to an additional 99 megawatts of generation above what was originally scheduled for Glen Canyon Dam for a duration of 2 hours or less. Under normal circumstances, calls for reserve generation occur fairly infrequently and are for much less than the required 99 megawatts.
Annual Operations-Coordinated Operation of Lake Mead and Lake Powell under Interim Guidelines for Water Year 2011
In August of 2010, the 24-Month Study was used to projected the January 1, 2010 elevations of Lake Powell and Lake Mead. Based on these projected elevations and pursuant to the December 2007 Record of Decision on Colorado River Interim Guidelines for Lower Basin Shortages and the Coordinated Operations of Lake Powell and Lake Mead (Interim Guidelines), the operating tier for water year 2011 was selected to be Upper Elevation Balancing.
Operation of Glen Canyon Dam under Upper Elevation Balancing can result in an annual release as low as 7.0 million acre feet (maf) when Balancing or as high as would be required to achieve Equalization which could be as low as 8.23 maf or as high as 13 maf or greater depending on system conditions. The operational outcome of Upper Elevation Balancing is largely dependent on system conditions at the end of the water year that are projected in the April 24-Month Study. For more information on the Interim Guidelines see:
http://www.usbr.gov/lc/region/programs/strategies/RecordofDecision.pdf
Current Inflow Forecasts and Model Projections
Snowpack conditions above Lake Powell have persisted to be above average since late December 2010. The overall snowpack above Lake Powell on May 2, 2011 was 160% of the seasonal average. The snowpack conditions have continued to build basin wide throughout April when typically snowpack begins to melt in April. Temperatures in the Colorado River Basin have remained below average and this has protected the snowpack beyond what was expected just one month ago. The Water Supply forecast for Lake Powell (April through July Unregulated Inflow Volume) was 9.7 maf (123% of average) at mid-April but has been increased substantially for May which is now 11.5 maf (145% of average). The 2011 water year projected unregulated inflow volume was projected in April to be 13.11 maf (109% of average). With the increased forecast for May the updated 2011 water year projected unregulated inflow volume is now 15.34 maf (127% of average). The last water year where Lake Powell observed that level of unregulated inflow was 1996 when the unregulated inflow volume was 17.05 maf (142% of average) and in the 47 years since the closure of Glen Canyon Dam there have only been 8 years where the unregulated inflow volume was at or above the level projected for this year.
The unregulated inflow forecast for Lake Powell over the next 3 months based on the May preliminary Water Supply forecast is as follows: May-3,000 kaf (130% of average); June-5,200 kaf (169% of average); July-2,300 kaf (148% of average). Incorporating these new forecasts the most probable Lake Powell unregulated inflow volume projection for water year 2011 is 15.34 maf (127% of average). This is the median unregulated inflow volume that is forecasted to occur for water year 2011. There is a 50% chance that the unregulated inflow volume will be higher or lower than this volume. A reasonable range of possible inflows is defined by a minimum probable inflow volume and a maximum probable inflow volume. These volumes represent what would be expected to be achieved or exceeded 90% of the time (minimum inflow volume) and 10% of the time (maximum inflow volume). The forecasted water year (2011) minimum probable inflow volume issued for May is 12.4 maf (103% of average). The forecasted water year (2011) maximum probable inflow volume issued for May is 18.1 maf (150% of average). Given this range, there is still a significant amount of uncertainty for how 2011 will play over the next 5 months.
The April 2011 24-Month Study, with the 2011 most probable inflow condition projects that Equalization will be required under the Interim Guidelines and the projected annual release volume from this study is projected to be 11.56 maf. The increased forecast will almost certainly increase the projected annual release volume when the May 24-month study is completed. As hydrologic conditions change during the remainder of the water year, this annual release volume projection will be adjusted to achieve the objectives of the Equalization Tier of the Interim Guidelines.
The April 2011 24-Month Study has been published and is available here:
http://www.usbr.gov/uc/water/crsp/studies/24Month_12.pdf
Updated elevation projections for Lake Powell through water year 2011 and 2012 based on the most recently published 24-Month Study are maintained at:
http://www.usbr.gov/uc/water/crsp/studies/lppwse.html
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 - 225%, January- 50%, February - 100%, March - 90%)
The Climate Prediction Center outlook (dated April 21, 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 in the northern reaches of the basin while below average in the southern reaches of the basin.
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 near or above average inflow conditions and net gains in storage to Lake Powell. 2011 will be another above average inflow year so drought conditions are easing somewhat in the Colorado River Basin. As of May 2, 2011 the storage in Lake Powell was approximately 12.95 million acre-feet (53.2 % of capacity) which is below desired levels. The overall reservoir storage in the Colorado River Basin as of April 18, 2011 is approximately 31.58 million acre-feet (53.1 % of capacity).
RRFW thanks Rick Clayton of US Bureau of Reclamation for this update.
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