Glen Canyon Dam Lake Powell
The unregulated inflow volume to Lake Powell for May 2011 was been 2.35 million acre feet (maf) (102% of average). Inflows have increased due to snowmelt runoff within the basin and observed inflows to Lake Powell are averaging nearly 75,000 cubic feet per second (cfs). Observed inflows will likely continue to increase in June and could reach levels near 140,000 cfs by late June. The spring runoff is well under way and there is still significant snow yet to melt at the higher elevations within the basin.
Inflows will likely be very high over the next 2 months. The forecasted unregulated inflow for June is 6.1 maf which would be the 2nd wettest June for Lake Powell since the initial operation of Glen Canyon Dam began in 1963.
The elevation of Lake Powell will increase significantly during the months of June and July, 2011. As of June 7, 2011 the elevation of Lake Powell was 3628.4 feet which is about 19 feet above the low elevation of the year (3609.7 feet on April 9, 2011). During June and July, the elevation will likely increase rapidly and is projected to reach a peak elevation for the year in early August 2011 of approximately 3660 to 3665 feet above sea level. This would be 35 to 40 feet below the full pool elevation of 3700 feet. The last time Lake Powell was at an elevation in this range was in October of 2001 near the beginning of the most current drought.
Current Dam Operations
The releases from Glen Canyon Dam are approximately 22,400 cfs which is very near the full capacity of the power plant allowing for 100 megawatts of reserve generation and 40 megawatts of regulation. As the elevation of Lake Powell increases, the capacity of the power plant will also increase and operation of the power plant will be adjusted day by day to release the maximum volume possible. It is anticipated that the release volume for June will be approximately 1,369 kaf with 7 generating units operating at full capacity. The actual release volume for June will largely depend on the generation unit efficiencies that occur throughout the month and could be higher or lower than this anticipated volume.
While the release rate over the next several months is likely to be steady at or near full power plant capacity, the instantaneous releases from Glen Canyon Dam may 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 Colorado River Storage Project (CRSP) power plants to participate in the electrical generation and transmissions system, these dams must maintain a level of generation capacity in reserve to assist the local control area to maintain electrical supply when unanticipated generation outages occur within the control area. The CRSP power plants are required to maintain 100 megawatts (approximately 2,675 cfs of release) of capacity in reserve for these unanticipated outages.
When an electrical outage occurs, CRSP power plants can be called upon to provide up to an additional 100 megawatts of generation above what was originally scheduled 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 100 megawatts. Because Glen Canyon Power plant is the largest facility of the CRSP power plants, typically most of the CRSP reserve requirement is maintained there rather than the other CRSP power plants.
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 project 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 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 click here:
Current Inflow Forecasts and Model Projections
Snowpack conditions above Lake Powell that occurred during the winter of 2010-2011 were well above average and resulted in a snowpack condition above Lake Powell on April 10, 2011 that was 119% of average. Typically the snowpack peaks during the first part of April and snow levels normally begin to decrease after about April 10th each year. Temperatures in April of 2011, however, were well below average which allowed the snowpack to continue to build through the month of April. On May 1, 2011 the snowpack above Lake Powell reached its peak level for the year which was approximately 130% of the average seasonal peak snowpack condition. Below average temperatures in the basin persisted in May and by June 1, 2011 only about 30% of the snowpack had melted. The as of June 8, 2011 about 50% of the accumulated snowpack for 2011 had melted. There is still significant snowpack within the basin which will support high inflow conditions for several months.
The Water Supply forecast for Lake Powell (April through July Unregulated Inflow Volume) is now 12.6 maf (159% of average) and the water year unregulated inflow to Powell for 2011 is projected to be 16.7 maf (138% of average). The last water year where Lake Powell observed that level of unregulated inflow was 1997 when the unregulated inflow volume was 17.4 maf (145% of average) and only 5 of 47 years since the closure of Glen Canyon Dam have had unregulated inflow volumes greater than what is 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: June-6,100 kaf (198% of average); July-3,300 kaf (212% of average);August-950 kaf (155% of average).
This forecast was last updated on June 3, 2011. Incorporating these new forecasts the most probable Lake Powell unregulated inflow volume projection for water year 2011 is 16.7 maf (138% 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 15.1 maf (125% of average). The forecasted water year (2011) maximum probable inflow volume issued for May is 18.5 maf (154% of average). Given this range, there is still a significant amount of uncertainty for how 2011 will play over the next 4 months. The May 2011 24-Month Study, with the 2011 most probable inflow condition projected the annual release volume to be 12.46 maf. This release volume is the estimated maximum volume that can be released through Glen Canyon Power plant by September 30, 2011. This annual release volume does not achieve the Equalization objective of the Interim Guidelines by September 30th, 2011. As hydrologic conditions change during the remainder of water year 2011, actual and projected releases from Glen Canyon Dam will be adjusted to achieve the Equalization objective of the Interim Guidelines as practicably as is possible by September 30th, 2011. If the Equalization objective is not achieved by September 30th, releases from Glen Canyon Dam will likely remain at power plant capacity until the Equalization objective is achieved.
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
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 - 85%, April - 155%, May - 155%) The Climate Prediction Center outlook (dated May 19, 2011) 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 has experienced a protracted multi-year drought since early 2000. During this drought, the inflows to Lake Powell have been below average in every year except water years 2005, 2008 and likely 2011. 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 have eased somewhat in the Colorado River Basin over the past 6 years. As of June 7, 2011 the storage in Lake Powell was approximately 14.68 million acre-feet (60.4 % of capacity) which is still below desired levels. The overall reservoir storage in the Colorado River Basin as of June 7, 2011 is approximately 33.82 million acre-feet (56.9 % of capacity).
RRFW thanks Rick Clayton of the USBOR for this information.
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