Glen Canyon Dam / Lake Powell
The unregulated inflow volume into Lake Powell so far for June (as of June 23rd) is 2.42 million acre feet (maf). Observed inflows peaked on June 12, 2010 at 57,600 cubic feet per second (cfs) and have declined since that time to 26,300 cfs on June 23, 2010. At the current rate of decline is it estimated that the unregulated inflow to Lake Powell for the month of June will be approximately 2.7 maf (88% of average). This will be well above the projected unregulated inflow from the June 24-month study which was 2.05 maf (66% of average). The Colorado Basin River Forecast Center did increase the June forecast to 2.45 maf (79% of average) on June 15th and this volume will now likely be exceeded on June 24th.
As a result of the additional volume of inflow that is being observed this month, the elevation of Lake Powell is much higher than what was projected in the June 24-Month Study. It is now likely that the peak elevation this summer will be in the range from 3638 to 3640 feet above sea level rather than the previously projected peak of 3634 to 3635 feet above sea level. The April through July forecasted unregulated inflow to Lake Powell is currently 5.45 maf (69% of average) based on the June mid month forecast. This forecast will be updated during the first week of July.
Releases from Glen Canyon Dam during the month of June will fluctuate each day for power generation between a peak hourly average release of about 12,500 cfs, during the morning and afternoon and a daily low hourly average release of 6,500 cfs during the late evening and early morning hours. The release volume scheduled for June is 600,000 acre-feet. The release volume scheduled for July is 800,000 acre-feet and daily fluctuations will range between 16,500 cfs during the afternoon and evening hours and 8.500 cfs during the late night and early morning hours. The release volume for August is projected to be 800,000 acre-feet and this will be confirmed in late July.
In addition to the daily fluctuation pattern, instantaneous releases from Glen Canyon Dam also fluctuate to provide approximately 40 megawatts (approximately 1,100 cfs) of system regulation to maintain stable conditions within the electrical generation and transmission system. This translates into momentary release fluctuations of about +/- 1100 cfs above or below the hourly average release rate. These momentary fluctuations for regulation are very short lived and typically balance out over the hour. When an unanticipated outage event occurs in the generation system, reserve generation at Glen Canyon Dam can also be called upon up to a limit of 83 megawatts (approximately 2,250 cfs of release) 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 limit of 83 megawatts.
On September 1, 2010 and continuing through October 31, 2010, the releases from Glen Canyon Dam will be steady with no fluctuations for power production (excluding system regulation and spinning reserves) for a steady flow experiment pursuant to the February 2008 Finding of No Significant Impact Experimental Releases from Glen Canyon Dam, Arizona 2008 through 2012. This year will be the third year of steady flows of the 5 year experiment. The projected release rate being targeted is 8,000 cfs which is equivalent to a monthly release volume of approximately 476,000 acre-feet in September 2010 and 491,000 acre-feet in October 2010. At the end of August, for a period of approximately 3 days, the daily fluctuation schedule will change each day in order to gradually transition to the steady release rate of 8,000 cfs to begin on September 1, 2010.
Pursuant to the Colorado River Interim Guidelines for Lower Basin Shortages and Coordinated Operations for Lakes Powell and Mead (Interim Guidelines) the operational tier for water year 2010 is Upper Elevation Balancing and the projected water year release volume is 8.23 maf. Under this operational tier there was a possibility that Equalization could occur in 2010 if the April 2010 24-Month Study ,with 8.23 maf release during water year 2010, projected that the elevation of Lake Powell on September 30 , 2010 was to be greater than 2010 equalization level (3642 feet above sea level). This condition, however, was not projected in the April 2010 24-Month Study. Therefore, the release volume for water year 2010 will be 8.23 maf. Projected monthly release volumes for the remainder of the water year will be scheduled to meet this annual release volume.
The June 2010 24-Month Study projects that operation tier for Glen Canyon Dam in water year 2011 will be Upper Elevation Balancing. The June 2010 24-Month Study also projects a shift to Equalization in April 2011 with a projected annual release volume of 11.15 maf. It should be cautioned however that at this time of year, the inflow scenario assumed in the 24-Month Study for the following water year is based on the statistical average and does not reflect a current projection from the Colorado Basin River Forecast Center (CBRFC). There is a high level of uncertainty about what the inflow conditions will be like in water year 2011. In August 2010 the CBRFC will issue an inflow projection for water year 2011 and the August 2010 24-Month Study will be based on this inflow projection rather than statistical average. It is currently forecasted that there is approximately a 62% probability that Equalization will occur in water year 2011. This forecast will be updated each month as conditions change.
The July 2010 24-Month Study will be published by July 12, 2010 and will be available here:
http://www.usbr.gov/uc/water/crsp/studies/24Month_07.pdf
Updated elevation projections for Lake Powell through water year 2010 based on the most recently published 24-Month Study are maintained at: Lake Powell Projected Elevations:
http://www.usbr.gov/uc/water/crsp/studies/lppwse.html
Upper Colorado River Basin Hydrology
In the Upper Colorado River Basin during water year 2009, the overall precipitation accumulated through September 30, 2009 was approximately 95% of average based on the 30 year average for the period from 1971 through 2000. For water year 2010 dry conditions have persisted. Estimated percentages of average precipitation for the months thus far in water year 2010 are as follows: October 85%, November 40%, December 130%, January 100% and February 100%, March 90%, April 120%, May 80%. The overall estimated precipitation percentage of average thus far in water year 2010 for the Upper Colorado River Basin is 87% of average.
The Climate Prediction Center outlook (dated May 20, 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 June 23, 2010 the storage in Lake Powell was 15.8 million acre-feet (64.8 % of capacity) which is still below desired levels while the overall reservoir storage in the Colorado River Basin as of June 23, 2010 is 34.65 million acre-feet (58.3 % of capacity).
RRFW thanks Rick Clayton of the USBOR for his assistance in providing information for this notification.