May 2, 2010
Glen Canyon Dam / Lake Powell
The unregulated inflow volume into Lake Powell in April 2010 will be well above the volume forecasted at the beginning of the month. The official forecasted unregulated inflow volume for April 2010 for Lake Powell was 650,000 acre feet (66% of average). As of April, 29, 2010 the unregulated inflow volume is tracking towards a monthly volume of 915,000 acre-feet (93% of average). This isn't necessarily an indication the runoff volume this year will be larger than expected, but is more an indication that the runoff is occurring earlier than expected. During the first week of May 2010, the Colorado River Basin River Forecast Center will update the forecasted runoff volume for the year and Reclamation will update the operatonal projections at that time. The current runoff forecast (April through July unregulated inflow volume) for Lake Powell (April mid month) is 5.2 million acre-feet (66% of average).
Because the inflow volume to Lake Powell during April 2010 has been larger than expected, the elevation of Lake Powell is about 2 feet higher than expected. Based on the forecasted inflow at the beginning of April 2010, the elevation was projected to be at 3618.5 feet above sea level (81.5 feet from full pool) on May 1, 2010. The actual elevation on May 1, 2010 will likely be about 3620.5 feet above sea level (79.5 feet from full pool). Given the current forecast, the projected peak elevation for water year 2010 will occur in July and is likely to be approximately 3632 feet above sea level (68 feet below full pool).
Releases from Glen Canyon Dam during the month of May 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 May is 600,000 acre-feet. The release volume projected for June is also 600,000 acre-feet and this will be confirmed in late May.
In addition to the daily fluctuation pattern, instantaneous releases from Glen Canyon Dam also fluctuate to provide approximately 40 megawatts 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 88 megawatts (approximately 2400 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 88 megawatts.
Based on the April Official Forecast, and a projected Lake Powell release volume of 8.23 maf pursuant to the Colorado River Interim Guidelines for Lower Basin Shortages and Coordinated Operations for Lakes Powell and Mead (Interim Guidelines), the April (Most Probable) 24-Month Study projects that the water year ending elevation of Lake Powell will be 3629.20. This elevation is 12.8 feet below the Equalization Level for water year 2010 as established in the Interim Guidelines. This condition does not meet the criteria for an April adjustment to the Equalization Tier from the remainder of water year 2010 (see Section 6.B.3 of the Record of Decision ). Therefore, the release volume for water year 2010 will be 8.23 maf and monthly release volume for the remainder of the water year will be scheduled to meet this annual release volume.
The May 2010 24-Month Study will be published on May 10, 2010 and will be available here:
http://www.usbr.gov/uc/water/crsp/studies/24Month_05.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 here:
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 the 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 85%.
The Climate Prediction Center outlook (dated April 15, 2010) for temperature over the next 3 months indicates that temperatures in the Upper Colorado River Basin are expected to be above average and precipitation over the next 3 months in the Upper Colorado River Basin is projected to also be above 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 April 29, 2010 the storage in Lake Powell was 13.80 million acre-feet (56.8 percent of capacity) which is still below desired levels while the overall reservoir storage in the Colorado River Basin as of April 29, 2010 is 32.85 million acre-feet (55.2 percent of capacity).
RRFW thanks Rick Clayton of the USBOR for his assistance in providing information for this notification.