Glen Canyon Dam Update - Apr 2011

Glen Canyon Dam / Lake Powell

The unregulated inflow to Lake Powell for March 2011 was 594 kaf (90% of average). Observed inflows to Lake Powell have increased over the past several weeks and are currently averaging about 10,400 cfs. With daily average release rate from Glen Canyon Dam that is about 16,000 cfs, the elevation of Lake Powell is still declining , but the rate of decline has been moderated by these increased inflow conditions. The elevation of Lake Powell at midnight on April 7, 2011 was 3609.84 feet above sea level (90.16 feet from full pool). The elevation of Lake Powell will begin to increase later in April when inflows exceed releases. It is projected that the elevation of Lake Powell could increase by more than 30 feet to a peak elevation of approximately 3643 feet above sea level by late July or early August.

Current Dam Operations

The release volume scheduled for April is 966 kaf. During the first 3 days of April, daily releases fluctuates for power production between an afternoon peak of approximately 19,600 cfs and an early morning low release of approximately 14,000 cfs. On April 4, 2011, Glen Canyon Powerplant Units 3 and 4 were taken out of service for approximately 6 weeks for annual maintenance. Releases from Glen Canyon Dam were set to 16,000 cfs with no fluctuations for power generation at that time due to limited capacity of the available generating units at Glen Canyon Power Plant. Releases of 16,000 cfs steady will likely continue until the end of April. In early May, releases will likely be steady at about 15,000 cfs for the first 13 days of the month. 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 peak releases will be about 22,000 cfs and off peak releases will be about 16,000 cfs. The projected release volume for May is approximately 1.10 maf.

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 releases adjustments maintain stable conditions within the electrical generation and transmission system and result in momentary release fluctuations within a range that is about 1100 cfs above or below the targeted release rate for a given hour of the day. These momentary fluctuations for regulation are very short lived and typically balance out over the hour. Spinning and non-spinning reserve generation can also occur at Glen Canyon Dam. When an unanticipated electrical outage event occurs within the electrical transmission system, reserve generation at Glen Canyon Dam can be called upon up to a maximum of 98 megawatts (approximately 2,600 cfs of release) for a duration of up to 2 hours. Under normal circumstances, calls for reserve generation occur fairly infrequently and are for much less than the required 98 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 model projected the January 1, 2010 elevation of Lake Powell and Lake Mead under the most probable inflow scenario. 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) and based on this August projection, the operational tier for water year 2011 was selected to be the Upper Elevation Balancing Tier. Under this operational tier, there is a possibility that the annual release volume from Lake Powell could be as low as 8.23 maf and there is also a possibility under this tier that Equalization or Balancing could occur which would result in an annual release volume greater than 8.23 maf.

The possibility of Equalization or Balancing in water year 2011 is dependent upon the reservoir conditions of Lake Powell and Lake Mead projected at the end of the water year in the Most Probable April 24-Month Study when the projected Glen Canyon Dam annual release condition is 8.23 maf . The April 24-Month Study, with a projected water year release of 8.23 maf projects the elevation of Lake Powell on September 30, 2011 (end of water year 2011) to be 3662.63 feet above sea level which is above the Equalization Level for 2011 (3643 feet). Based on this model projection and consistent with the Interim Guidelines, the Equalization Tier will govern the operation of Lake Powell for the remainder of water year 2011.

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 April 7, 2011 was 115% of average. The current Water Supply forecast for Lake Powell (April through July Unregulated Inflow Volume) is 9.5 maf (120% of average) and this forecast was issued by the CBRFC on April 4, 2011.

The unregulated inflow forecast for Lake Powell over the next 3 months is as follows: April-1,100 kaf (112% of average); May-3,000 kaf (130% of average); June-3,850 kaf (125% of average). Incorporating these new forecasts with the current Water Supply forecast, the projected unregulated inflow volume to Lake Powell during water year 2011 is now 13.11 maf (109% of average). These forecasts combined with projected inflows in August and September of 2011 make up the 2011 Most Probable water year inflow condition. The Most Probable inflow condition has a statistical probability of being achieved that is 50%. In other words, there is a 50% chance that the unregulated inflow volume for water year 2011 for Lake Powell will be 13.11 maf or greater.

A Minimum Probable water year inflow conditions has also been developed for water year 2011. The Minimum Probable inflow condition has a statistical probability of being achieved that is 90%. The 2011 Minimum Probable water year inflow condition is currently 10.5 maf (87% of average). A Maximum Probable water year inflow condition has also been developed for water year 2011. The 2011 Maximum Probable inflow condition is currently 16.2 maf (135% of average).

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 is projected to be 11.56 maf. As hydrologic conditions change during the remainder of the water year, this annual release projection will be adjusted to achieve the Equalization Tier of the Interim Guidelines.

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 March 17, 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 net gains in storage to Lake Powell. As of April 7, 2011 the storage in Lake Powell was approximately 12.71 million acre-feet (52.2 % of capacity) which is below desired levels. The overall reservoir storage in the Colorado River Basin as of April 7, 2011 is approximately 31.42 million acre-feet (52.8 % of capacity).

RRFW thanks Rick Clayton, US Bureau of Reclamation for this update.