Glen Canyon Dam Update Feb 2012

Glen Canyon Dam, Lake Powell

In January 2012 the unregulated inflow volume to Lake Powell was 356 thousand acre feet (kaf), or 99% of average. This volume is very close to the final forecast for January issued by the Colorado Basin River Forecast Center on January 5th which was 375 kaf. The elevation of Lake Powell on January 31st, 2012 was 3636.90 feet above sea level (63.10 feet below full pool). During January, the elevation of Lake Powell decreased by 3.52 feet and it is likely that the elevation will continue to decrease near this rate for approximately 2 more months. By late March or early April, when the snowpack begins to melt, inflows will likely increase to a point where they are greater than releases and the elevation will begin to rise. Snowpack conditions above Lake Powell are 77% of average as of February 8, 2012.

The current Water Supply forecast (April through July Unregulated Inflow Volume) for Lake Powell for 2012 is 5.05 million acre feet (maf) which is 71% of average. Based on this inflow forecast, it is currently projected that the most probable annual release from Glen Canyon Dam in water year 2012 will be 9.46 maf. At this time of year however, there is a high level of uncertainty in hydrologic forecasts and the annual release volume from Glen Canyon Dam in WY2012 will ultimately be based on the actual inflows that occur during 2012 rather than this Water Supply forecast.

Current Dam Operations

In August 2011, pursuant to the Interim Guidelines, the Operating Tier for Glen Canyon Dam was established to be the Equalization Tier. Under the Equalization Tier for 2012, with 1.233 maf of release carried over from 2011 to 2012, the annual release volume for 2012 could be as low as 9.46 maf or higher depending on actual inflow conditions. As hydrologic conditions for Lake Powell and Lake Mead change throughout the year, Reclamation will adjust operations of Glen Canyon Dam to release the appropriate annual volume during 2012 to achieve Equalization objectives as practicably as possible by September 30, 2012.

Releases from Glen Canyon Dam are currently averaging about 12,500 cubic feet per second (cfs) with fluctuations for power generation throughout the day that peak near 13,000 cfs in the afternoons and with early morning low level releases are about 7,000 cfs. This operation is consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The release volume for February is scheduled to be 650 kaf. In March, the monthly release volume will likely be 600 kaf and fluctuating releases from Glen Canyon Dam will likely average about 10,100 cfs each day with scheduled daily fluctuations occurring within the range from 7,000 cfs to 13,000 cfs.

In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 megawatts (MW) of system regulation. These instantaneous release adjustments stabilize the electrical generation and transmission system and translate to a range of about 1100 cfs above or below the hourly scheduled release rate. Typically, fluctuations for system regulation are very short lived and balance out over the hour and do not have noticeable impacts on downstream river flow conditions.

Releases from Glen Canyon Dam can also fluctuate beyond scheduled fluctuations for power generation when called upon as a partner that shares reserve requirements within the electrical generator community (i.e. balancing area). There are many generators that supply electricity to the transmission system within the balancing area. At times, a participating generator may experience operating conditions such that it cannot make its scheduled delivery of electricity to the system (i.e. unscheduled outage). To provide system reliability, all participating electricity generators within the balancing area maintain a specified level of generation capacity (i.e. reserves) that can be called upon when an unscheduled outage occurs. Glen Canyon Dam typically maintains 113 MW of reserves for this purpose.

Reserve agreements allow the controllers of the balancing area to call upon Glen Canyon Dam to produce up to an additional 113 MW of electricity beyond what is originally scheduled for a given hour. Reserve calls can be maintained for a maximum of 2 hours after which time the generation rate should be returned to the original schedule. The 113 MW reserve requirement for Glen Canyon Dam translates to approximately 2,800 cfs of flow in the river. When the balancing area controllers call for reserve generation from Glen Canyon Dam, releases from the dam can exceed scheduled levels and have a noticeable impact on the river downstream from Glen Canyon Dam. But these calls for reserves are fairly infrequent and typically are for much less than the required level of 113 MW.

Current Inflow Forecasts and Model Projections

Over the next three months (February, March and April) the forecasted unregulated inflow volume to Lake Powell is projected to be 390 kaf (99% of average), 550 kaf (83% of average) and 800 kaf (76% of average), respectively. These percent of averages are all based on the historic period from 1981 through 2010. Combining this forecast with the February Water Supply Forecast and extending projections to the end of WY2012, the most probable (i.e. 50% likely to be exceeded) unregulated inflow volume for WY2012 is projected to be 8.48 maf (78% of average). There is significant uncertainty associated with this forecast. Recent analysis indicates that it is reasonably possible for the actual unregulated inflow volume to be as low as 5.48 maf (51% of average) or as high as 12.65 maf (117% of average) depending on the range of precipitation patterns that could occur over the next several months.

Based on the reasonable range inflow conditions that could occur this year, the annual release volume from Glen Canyon Dam could be as low as 9.46 maf to as high as 12.92 maf. Under the most probable inflow condition, the annual release volume is projected to be 9.46 maf and the elevation of Lake Powell at the end of WY2012 is projected to be 3643.2 feet above sea level. This elevation corresponds to a live storage volume of 16.38 maf (67% of full capacity).

Upper Colorado River Basin Hydrology

Since water year 2005, hydrologic conditions in the Upper Colorado River Basin have been near average with significant variability from year to year. The unregulated inflow to Lake Powell, which is a good measure of the hydrologic condition in the Colorado River Basin, has averaged 10.98 maf (101% of average (period 1981-2010)) per year during the period from 2005 through 2011. The hydrologic variability during this period has been from a low water year unregulated inflow of 8.62 maf (80% of average) in water year 2006 to a high of 15.97 maf (147% of average) which occurred in water year 2011.

Overall reservoir storage in the Colorado River Basin has increased by nearly 10 maf since the beginning of water year 2005 and this is a significant improvement over the drought conditions during water years 2000 through 2004. On October 1, 2004, the beginning of water year 2005, the total reservoir storage in the Colorado River Basin was 29.84 maf (50.2% of capacity). As of January 30, 2012 the total reservoir storage in the Colorado River Basin was 38.35 maf (64.3% of capacity).

RRFW thanks Rick Clayton of the USBOR for this information.