GCPBA note: We are beginning to get an idea of the flows we will have in Grand Canyon for 2012.
Glen Canyon Dam Update
During December 2011 the unregulated inflow volume to Lake Powell was 359 kaf (99% of average based on the historic period from 1981 through 2010). This was well below the volume forecasted for the month of December which was 500 kaf (138% of average). As a result, the elevation of Lake Powell at the end of December was 3639.7 feet above sea level which was 1.1 feet lower than projected at the beginning of December.
Snowpack conditions above Lake Powell are well below average for this time of year. As of January 9, 2012 the overall snowpack above Lake Powell was only 60% of average. Reclamation has received the first Water Supply forecast for 2012 and the April through July unregulated inflow to Lake Powell is projected to be 5.05 maf which is 71% of average. Based on this forecast, the projected most likely (i.e. 50% likely to be exceeded) annual release volume 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 actual hydrology rather than forecasted hydrology.
Current Dam Operations
In August 2011, as part of the Colorado River Annual Operating Plan process, the Operating Tier for Glen Canyon Dam was determined to be the Equalization Tier under the 2008 Interim Guidelines. Under the Equalization Tier, with 1.233 maf of release volume carried over from 2011 to 2012, the annual release volume for WY2012 could be as low as 9.46 maf to as high as 13.1 maf or higher depending on actual inflow conditions. As inflow and storage conditions for Lake Powell and Lake Mead change throughout the year, Reclamation will make practicable adjustments to the operation of Glen Canyon Dam to release the appropriate annual volume during 2012. The overall goal during 2012 in terms of annual release will be to achieve the objectives of the Equalization Tier of the Interim Guidelines.
In response to the Water Supply forecast issued for Lake Powell in January, Reclamation has reduced the release volume for January from 1000 kaf to 850 kaf. This reduction began on January 10, 2012. Releases from Glen Canyon Dam are currently averaging about 13,200 cfs with fluctuations for power generation throughout the day that peak near 17,500 cfs in the afternoons. Early morning low releases are about 9,500 cfs. This operation is consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997).
In addition to hourly release fluctuations for load following power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate somewhat to provide 40 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 release rate that is scheduled for a given hour. Typically, fluctuations for system regulation are very short lived and balance out over the hour and have minimal impacts on river flow conditions.
Releases from Glen Canyon Dam can also fluctuate beyond the typical load following pattern when called upon as a partner that shares reserve requirements within the electrical generator community (i.e. control area). There are many generators that supply electricity to the transmission system within the control 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. outage). To provide system reliability, all participating electricity generators within the control area maintain a specified level of generation capacity (i.e. reserves) that can be called upon when an outage occurs. Glen Canyon Dam typically maintains 109 MW of reserves for this purpose.
Reserve agreements allow the controllers of the transmission system to call upon Glen Canyon Dam for up to 109 MW of additional generation beyond what is originally scheduled for a given hour. These calls for reserve generation can be maintained for up to 2 hours in total duration. The 109 MW reserve requirement for Glen Canyon Dam translates to approximately 2,700 cfs of flow in the river and calls for reserves can have noticeable impacts on river flow conditions. Calls for reserves are fairly infrequent and typically are for much less than the maximum requirement of 109 MW.
Current Inflow Forecasts and Model Projections
Over the next three months (January, February, March) the forecasted unregulated inflow volume to Lake Powell is projected to be 375 kaf (104% of average), 410 kaf (104% of average) and 575 kaf (86% of average), respectively. These percent of averages are all based on the historic period from 1981 through 2010. Combining this forecast with the January 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 now projected to be 8.55 maf (79% of average). The minimum probable (i.e. 90% likely to be exceeded) unregulated inflow volume for WY2012 is now projected to be 5.48 maf (51% of average). The maximum probable (i.e. 10% likely to be exceeded) unregulated inflow volume for WY2012 is now projected to be 12.65 maf (117% of average).
Based on this range of possible inflow conditions, the January 24-Month Study projects the annual release volume for WY2012 to be as lows as 9.46 maf (under the minimum probable inflow condition) to as high as 13.01 maf (under the maximum probable inflow condition). 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 3643.9 feet above sea level. This elevation corresponds to a live storage volume of 16.47 maf (68% 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 10, 2012 the total reservoir storage in the Colorado River Basin was 38.54 maf (64.8% of capacity).
Updated: January 11, 2012
This information is from Rick Clayton, Hydraulic Engineer/Glen Canyon Dam Upper Colorado Region Bureau of Reclamation (801)524-3710, posted here by GCPBA.
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