RRFW Riverwire – Glen Canyon Dam Update
October 13, 2011
Glen Canyon Dam Lake Powell
During September 2011 the unregulated inflow volume to Lake Powell was 532 thousand acre feet (kaf) or 112% of average. This was approximately 93 kaf below what was projected in the September 24-Month Study and resulted in the elevation of Lake Powell ending September about 0.92 feet below what was projected in the September 24-Month Study. The September 30th, 2011 elevation of Lake Powell was 3653.01 feet above sea level which corresponds to a live storage of approximately 17.59 million acre feet (maf) and 72.3% of the full capacity of 24.32 maf.
For water year 2011, the observed unregulated inflow volume to Lake Powell was 16.77 maf (139% of average for the 1971-2000 historical period of record). The 2011 water year unregulated inflow volume was the 6th wettest out of 48 years since the closure of Glen Canyon Dam (1963). Water year unregulated inflow volumes of the magnitude observed in water year 2011 (or greater) would statistically be expected to occur in about 12-14% of all years.
The 2011 water year release volume from Glen Canyon Dam was 12.52 maf and this was the largest water year release volume made from Glen Canyon Dam since water year 1998. During water year 2011 the above average inflow volume combined with the large water year release volume from Glen Canyon Dam resulted in Lake Powell realizing a net gain in elevation (year over year) of 19.35 feet which translates to an increase in live storage in Lake Powell of 2.32 maf.
Current Dam Operations
Releases from Glen Canyon Dam are now being made for a steady flow experiment that will continue to the end of October. Releases from Glen Canyon Dam are steady at approximately 15,500 cfs and will likely remain at the level through October 31, 2011 to complete the 2 month steady flow experiment.
In early November through November 10, 2011, releases will likely continue to be steady near 15,500 cfs due to ongoing maintenance work at Glen Canyon Power-plant on units 5 and 6. On or about November 11, 2011 releases will likely be increased to approximately 22,600 cfs when units 5 and 6 are returned to service. The projected release volume for November is 1200 kaf.
While the release rate from Glen Canyon Dam over the next several months will likely be near steady, the instantaneous releases from Glen Canyon Dam may fluctuate somewhat to provide 40 MW of system regulation. These instantaneous release adjustments maintain stable conditions within the electrical generation and transmission system and result in momentary release fluctuations within a range of about 1100 cfs above or below the targeted hourly release rate. The momentary fluctuations for regulation are very short lived and typically balance out over the hour.
Spinning and non-spinning reserve generation is also maintained at Glen Canyon Dam. In order for Colorado River Storage Project (CRSP) power-plants to participate in the electrical generation and transmissions system, these power-plants must maintain a level of generation capacity available in reserve to assist the local control area for when unanticipated generation outages occur. The current CRSP power plant reserve requirement is 100 MW (equivalent to approximately 2,675 cfs of release from Glen Canyon Dam). When an electrical outage occurs within the control area, CRSP power-plants can be called upon to provide up to 100 MW of additional generation for up to 2 hours. Under normal circumstances, calls for reserves are infrequent and for much less than the required 100 MW. Because Glen Canyon Power-plant is the largest facility of the CRSP power-plants, most of the CRSP reserve requirement is maintained at Glen Canyon Dam.
Current Inflow Forecasts and Model Projections
Over the next three months (October, November and December) the forecasted unregulated inflow to Lake Powell is projected to be above average with monthly percent of average forecasts of 119%, 115% and 120%, respectively. The hydrologic outlook forecast for water year 2012 has been revised in October and now projects that the most probable (median) unregulated inflow volume to be 11.6 maf (96% of average based on the period from 1971 through 2000). Based on this revised hydrologic outlook forecast, the October 24-Month Study projects the annual release volume for water year 2012 will likely be 12.26 maf. The October 24-Month Study also projects that the end of water year reservoir elevation and storage for Lake Powell will likely be 3645.00 feet (55.00 feet from full pool) and 16.60 maf (68% of capacity), respectively.
Upper Colorado River Basin Hydrology
Since water year 2005, hydrologic conditions in the Upper Colorado River Basin have been slightly below 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 11.15 maf per year during the period from 2005 through 2011. This is slightly below the official average of 12.04 maf per year. The hydrologic variability during this period has been from a low water year unregulated inflow of 8.40 maf (70% of average) in water year 2006 to a high of over 16.77 maf (139% 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 October 12, 2011, the total reservoir storage in the Colorado River Basin was 38.62 maf (64.9% of capacity).
RRFW thanks Rick Clayton of the USBOR for this information.
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