Glen Canyon Dam Update September 28, 2010

Glen Canyon Dam Lake Powell 
During September 2010 through September 26, 2010 the unregulated inflow
volume to Lake Powell is trending towards 266 thousand acre feet (kaf) (56%
of average). This will be approximately 144 kaf below what was projected in
the September 24-Month Study and as a result the elevation of Lake Powell at
the end of September will be about 1 foot lower than what was projected in
the September 24-Month Study. The September 30th elevation of Lake Powell
will likely be approximately 3633.7 feet above sea level. This projected
ending elevation corresponds to a live storage of 15.27 million acre feet
(maf) which is 62.8% of the full capacity of 24.32 maf.
During August the release volume from Glen Canyon Dam was 801.7 kaf and the
hourly releases during most days fluctuated between a peak of 16,500 cubic
feet per second (cfs) during the day and a low of 8,500 cfs during the
evening and early morning for power generation. On September 1, 2010 and
continuing through October 31, 2010, the releases from Glen Canyon Dam will
be steady with no fluctuations for power production (excluding system
regulation and spinning reserves) for the steady flow experiment pursuant to
the February 2008 Finding of No Significant Impact Experimental Releases
from Glen Canyon Dam, Arizona 2008 through 2012. This will be the third year
of steady flows of the 5 year experiment. The steady release rate is 8,000
cfs which is equivalent to a monthly release volume of approximately 476,000
acre-feet in September 2010 and 492,000 acre-feet in October 2010.
During the steady flow experiment the instantaneous releases from Glen
Canyon Dam may fluctuate somewhat to provide approximately 40 megawatts
(approximately 1,100 cfs) 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
targeted steady release target (8000 cfs). These momentary fluctuations for
regulation are very short lived and will typically balance out over the
hour. Spinning and non-spinning reserve generation will also be carried at
Glen Canyon Dam during the steady flow experiment. 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 83 megawatts
(approximately 2,250 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 83 megawatts.
The August 2010 24-Month Study (most probable inflow scenario) projected the
January 1, 2011 elevation of Lake Powell to be 3628.73 feet.  Pursuant to
the Interim Guidelines, the determination is that the Operational Tier for
water year 2011 will be the Upper Elevation Balancing Tier.  Under this
Operational Tier, there is a possibility that the annual release volume from
Lake Powell could be 8.23 maf. There is also a possibility that Equalization
or Balancing could occur in 2011 which would result in an annual release
volume greater than 8.23 maf. The possibility of Equalization or Balancing
in 2011 will depend on the reservoir conditions projected for the end of
water year 2011 in the April 2011 24-Month Study with the most probable
inflow scenario and 8.23 maf projected for release from Lake Powell. The
September 2010 24-Month Study indicates that Equalization is likely to be
triggered in April 2011 and the annual release volume for water year 2011 is
projected to be 11.28 maf.
There is a high level of uncertainty regarding the hydrologic conditions
that will be experienced in water year 2011. Each month, the 24-Month Study
will be updated to reflect current reservoir conditions and the most
probable inflow forecast. The projected annual release volume for water year
2011 in the 24-Month Study will reflect the implementation of the Upper
Elevation Balancing Tier with updated hydrologic conditions and is therefore
likely to change each month. It is possible that a relatively small change
in the forecast could have a large impact on the projected annual release
volume.   Based on the current inflow forecast (dated September 1, 2010),
there is approximately a 58% probability that Equalization will occur in
water year 2011.
The current  inflow forecast for Lake Powell projects the most probable
unregulated inflow volumes for the next 3 months as follows: Sepember-400
kaf (84% of average); October-475 kaf (87% of average; November-460 kaf (84%
of average). The outlook for water year 2011 (dated August 3, 2010)
projected the most probable  unregulated inflow volume to Lake Powell during
water year 2011 to be 10.75 maf (89% of average).  It is likely the
unregulated volume of inflow to Lake Powell in water year 2011 will be
greater than or less than the most probable projection.  The range of
possible unregulated inflow volumes to Lake Powell is currently projected to
be as dry as 5.0 maf (40% of average) to as wet as 17.1 maf (142% of
average).  In October, this hydrologic outlook for water year 2011 will be
The September 2010 24-Month Study has been published and is available here:
Updated elevation projections for Lake Powell through water year 2010 based
on the most recently published 24-Month Study are maintained at:
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 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 90%, April 120%, May 75%, June 100%, July 95%. The
overall estimated precipitation percentage of average thus far in water year
2010 for the Upper Colorado River Basin is 96% of average.
The Climate Prediction Center outlook (dated August 19, 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 below 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 maf, 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 maf (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 September 1,
2010 the storage in Lake Powell was 15.36 maf (63.1 % of capacity) which is
still below desired levels while the overall reservoir storage in the
Colorado River Basin as of September 1, 2010 is 33.73 maf (56.7 % of
RRFW thanks Rick Clayton of the USBOR for his assistance in providing
information for this notification.
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