Glen Canyon Dam Update December 21, 2010

During December 2010, through the first 19 days of the month, the
unregulated inflow to Lake Powell has been tracking towards a monthly volume
of 383 kaf (88% of average). This volume is slightly above the volume
forecasted for December by the Colorado Basin River Forecast Center on
December 1, 2010 which was 360 kaf (83% of average). Although the inflow to
Lake Powell during December is projected to be above what was forecasted at
the beginning of December, the water surface elevation of Lake Powell at the
end of December will likely be about 0.3 feet below what was projected in
the December 24-Month Study. The December 24-Month Study projected that Lake
Powell would end December at an elevation of 3626.4 feet above sea level.
The elevation of Lake Powell at the end of the day on December 31, 2010 will
likely be about 3626.1 feet above sea level. The end of month storage in
Lake Powell for December 2010 will likely be about 14.6 million acre feet
(maf) which is 60.1% of the full capacity of 24.32 maf.

The release volume scheduled for December is 845 thousand acre feet (kaf)
which is equivalent to an average daily release rate of approximately 13,750
cubic feet per second (cfs). Daily fluctuations during December have been
and will continue to peak near 16,000 cfs during the morning and early
evening hours. Daily low releases have been and will continue to occur
during the early morning hours (i.e. midnight to about 6:00 am) to about
8,500 cfs. The projected release volume for January is currently 865 kaf
which is equivalent to an average daily release rate of approximately 14,050
cfs. The daily peak and low release rate in January will likely also range
from 16,000 cfs to about 9,500 cfs respectively.

In addition to the daily fluctuation pattern for 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 is also
maintained at Glen Canyon Dam. When an unanticipated electrical outage event
occurs within the electrical transmission system, this reserve generation at
Glen Canyon Dam can be called upon up to a limit 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 limit of 98 megawatts.

In August of 2010, the August 2010 24-Month Study Model was used to project
the January 1, 2010 elevation of Lake Powell and Lake Mead under the most
probable inflow scenario. Pursuant to the 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 the Upper Elevation
Balancing Tier, there is a possibility that the annual release volume from
Lake Powell could be 8.23 maf. There is also a possibility under this tier
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 is dependent on the end
of water year 2011 reservoir conditions projected in the April 2011 24-Month
Study under the most probable inflow scenario and with 8.23 maf projected
for release from Lake Powell during water year 2011.  For this reason it
will not be known for certain whether Equalization or Balancing will occur
in water year 2011 until April 2011.  24-Month Studies prior to April 2011
can project that Equalization or Balancing are likely to occur, but these
projections are subject to change with changes in the forecasted hydrology
of the Colorado River Basin.  It is possible that a relatively small change
in forecasted hydrology can have a large impact on the projected annual
release volume.

The December 2010 24-Month Study with the most probable inflow scenario for
water year 2011 did project that Balancing is likely to occur in 2011.   For
this reason, the projected most probable annual release volume for water
year 2011 in the December 24-Month Study was 9.00 maf. Given the current
range of uncertainty of the forecasted hydrology for water year 2011, it is
possible that Equalization could also occur in water year 2011 which would
result if the annual release being greater than about 10.7 maf.  Each month
the 24-Month Study is updated to reflect the most probable inflow scenario
which is based on the most recent forecast from the Colorado River Basin
Forecast Center (CBRFC). In January, the CBRFC will issue the first water
supply forecast for 2011. This new forecast could potentially change the
most probable inflow scenario and projected annual release from Glen Canyon the January 24-Month Study. Analysis of the probable range of inflows
that could occur during water year 2011 indicates that the probability of
Equalization occurring in 2011 is currently about 48%.  This probability
will be updated during the first part of January 2011.

The unregulated inflow forecast for Lake Powell over the next 3 months is as
follows: December-360 kaf (83% of average); January-350 kaf (86% of
average); February-350 kaf (83% of average). The outlook for water year
2011, incorporating this new forecast, projects the most probable
unregulated inflow volume to Lake Powell during water year 2011 to be 9.66
maf (80% of average). It is possible that the unregulated volume of inflow
to Lake Powell in water year 2011 will be greater than or less than the most
probable projection. The probable range of unregulated inflow volumes to
Lake Powell during water year 2011 is currently projected to be as dry as
4.5 maf (37% of average) to as wet as 15.8 maf (131% of average).

The December 2010 24-Month Study has been published and is available here:

Updated elevation projections for Lake Powell through water year 2011 and
2012 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 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%)

The Climate Prediction Center outlook (dated November 18, 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.

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 December 19, 2010 the storage in Lake Powell was
approximately 14.61 million acre-feet (60.1 % of capacity) which is below
desired levels. The overall reservoir storage in the Colorado River Basin as
of December 19, 2010 is approximately 32.19 million acre-feet (54.1 % of

RRFW thanks Rick Clayton of the USBOR for this update.


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