RRFW Riverwire - Glen Canyon Dam Update
February 10, 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.
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