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Old 04-27-2016, 10:35 AM
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T Gunn T Gunn is offline
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Lees Ferry Water Flow Update 4/27/16

Dam Operations

APRIL 27, 2016

CURRENT STATUS

The unregulated inflow volume to Lake Powell in March was 553 thousand
acre-feet (kaf) (83 percent of average). The release volume from Glen
Canyon Dam in March was 694 kaf. The end of March elevation and storage
of Lake Powell were 3,592 ft (108 feet from full pool) and 11.02 maf
(45% of full capacity), respectively. The reservoir elevation is near
the anticipated seasonal low and will soon begin increasing as spring
runoff enters the reservoir.

CURRENT OPERATIONS

In April, the release volume will be approximately 664 kaf, with
fluctuations anticipated between about 8,000 cfs in the nighttime to
about 14,000 cfs in the daytime and consistent with the Glen Canyon
Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997).
The anticipated release volume for May is 700 kaf with daily
fluctuations between approximately 8,000 cfs and 14,000 cfs. The
expected release for June is 800 kaf with daily fluctuations between
approximately 9,000 cfs and 17,000 cfs. The anticipated release volume
for July 2016 is 950,000 kaf. This will be confirmed in a subsequent
directive toward the end of May.

The operating tier for water year 2016 was established in August 2015
as the Upper Elevation Balancing Tier. In the Upper Elevation Balancing
Tier the initial water year release volume is 8.23 maf; however, there
is the possibility for an April adjustment to equalization or balancing
operations to govern for the remainder of the water year. This April
2016 24-Month Study establishes that Lake Powell operations will shift
to “balancing releases” for the remainder of water year 2016. Under
Balancing, the contents of Lake Powell and Lake Mead will be balanced
by the end of the water year, but not more than 9.0 maf and not less than
8.23 maf shall be released from Lake Powell. Based on the most probable
inflow forecast, this April 24-Month Study projects a balancing release
of 9.0 maf in water year 2016; the actual release in water year 2016,
however, will depend on hydrology in the remainder of water year and
will range from 8.23 to 9.0 maf. The projected release from Lake Powell
in water year 2016 will be updated each month throughout the remainder
of the water year. Reclamation will schedule operations at Glen Canyon
Dam to achieve as practicably as possible the appropriate total annual
release volume by September 30, 2016.

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 1,200 cfs above or below the
hourly scheduled release rate. Under system normal conditions,
fluctuations for regulation are typically short lived and generally
balance out over the hour with minimal or no noticeable impacts on
downstream river flow conditions.

Releases from Glen Canyon Dam can also fluctuate beyond scheduled
releases when called upon to respond to unscheduled power outages or
power system emergencies. Depending on the severity of the system
emergency, the response from Glen Canyon Dam can be significant, within
the full range of the operating capacity of the power plant for as long
as is necessary to maintain balance in the transmission system. Glen
Canyon Dam typically maintains 30 mw (approximately 880 cfs) of
generation capacity in reserve in order to respond to a system
emergency even when generation rates are already high. System emergencies occur
fairly infrequently and typically require small responses from Glen
Canyon Dam. However, these responses can have a noticeable impact on
the river downstream of Glen Canyon Dam.

INFLOW FORECASTS AND MODEL PROJECTIONS

The April to July 2016 water supply forecast for unregulated inflow to
Lake Powell, issued on April 4, 2016, by the Colorado Basin River
Forecast Center, projects that the most probable (median) unregulated
inflow volume will be 5.3 maf (74 percent of average based on the
period 1981-2010). The forecast decreased by 400 kaf since last month. At this
point in the season, there is still uncertainty regarding this year’s
water supply and the total inflow to Lake Powell. The spring runoff
forecast ranges from a minimum probable of 3.85 maf (54 percent of
average) to a maximum probable of 7.65 maf (107 percent of average).
There is 10% chance that inflows could be higher than the maximum
probable and a 10% chance they could be lower than the minimum
probable.

As determined in the August 2015 24-Month Study, and documented in the
2016 Annual Operating Plan, Lake Powell’s operations in water year 2016
will be governed by the Upper Elevation Balancing Tier. In this tier,
the initial water year release volume is 8.23 maf, however, there is
the potential for an April adjustment to equalization or balancing releases
in April 2016. This April 2016 24-Month Study projects the end of water
year elevation at Lake Powell to be above 3,575 ft and the end of water
year elevation at Lake Mead to be below elevation 1,075.0 ft.
Therefore, in accordance with Section 6.B.4 of the 2007 Interim Guidelines, Lake
Powell operations will shift to “balancing releases” for the remainder
of water year 2016. Under Section 6.B.4, the contents of Lake Powell
and Lake Mead will be balanced by the end of the water year, but not more
than 9.0 maf and not less than 8.23 maf shall be released from Lake
Powell.

Based on the April most probable inflow forecast, the annual release
volume from Lake Powell during water year 2016 is projected to be 9.0
maf. Under the minimum probable inflow scenario, the water year release
is projected to be 9.0 maf. Under the maximum probable inflow scenario,
the release is projected to be 9.0 maf. There is 10% chance that
inflows will be lower than the current minimum probable forecast, potentially
resulting in lower releases. If inflows are less than the minimum
probable forecast, the water year 2016 annual release could be as low
as 8.23 maf. If inflows are greater than the current forecasted maximum
probable inflow, the annual release will be 9.0 maf. The projected
release from Lake Powell in water year 2016 will be updated each month
throughout the remainder of the water year.

Based on the current forecast, the April 24-Month Study projects Lake
Powell elevation will end water year 2016 near 3,600 ft with
approximately 11.75 maf in storage (48% capacity). Projections of
elevation and storage still have significant uncertainty at this point
in the season, primarily due to uncertainty regarding spring runoff and
the resulting inflow to Lake Powell. Under the minimum probable inflow
scenario, updated in April, the projected end of water year elevation
and storage are 3585 ft and 10.35 maf (43% capacity), respectively.
Under the maximum probable inflow scenario, updated in April, the
projected end of water year elevation and storage are 3622 ft and 14.01
maf (58% capacity), respectively. Modeling of projected reservoir
operations based on the minimum and maximum scenarios will be updated
again in August.

UPPER COLORADO RIVER BASIN HYDROLOGY

The Upper Colorado River Basin regularly experiences significant year
to year hydrologic variability. During the 16-year period 2000 to 2015,
however, the unregulated inflow to Lake Powell, which is a good measure
of hydrologic conditions in the Colorado River Basin, was above average
in only 3 out of the past 16 years. The period 2000-2015 is the lowest
16-year period since the closure of Glen Canyon Dam in 1963, with an
average unregulated inflow of 8.39 maf, or 78% of the 30-year average
(1981-2010). (For comparison, the 1981-2010 total water year average is
10.83 maf.) The unregulated inflow during the 2000-2015 period has
ranged from a low of 2.64 maf (24 percent of average) in water year
2002 to a high of 15.97 maf (147 percent of average) in water year 2011. The
water year 2015 unregulated inflow volume to Lake Powell was 10.174 maf
(94 percent of average), which, though still below average, was
significantly higher than inflows observed in 2012 and 2013 (45% and 47
percent of average, respectively).

Under the current most probable forecast, total water year 2016
unregulated inflows to Lake Powell is projected to be 8.44 maf (78
percent of average), and ranges from a minimum probable inflow of 6.86
maf (63%) and maximum probable inflow of 11.13 maf (103%).

At the beginning of water year 2016, total system storage in the
Colorado River Basin was 30.3 maf (51% of 59.6 maf total system
capacity). This is nearly the same as the total storage at the
beginning of water year 2015 which began at 30.1 maf (50% of capacity). Since the
beginning of water year 2000, total Colorado Basin storage has
experienced year to year increases and decreases in response to wet and
dry hydrology, ranging from a high of 94% of capacity at the beginning
of 2000 to a low of 50% of capacity at the beginning of water year
2014.
One wet year can significantly increase total system reservoir storage,
just as persistent dry years can draw down the system storage. Based on
current inflow forecasts, the current projected end of water year 2016
total Colorado Basin reservoir storage is approximately 29.0 maf (49%
of capacity).

The actual end of water year storage may vary from this projection,
primarily due to uncertainty regarding this season’s runoff and
resulting reservoir inflow. Based on the April minimum and maximum
probable inflow forecasts and modeling the range is approximately 27.6
maf (46%) to 31.4 maf (53%), respectively.

This update courtesy of Paul Davidso
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