RRFW Riverwire – Glen Canyon Dam Update: Bug Flows

April 18, 2018

Current Status

The 2016 Glen Canyon Dam Long Term Experimental and Management Plan Record of Decision (LTEMP ROD) provides the framework for implementing flow-based experiments at Glen Canyon Dam when resource conditions warrant. The purpose of LTEMP experiments is to learn, through adaptive management, how to better protect, mitigate adverse effects to, and improve resources downstream of Glen Canyon Dam, while complying with relevant laws. Ongoing research and monitoring through the Glen Canyon Dam Adaptive Management Program ensures the best science and data is available for making decisions related to experimental releases.

Under the LTEMP, the Department may conduct flow-based experiments (High Flow Experiments, Bug Flows, Trout Management Flows, and Low Summer Flows) at Glen Canyon Dam when resource conditions warrant and if it is determined that there will not be unacceptable adverse impacts on other resources. This is the first year of implementing flow-based experiments under LTEMP, and Reclamation has been following a process similar to that established for High Flow Experiments under prior operational decisions.

The Glen Canyon Dam Experimental Technical Team (Technical Team) recommends that experimental Macroinvertebrate Production Flows (Bug Flows) be implemented at Glen Canyon Dam beginning May 1 through August 31, 2018.

The Bug Flow experiment consists of steady weekend releases from Glen Canyon Dam and normal fluctuating releases during the weekdays. The steady weekend flows are expected to provide favorable conditions for insects to lay eggs along the Colorado River downstream of Glen Canyon Dam, while the minimum flows on weekdays are designed to be similar to flows on the weekends, thus preventing the eggs from drying out. Performing this experiment will not affect the Monthly or Weekly planned release volumes. The affect will only be to the daily distribution volumes, and the peak and low daily flow rates. At this time we do not have a firm confirmation for a Bug Flow event. However, a final determination for this experiment will likely be made near the end of April.

The unregulated inflow volume to Lake Powell in March was 332 thousand acre-feet (kaf) (50 percent of average). The release volume from Glen Canyon Dam in March was 800 kaf. The end of March elevation and storage of Lake Powell were 3,612 feet (88 feet from full pool) and 13.0 million acre-feet (maf) (53 percent of full capacity), respectively. With a much below average snowpack this water year, the anticipated spring inflows will not be enough to arrest the falling elevation levels on the lake. The reservoir is projected to reach new seasonal low elevations as time progresses on through the end of this year. The basin snowpack reached a peak of 72 percent of the seasonal medium peak on April 10th.

To view the most current reservoir elevation, content, inflow and release, click on:

https://www.usbr.gov/rsvrWater/rsv40Day.html?siteid=919&reservoirtype=Reservoir

To view the most current reservoir elevation projections, click on:

https://www.usbr.gov/uc/water/crsp/studies/images/PowellElevations.png

To view the 2017 progression of snowpack above Lake Powell, click on:

https://www.usbr.gov/uc/water/notice/Graphs/Upper_Colorado.png

To view the current inflow forecast relative to past inflows, click on:

https://www.usbr.gov/uc/water/crsp/studies/images/PowellForecast.png

Current Operations

The operating tier for water year 2018 was established in August 2017 as the Upper Elevation Balancing Tier. In the Upper Elevation Balancing Tier the initial water year release volume is 8.23 million acre feet (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 2018 24-Month Study establishes that Lake Powell operations will shift to “balancing releases” for the remainder of water year 2018. 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 2018; the actual release in water year 2018, 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 2018 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, 2018.

In April, the release volume will be approximately 705 thousand acre feet (kaf), with fluctuations anticipated between about 8,650 cubic feet per second (cfs) in the nighttime to about 15,000 cfs in the daytime and consistent with the Glen Canyon Dam, Record of Decision (dated December 2016). The anticipated release volume for May is 705 kaf with daily fluctuations between approximately 7,050 cfs and 13,390 cfs. The expected release for June is 760 kaf with daily fluctuations between approximately 8,850 cfs and 16,450 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 up to 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 currently maintains 27 MW (approximately 800 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 2018 water supply forecast for unregulated inflow to Lake Powell, issued on April 3, 2018, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 3.1 maf (43 percent of average based on the period 1981-2010). The projected water year 2018 inflow is 5.6 maf (124 percent). At this early point in the season, there is still significant uncertainty regarding this year’s water supply. The April-July forecast ranges from a minimum probable of 1.9 maf (27 percent) to a maximum probable of 4.9 maf (68 percent). There is a 10 percent chance that inflows could be higher than the current maximum probable forecast and a 10 percent chance that inflows could be lower than the minimum probable forecast.

As determined in the August 2017 24-Month Study, and documented in the 2018 Annual Operating Plan, Lake Powell’s operations in water year 2018 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 2018.  This April 2018 24-Month Study projects the end of water year elevation at Lake Powell to be above 3,575 feet and the end of water year elevation at Lake Mead to be below elevation 1,075.0 feet. 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 2018. 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 2018 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 a 10 percent 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 2018 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 2018 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 2018 near 3,598 feet with approximately 11.6 maf in storage (48 percent 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 3,589 feet and 10.7 maf (44 percent capacity), respectively. Under the maximum probable inflow scenario, updated in April, the projected end of water year elevation and storage are 3616 feet and 13.3 maf (55 percent 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 18-year period 2000 to 2017, 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 4 out of the past 18 years. The period 2000-2017 is the lowest 18-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.76 maf, or 81 percent 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 2000-2017 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. In water year 2017 unregulated inflow volume to Lake Powell was 11.9 maf (110 percent of average), the fourth year to be above average. Under the current most probable forecast, the total water year 2018 unregulated inflow to Lake Powell is projected to be 5.6 maf (52 percent of average). At the beginning of water year 2018, total system storage in the Colorado River Basin was 32.9 maf (55 percent of 59.6 maf total system capacity). This is an increase of 2.7 maf over the total storage at the beginning of water year 2017 when total system storage was 30.2 maf (51 percent 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 percent of capacity at the beginning of 2000 to a low of 50 percent of capacity at the beginning of water year 2005.

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 total Colorado Basin reservoir storage for water year 2018 is approximately 28.9 maf (48 percent of total system capacity). The actual end of water year 2018 system storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and reservoir inflow. Based on the April minimum and maximum probable inflow forecasts and modeling, the range of end of water year 2018 total system capacity is approximately 27.7 maf (46 percent of capacity) to 30.9 maf (52 percent of capacity), respectively.

This release courtesy Bureau of Reclamation

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