Home > Uncategorized > Mead and Powell: The Current Status Tango

Mead and Powell: The Current Status Tango

2010 October 5

The United States Bureau of Reclamation Glen Canyon Dam / Lake Powell Current Status
http://www.usbr.gov/uc/water/crsp/cs/gcd.html

Glen Canyon Dam / Lake Powell –During September 2010 through September 26, 2010 the unregulated inflow volume to Lake Powell is trending towards 266 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 maf which is 62.8% of the full capacity of 24.32 maf.

A bunch of key phrases to catch here. The September 24-month study was optimistic in regards to input. Inflow was an actual 266 thousand acre-feet in contrast with a projected 410 kaf. The current elevation is 3633.7′. Current elevations are important as they are used (in conjuction with Lake Mead elevations) to determine Lake Powell releases. The current 62.8% capacity is much improved over the 2005 capacity.

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 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.

Daily flows are of interest to river rafters, fishermen, and river ecologists. Likewise, it is interesting to note the 5 year steady-flow experiment.

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.

So ‘steady’ for a sufficiently vague sense of the term. ;-)

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.

This is the meat of the status report as far as Lake Mead elevations are concerned. The way I read this is as follows: The Lake Powell August 2010 24-Month Study is used to project January elevations in that reservoir. That elevation is used to determine a “Balancing Tier.” (see below) These tiers are used in conjuction with projected Lake Mead elevations to determine scheduled Lake Powell outflows. The likely ‘baseline’ for 2011 will be 8.23 million acre-feet. An April 2011 projection of Lake Mead at < 1075 in September 2011 could trigger additional releases which could bring the total yearly release to 11.28 maf.

This is different from my understanding in the previous Lake Mead post. I’ve learned an additional nuance to the 1075′ trigger.

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.

If I read this right, the USBR is projecting a 58% probability that the April 2011 24-month study will project that September 2011 Lake Mead water level will be below 1075′. But I’m groping in the dark. This language is still new to me.

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 updated.

Wow. That’s quite a range. 5 maf to 17.1 maf with a projected 10.75 maf at 89% of average.

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: Lake Powell Projected Elevations.

You can find the 24-month studies here:
http://www.usbr.gov/lc/region/g4000/24mo/

A snapshot of the Lake Powell elevation tiers from the Interim Guidelines

lake-powell-tiers.jpg

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  1. 2010 October 12 at 5:01 am | #1

    I first came across the acre-foot when I was designing contactless water meter protocols, and I thought it was a joke unit of volume. I still can’t take it seriously (not your fault, obviously).

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