The unregulated inflow volume to Lake Powell in May was 1,121 thousand acre-feet (kaf) (48% of average). The release volume from Glen Canyon Dam in May was 602 kaf. The end of May elevation and storage of Lake Powell were 3599.4 feet (100.6 feet from full pool) and 11.68 million acre-feet (maf) (48% of full capacity). The reservoir elevation is expected to remain within several feet of the current elevation throughout spring and summer as inflow from runoff roughly matches reservoir releases. In late summer, the reservoir elevation will begin to decline again.
Releases from Glen Canyon Dam in June are currently averaging approximately 13,000 cubic feet per second (cfs) with daily fluctuations between approximately 9,000 cfs at nighttime and approximately 17,000 cfs during the daytime and consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The scheduled release volume for June 2013 is 800 thousand acre feet (kaf).
In July, the release volume will likely be about 847 kaf, with daily fluctuations for hydropower between approximately 10,000 cfs in the nighttime and approximately 18,000 cfs in the daytime. In August, the release volume will likely be about 800 kaf with daily fluctuations between approximately 9,000 cfs and 17,000 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 1,100 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). Reserves provide system reliability in the event of an unscheduled outage. Glen Canyon Dam typically maintains 43 MW of reserves (approximately 1,100 cfs). Reserve calls can be maintained for a maximum of 2 hours after which time the generation rate should be returned to the original schedule. If reserves from Glen Canyon Dam are called upon, releases from the dam can exceed scheduled levels and can have a noticeable impact on the river downstream from Glen Canyon Dam. Calls for reserves are fairly infrequent and typically are for much less than 43 MW.
The operating tier for 2013 is the Upper Elevation Balancing Tier, as establish in August 2012 and pursuant to the Interim Guidelines. Since the April 2103 projected end of water year elevation at Lake Powell was below the 2013 Equalization Elevation of 3,646.0 feet and the projected end of water year elevation at Lake Mead was above elevation 1,075.0 feet, Section 6.B.1 and 6.B.4 of the Interim Guidelines provide for an annual release volume of 8.23 maf from Lake Powell during water year 2013. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible an 8.23 maf annual release volume by September 30, 2013.
Inflow Forecasts and Model Projections
The hydrologic forecast for Lake Powell for the April to July water supply season projects that the most probable (median) unregulated inflow volume will be 3.0 maf (42% of average based on the period 1981-2010). The April-July most probable forecast did not change from last month and the overall water supply outlook remains significantly below average. The minimum probable and maximum probable forecasts are 2.48 maf (35%) and 3.48 maf (49%), respectively. Based on the current forecast, the June 24- Month study projects Lake Powell elevation will decline approximately 11 feet from June to September and end the water year at 3588.8 feet with 10.71 maf in storage (44% capacity). The annual release volume from Lake Powell during water year 2013 is scheduled to be 8.23 maf. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible an 8.23 maf annual release volume by September 30, 2013.
Consistent with Section 6.C.1 of the Interim Guidelines, if the August 24-Month study projects the January 1, 2014, Lake Powell elevation to be less than 3,575.0 feet and at or above 3,525.0 feet and the Lake Mead elevation to be at or above 1,025.0 feet, the operational tier for Lake Powell in water year 2014 would be the Mid-Elevation Release Tier and the water year release volume from Lake Powell would be 7.48 maf. This June 2013 24-Month study projects that, with an 8.23 maf annual release pattern in water year 2014, the January 1, 2014, Lake Powell elevation would be 3,577.05 feet and the Lake Mead elevation would be 1,105.27 feet.
Therefore, the 2014 Lake Powell operational tier is currently projected to be the Upper Elevation Balancing Tier with an annual release volume of 8.23 maf and no projected shift in April to the Equalization Tier. However, if hydrology should become slightly drier than is currently projected and the August 24- Month Study projects the January 1, 2014 Lake Powell elevation to be less than 3,575.00 feet, the Mid-Elevation Release Tier will govern and the annual release volume from Lake Powell will be 7.48 maf. Based on analysis of a range of inflow scenarios, the current probability of realizing an inflow volume that would result in the Mid-Elevation Release Tier and a 7.48 maf annual release is approximately 45 percent.
Upper Colorado River Basin Hydrology
Since 2005 the Upper Colorado River Basin has experienced significant year to year hydrologic variability. During the period 2005 through 2012, the unregulated inflow to Lake Powell, which is a good measure of hydrologic conditions in the Colorado River Basin, averaged a water year volume of 10.22 maf (94% of average (period 1981-2010)). The unregulated inflow has ranged from a low of 4.91 maf (45% of average) in water year 2012 to a high of 15.97 maf (147% of average) in water year 2011. This has been an improvement over the persistent drought conditions of 2000 to 2004, which averaged a water year unregulated inflow of 5.73 maf. However, based on observed inflows and current forecasts, water year 2013 unregulated inflow is expected to be 4.77 maf (44% of average), which would be a second below-average year in a row.
At the beginning of water year 2013, total system storage in the Colorado River Basin was 33.9 maf (57 % of capacity), which was an increase of about 4 maf since water year 2005 which began at 29.8 maf (50% of capacity). Since 2005, however, total Colorado Basin storage has experienced year to year increases and decreases in response to wet and dry hydrology. In addition, conditions in both 2012 and 2013 have been significantly drier than average and given observed inflows and current forecasts, the current projected end of water year 2013 total reservoir storage is approximately 29.3 maf (49% of capacity).
This update courtesy of Katrina Grantz, Bureau of Reclamation