Glen Canyon Dam, Lake Powell
Snowpack conditions above Lake Powell have been well below average all year and are now nearly melted out. The runoff from the melting snow has been less than impressive and the inflow to Lake Powell so far has peaked at just over 15,000 cubic feet per second (cfs). It is possible that this peak could be exceeded if temperature conditions warm quickly in the coming weeks. The Colorado Basin River Forecast Center will update the Water Supply Forecast for Lake Powell (April through July Unregulated Inflow Volume) in early May. In April this forecast was for 3.50 maf (49% of average) of unregulated inflow to Lake Powell.
The monthly unregulated inflow volume to Lake Powell for April was 764 thousand acre-feet (kaf) (72% of average). This was 36 kaf below what was forecasted in early April. The release volume from Glen Canyon Dam in April was 606 kaf which was 6,000 acre-feet above what was scheduled for release during the month. As a result of the difference between the projections made in early April and actual conditions and operations that occurred in April, the elevation of Lake Powell at the end of April was 0.17 feet (about 2 inches) higher than projected. On April 30, 2012 the elevation of Lake Powell was 3635.76 feet above sea level (64.24 feet below full pool).
Current Dam Operations
Releases from Glen Canyon Dam are now averaging about 10,050 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 and this operation is consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The release volume for May is scheduled to be 600 kaf. In June, the monthly release volume will likely be about 714 kaf. Release fluctuations in June are projected to be in the range from about 9,000 cfs during the early morning hours to an afternoon peak of about 15,000 cfs.
In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 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 megawatts (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.
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 million acre feet (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.
Current Inflow Forecasts and Model Projections
Over the next three months (May, June and July) the forecasted unregulated inflow volume to Lake Powell is projected to be 1,050 kaf (45% of average), 1,150 kaf (43% of average) and 500 kaf (46% of average), respectively. These percent of averages are all based on the historic period from 1981 through 2010. Combining this forecast with the April 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 6.79 maf (63% of average).
Comparing this projected water year unregulated inflow volume to the driest year on record (2002) in which the unregulated inflow volume was only 2.64 maf (24% of average), water year 2012 will likely be very dry yet not close to the driest year on record. There is still a fair amount of uncertainty associated with the forecast conditions for the remainder of the water year.. Recent analysis indicates that it is reasonably possible for the actual unregulated inflow volume for water year 2012 to be as low as 4.9 maf (45% of average) or as high as 9.2 maf (85% 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 9.81 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 3632.5 feet above sea level. This elevation corresponds to a live storage volume of 15.14 maf (62 % of full capacity). These projections are based on conditions in the April 24-Months Study.
The April 2012 24-Month Study has been published and is available here:
http://www.usbr.gov/uc/water/crsp/studies/24Month_04.pdf
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 a water year volume of 10.98 maf (101% of average (period 1981-2010)) during the period from 2005 through 2011. The hydrologic variability during this period has been from a low water year unregulated inflow volume of 8.62 maf (80% of average) in water year 2006 to a high water year unregulated inflow volume of 15.97 maf (147% of average) which occurred in water year 2011.
Overall reservoir storage in the Colorado River Basin has increased by over 8 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). On October 1, 2011, the beginning of water year 2012, the total reservoir storage in the Colorado River Basin was 38.66 maf (64.8% of capacity). As of May 2, 2012 the total reservoir storage in the Colorado River Basin was 37.17 maf (62.3% of capacity).
RRFW thanks Rick Clayton of the USBOR for this information.