Glen Canyon Dam and Lake Powell
The unregulated inflow into Lake Powell during July was 1.40 million acre-feet (maf) which was 90% of average based on the period from 1971-2000. The most probable (median) inflow forecast for July was 1.13 maf so the actual unregulated inflow volume was 270,000 acre-feet above what was expected for the month of July. The end of month elevation of Lake Powell for July was about 0.5 feet lower than what was projected in the July 24-month study at 3,641.14 feet above sea level. The April through July inflow for Water Year 2009 was 7.81 maf (98% of average). The forecasted unregulated inflow for August and September are 550,000 acre-feet and 450,000 acre feet respectively. Based on these inflows and the projected water year release of 8.23 maf, the end of water year elevation is projected to be 3639.4 feet above sea level (60.6 feet from the top of the spillway gates). The storage in Lake Powell at the end of the water year (September 30, 2009) is projected to be 15.94 maf which is 65.8% of the full capacity (24.322 maf).
During August 2009, the scheduled release volume for Lake Powell is 800,000 acre-feet which will result in a daily average release of approximately 13,000 cfs. During weekdays, the afternoon peak release from Glen Canyon Dam will be approximately 16,500 cfs with morning lows of approximately 8,500 cfs. During weekends the afternoon peak release will be approximately 16,000 cfs with morning lows of approximately 8,500 cfs. During the last 3 days of August, daily fluctuations of releases from Glen Canyon Dam will be moderated in order to transition to steady flows beginning September 1, 2009.
On September 1, 2009 and continuing through October 31, 2009 the releases from Glen Canyon Dam will be steady with no fluctuations for power production (excluding system regulation and spinning reserves) for a steady flow experiment pursuant to the February 2008 Finding of No Significant Impact Experimental Releases from Glen Canyon Dam, Arizona 2008 through 2012. The projected release rate currently being targeted is 10,000 cfs which is equivalent to a monthly release volume of approximately 595,000 acre-feet in September and 615,000 acre-feet in October.
The water year release volume for 2009 is 8.23 maf pursuant to the Interim Guidelines. At the end of August, depending the remaining release volume required to achieve 8.23 maf, the release volume scheduled for September could be moderately adjusted which could impact the steady flow rate targeted for this year.
Upper Colorado River Basin Hydrology
The overall precipitation rates during October and November 2008 were well below average at approximately 55% and 80% respectively. In December, however, conditions improved significantly with precipitation measuring approximately 185% of average. Unfortunately this wetter trend did not continue with precipitation in January, February and March measuring 95%, 75% and 65% of average respectively. In April and May conditions returned to a wetter pattern with precipitation estimates of 120% and 105% of average, respectively. Precipitation in June was well above average and estimated to be about 215% of average while in July the precipitation is estimated to be only 65% of average. The overall water year precipitation rate through August 10, 2009 is 102% of average.
The Climate Prediction Center outlook (dated July 16, 2009) for temperature over the next 3 months indicates that temperatures in the southwest have an increased probability of being above average while precipitation is projected to be near average in the Upper Colorado River Basin.
Upper Colorado River Basin Drought
The Upper Colorado River Basin continues to experience a protracted multi-year drought. Since 1999, inflow to Lake Powell has been below average in every year except water years 2005 and 2008. In the summer of 1999, Lake Powell was close to full with reservoir storage at 23.5 million acre-feet, or 97 percent of capacity. During the next 5 years (2000 through 2004) unregulated inflow to Lake Powell was well below average. This resulted in Lake Powell storage decreasing during this period to 8.0 million acre-feet (33 percent of capacity) which occurred on April 8, 2005. During 2005 and 2008 drought conditions eased somewhat with net gains in storage to Lake Powell. As of August 10, 2009 the storage in Lake Powell was 16.0 million acre-feet (66 percent of capacity) which is still below desired levels while the overall reservoir storage in the Colorado River Basin as of August 10, 2009 is 35.2 million acre-feet (59.5 percent of capacity).
RRFW thanks Rick Clayton of the USBOR for his assistance in providing information for this notification.
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