Drought In the Upper Colorado River Basin
Current Conditions and Projections - AUGUST 2011
The unregulated inflow volume to Lake Powell in July was 4.35 maf (279 percent of average). This year the unregulated inflow in July was the second wettest July since the closure of Glen Canyon Dam (1963). The April through July unregulated inflow to Lake Powell for 2011 was 12.92 maf (163 percent of average) and this was the third wettest April through July period since the closure of Glen Canyon Dam.
In terms of reservoir elevation and storage, Lake Powell reached its peak for water year 2011 on July 30, 2011 which was 3660.9 feet (39.1 feet from full pool) and 18.61 maf (76.5 percent of capacity), respectively. Releases now exceed inflows and the elevation and storage are now slowly decreasing each day. The peak reservoir elevation and storage that occurred in water year 2010 was 3638.8 feet (61.2 feet below full pool) and 15.86 maf (65.2 percent of capacity) and this occurred on July 5, 2010. As of August 11, 2011 the reservoir elevation and storage in Lake Powell was 3659.64feet (40.36 feet from full pool) and 18.45 maf (75.8 percent of capacity), respectively.
Releases from Glen Canyon Dam are approximately 24,600 cfs which is very near the full capacity of the powerplant. The release volume for August will likely be about 1.45 maf. In late August, releases from Glen Canyon Dam will be reduced over three days to approximately 14,800 cfs on September 1, 2011. During the transition to the lower level, releases will likely fluctuate for power generation. Beginning on September 1, 2011 a steady flow experiment will begin and releases will be maintained steady at approximately 14,800 cfs until November 1, 2011. This will be the fourth year of a five year program of steady flows in September and October. The targeted release rate of 14,800 cfs is the current estimate of the maximum sustainable release rate for all 61 days during September and October given the current maintenance schedule for Glen Canyon Dam. It is possible that the actual maximum sustainable release rate during September and October could be somewhat higher. Reclamation will attempt to set the targeted release rate for the steady flow experiment at the highest level possible and this will be determined near the end of August.
The Operating Tier for coordinated operations of Lake Powell and Lake Mead during water year 2011, pursuant to the Interim Guidelines has been the Upper Elevation Balancing Tier. In April, based on the projected end of water year elevation of Lake Powell, a shift was made such that the Equalization Tier has governed operation of Lake Powell since then. Reclamation has been operating Glen Canyon Dam with the intent of achieving Equalization by September 30, 2011. Due to significantly high inflows this spring and summer however, it is very likely that Equalization will not be fully achieved by September 30, 2011. Reclamation will continue to operate Glen Canyon Dam to release water at near full capacity of the Glen Canyon Powerplant until Equalization is fully achieved for 2011.
The August 24-Month Study projects the annual release volume for water year 2011 will be 12.45 maf and the end of water year reservoir elevation and storage for Lake Powell will be 3656.91 feet (43.09 feet from full pool) and 18.092 maf (74.4 percent of capacity), respectively. The hydrologic outlook forecast for water year 2012 projects that the most probable (median) unregulated inflow volume will be 12.6 maf (105 percent of average based on the period from 1971 through 2000). Based on this hydrologic outlook, the August 24- Month Study projects the annual release from Lake Powell during water year 2012 will be 13.57 maf and the end of water year 2012 reservoir elevation and storage for Lake Powell to be 3646.4 feet (53.6 feet from full pool) and 16.77 maf (69 percent of capacity), respectively.
Beginning of the Drought
The Upper Colorado River Basin is experiencing a protracted multi-year drought which began in October 1999. Lake Powell was essentially full during the summer of 1999 with reservoir storage at 97 percent of capacity. However, it became evident with the low precipitation totals of October, November and December, at only 30 percent of average, that the stage was set for the first year of low runoff which occurred in 2000.
Inflow to Lake Powell provides a useful barometer of drought conditions in the Colorado River Basin. In the late 1990’s, inflow to Lake Powell was above average and the lake stayed full from 1995 through 1999. As late as September 1999, Lake Powell was still 95 percent full. Inflow into Lake Powell from water years 2000 through 2004 was about half of what is considered average. The 2002 inflow was the lowest ever recorded since Lake Powell began filling in 1963.
2007 - 68% 2008 - 102% 2009 - 88% 2010 - 72.5%
How Lake Powell Functions
It is important to understand that to provide for the water and power needs of the Southwest, Lake Powell functions essentially as a bank account of water that is drawn upon in times of drought. In 1922, the Colorado River Basin States signed the historic Colorado River Compact. Under the Compact, the states of Colorado, Utah, New Mexico and Wyoming received an entitlement to use 7.5 million acre-feet (MAF)of Colorado River water per year. The Compact also requires that these states deliver 75 MAF over any ten-year period at Lee Ferry (16 river miles below Glen Canyon Dam) for use by the downstream states of Arizona, California, and Nevada. Without the bank account of water stored in Lake Powell, water users in Colorado, Utah, New Mexico and Wyoming might have to curtail uses during periods of drought to meet this delivery requirement. Instead, storage in Lake Powell is used resulting in lower storage levels during periods of drought. The system is designed to function this way, and it is working well. There have been water shortages caused by the drought in the Upper Colorado River Basin the past five years (particularly in 2002), but water users above Lake Powell have not had to curtail use to meet deliveries to water users in the Lower Colorado River.
The Colorado River system of reservoirs was designed to protect against drought. Inevitably however, there are impacts associated with droughts, particularly when the drought is severe as is the current drought. While most marinas remain open, reservoir recreation has been impacted as well as downstream whitewater rafting. Additionally, there have been water shortages to water users in the Upper Colorado River Basin in the past five years, (particularly in areas where there is limited reservoir storage), and power generation has also been affected.
No one can say with certainty when this drought will end. River flow records of the Colorado River Basin reveal significant variability during multi-year periods of above and below average river flows. Inevitably, there will be a shift to wetter hydrologic conditions with an end to this drought and a recovery to water storage in Lake Powell.
Drought is a normal part of natural climate variations. There is nothing unusual about the occurrence of a drought, which should not be confused with aridity. Droughts are merely temporary abnormalities determined by deficient precipitation and depend on the area in question. In one area of the world a drought may be a week without rain, while in drier parts of world like the American Southwest, it might be an annual rainfall deficit of less than four inches.
A drought is also defined by its effect on the intended use of the water. Often, the the impacts of a drought are exacerbated because of increasing demands on a limited supply of water. Recent well documented droughts around the world have resulted in significant economic and environmental impacts.
What is Normal?
Droughts have occurred frequently throughout history. Whether or not a drought exists is determined by comparison with “normal” hydrology for an area. Normal is defined as a long-term average of highs and lows, which may include droughts and extremely wet periods. No single year will ever be normal due to the shear complexity of weather patterns. Because the occurrence of a drought affects this average, the definition of normal for the American Southwest, will be altered for the next several decades.