Drought In the Upper Colorado River Basin
Current Conditions and Projections - JANUARY 2010
In the Upper Colorado River Basin during water year 2009, the overall precipitation accumulated through September 30, 2009 was approximately 95 percent of average based on the 30 year average for the period from 1971 through 2000. For water year 2010 the dry conditions have continued. Precipitation for October 2009 was 85 percent of average and for November, precipitation was estimated to be only 40 percent of average. In December 2009, the estimated precipitation above Lake Powell was 115 percentof average.
The Climate Prediction Center outlook (dated December 17, 2009) for temperature over the next three months indicates that temperatures in the northern reaches of the Upper Colorado River Basin have an increased probability of being above average. Accumulated precipitation over the next three months has an increased probability of being above average in the southern reaches of the Upper Colorado River Basin.
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%
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.