A new tree-ring-based reconstruction of 508 years of Colorado River streamflow confirms that droughts more severe than the 2000-2004 drought occurred before stream gages were installed on the river.
The new research also confirms that using stream gage records alone may overestimate the average amount of water in the river because the last 100-year period was wetter than the average for the last five centuries.
"This work updates the original landmark Colorado River reconstruction that was done at The University of Arizona's Laboratory of Tree-Ring Research," said David M. Meko, a UA associate research professor of dendrochronology, the science of tree-ring dating.
"The main points of the 1976 research hold up. Droughts more severe and intense than we've seen in the gaged record occurred in the past, and the long-term mean flow is lower than the gaged mean flow."
Connie A. Woodhouse said, "The updated reconstruction for Lee’s Ferry indicates that as many as eight droughts similar in severity, in terms of average flow, to the 5-year 2000-2004 drought have occurred since 1500." Woodhouse, who led the research team, is a physical scientist at the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center Paleoclimatogy Branch in Boulder, Colo.
Allocations of Colorado River water made in the 1922 Colorado River Compact between the states of Arizona, California, Colorado, Nevada, New Mexico, Wyoming and Utah therefore overestimate the amount of river water available. Los Angeles, Las Vegas, Denver, Phoenix, Tucson and Albuquerque are among the many cities dependent on Colorado River water.
"The long-term perspective provided by tree-ring reconstructions points to a looming conflict between water demand and supply in the upper Colorado River basin," the researchers wrote in their report.
Woodhouse and Meko collaborated with Stephen T. Gray of the U.S. Geological Survey in Tucson, Ariz., and Jeffrey Lukas of the Institute of Arctic and Alpine Research at the University of Colorado in Boulder. The team's research article, “Updated streamflow reconstructions for the Upper Colorado River Basin," is published in the May 2006 issue of Water Resources Research.
Funding for the study was provided by NOAA, USGS, the U.S. Bureau of Reclamation, the University of Arizona Water Sustainability Program and the University of Colorado Western Water Assessment.
The new research updates the first tree-ring based reconstruction of streamflows at Lee’s Ferry and other Colorado River basin gages, which was published in 1976. The new research improves on the previous work by using an expanded network of tree-ring sites, and because the scientists could incorporate an additional 34 years of tree-ring records to compare to the stream gage record for four gages in the Upper Colorado River basin.
Researchers were able to statistically recreate flows back four centuries prior to the gage record by comparing tree-ring widths from 1906 to 1995 with naturalized gaged streamflows (i.e., streamflows adjusted to remove the impacts of humans) during the same period. The streamflows were reconstructed by using cores taken from approximately 1,200 trees in 60 locations throughout the Colorado River basin area.
Streamflow was reconstructed for Lee’s Ferry, Ariz., a critical measuring location and the dividing point between the Upper Basin and the Lower Basin of the Colorado River as defined by the 1922 Colorado River Compact.
The Lee’s Ferry streamflows are of particular interest to water managers in California, Arizona, Nevada, Utah, New Mexico, Wyoming and Colorado, the seven signatory states to the Compact, because the Colorado River supplies drinking water to approximately 30 million people and irrigates 3.5 million acres of farmland. Historic stream flows for other tributaries to the Colorado River were reconstructed as well.
The underlying message from these new reconstructions remains the same: that Colorado River Compact allocations were based on one of the wettest periods in the past five centuries, and that droughts more severe than any in the last 100 years occurred before stream gages were installed. The most severe sustained drought (based on the lowest 20-year average) in the Upper Colorado River basin occurred in the last part of the 16th century. This reconstruction also shows that average annual flows on the Upper Colorado regularly vary from one decade to the next by more than 1 million acre-feet.
According to Eric Kuhn, general manager of the Colorado River Water Conservation District and an expert on Colorado River issues, “Water managers have always made critical water decisions based on a relatively short and often incomplete gaged record for the Colorado River. This study should be of keen interest because it shows that there were likely a number of long-term droughts more severe than what we experienced in the 1900s and during this century. The study should have enormous implications on how the river is managed."
The new reconstructions do indicate the river may have a higher long-term average flow, 14.6 million acre-feet, than did the 1976 reconstruction, which estimated a long-term average flow of 13.5 million acre-feet. However, the new average for the past 500 years is still lower than the average of 15.2 million acre-feet recorded by stream gages from 1906 to 1995. An acre-foot is approximately 325,000 gallons and is enough water to meet the needs of two four-person families for a year.
The scientists' next step is understanding the source of the differences in the means between the new reconstruction and the 1976 work.
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