Although the current drought in the southwestern United States is the most severe in the nearly 100-year historical record, researchers say six other droughts that occurred during the past 500 years have been worse in both length and magnitude.
Using historical streamflow records, drought indices, and tree ring data from the Upper Colorado River Basin (UCRB), a team of researchers from University of Nevada , Las Vegas, and Scripps Institute of Oceanography concluded that the worst drought occurred at the end of the 16 th century and was two and a half to four times worse than current conditions. Prior to the study, the current drought was considered to be the worst in the past 500 years.
The UCRB, which extends through five states ( Colorado , Wyoming , Utah , Arizona , and New Mexico ), encompasses approximately 17,800 square miles and is the primary water producer for the Colorado River due to runoff from snowmelt.
The study was conducted by Tom Piechota, UNLV assistant professor of civil and environmental engineering; Hugo Hidalgo of the Scripps Institute of Oceanography at the University of California, San Diego; and Glenn Tootle and Janak Timilsena, graduate research assistants at UNLV.
The research team also says that the consequences of the current drought have been greatly exacerbated by increased water demand due to unprecedented population growth in the southwest United States .
"A moderate hydrologic drought combined with the overuse of water supply can cause low reservoir levels and extreme drought conditions in a basin," said Piechota, who was the principal investigator of the study. "We believe the results of this study highlight the importance of enacting greater conservation measures in case the current drought continues."
Other severe droughts identified by the study occurred during the following years and are listed in descending order of severity: 1658-1677, 1495-1507, 1870-1884, 1900-1905. Since the current drought began in 1999, the elevation of Lake Mead has declined by more than 75 feet, or approximately three water years of allocation for the state of California . Lake Powell is also at historic low levels, with only 40 percent of its water storage available.
The study also suggests that estimates of water availability used to establish the Colorado River Compact of 1922 were based on the highest period of streamflow in the 500 year period that was studied, therefore resulting in false projections of available resources. The compact is the initial piece of legislation that comprises the "Law of the River" - an assemblage of state and federal laws, international treaties, and court decisions that still governs water allocation today.
Research procedures included the study of hydrologic drought indices and historical records from two locations in the UCRB to determine the average streamflow on the Colorado River from 1923-2003. Analysis of tree ring data from 17 other locations in the basin was used to ascertain hydrological and climatic conditions for the previous 423 years, for which no data existed.
The findings will be published in the Aug. 10 issue of EOS, a publication of the American Geophysical Union.
The study is part of ongoing research funded by the USGS Water Resources Research Program, the National Science Foundation, the California Climate Change Center at the Scripps Institute of Oceanography, and the U.S. Department of Energy.
