Sunday, May 28, 2006 | 7:41 a.m.
Droughts have come more frequently to the West than once thought, according to a new University of Colorado study published this month.
That's a drop of bad news for water-system managers along the Colorado River. About 25 million people, including everyone in urban Las Vegas and its environs, depend on water from the river. Agencies running water systems like a steady, dependable and predictable supply.
The West and the river, however, are extremely variable, according to tree ring studies going back more than 500 years, the new study says.
Eric Kuhn, general manager of the Colorado River Water Conservation District and an expert on Colorado River issues, says the study "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."
Connie Woodhouse, lead investigator of the study published in a journal of the American Geophysical Society, says the work conducted over the last nine years corroborates similar studies but expands the scope with numerous studies of tree rings throughout the Colorado River basin.
The rings from low-elevation trees - Ponderosa pines, Douglas firs and pinion pines - provide a virtual time machine that goes back to about 1490, two years before Columbus came to the Americas.
Paradoxically, good weather - that is, wet weather in the West - during the last 100 years or so is bad news, because it gives an unrealistic picture of the long-term climate, Woodhouse says. The years before construction started on Hoover Dam in 1931 were wetter than average, and have provided federal river managers a historic gauge of river flows for 70 years.
"The early 20th century was one of the wettest periods in the last 500 years," says Woodhouse, a University of Colorado researcher.
The research indicates that the five-year drought, which began in 2000, was not, as water-agency officials in Las Vegas and elsewhere believed, the worst in the last 500 years or even in the historical record. A similar five-year drought hit from 1843 to 1848, she says, "so it wasn't that long ago."
There have been at least eight droughts as long or longer during the 500-year study period, she says. Some lasted more than a decade.
Water officials in Las Vegas say that they used the best information available at the time when they described the ongoing drought a few years ago. Kay Brothers, deputy general manager of the Southern Nevada Water Authority, which supplies all of urban Las Vegas' wholesale water, says indications of high variability in precipitation in the river basin back up the plans and existing infrastructure built by water agencies.
"All the climate studies that we've looked at show great variability," she says. "That's why we have the storage capacity on the Colorado River."
The storage is built into two huge reservoirs: Lake Powell, in southern Utah, and Lake Mead. Together along with several smaller reservoirs, the Colorado River holds about 20 trillion gallons. Although down more than a third from 1999 levels, officials say that without the storage capacity, lower basin river users would have been left high and dry.
Officials hope that the last two winters of relatively healthy precipitation in the West signal a return to normal. But they know that next year, particularly in a time of shifting global climates, could bring a return to drought conditions and further strain stored water resources.
Because of that uncertainty, the Water Authority does not want to be dependent on the Colorado River alone for future water supplies. Brothers was to speak Friday to a community gathering in Ely, where plans by the Water Authority to drill for groundwater and pipe it to Las Vegas have engendered opposition among ranchers and conservationists.
She says the University of Colorado study indicates the importance of diverse water sources. While drought can affect groundwater, it takes much longer to do so, making wells a more stable source, Brothers says: "Having additional sources is very key."
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