May 27, 2006

Severe Drought Traced Along Colorado River

Analysis of tree rings from 1490 to 1997 shows that dry spells are a 'defining feature.' It raises questions about growth in the region.

By Bettina Boxall, Staff Writer
Los Angeles Times

A new study of tree ring records in the Colorado River basin reaffirms that one of the West's most important water sources is no stranger to severe drought.

Analyzing tree cores that reflect how wet or dry the climate was, scientists reconstructed the Colorado River's flow from 1490 to 1997. During that period, they found evidence of as many as eight severe droughts that lasted five consecutive years.

In a dry spell from 1844 through 1848, the average Colorado flow was even less than during the drought that gripped the basin recently and left some of its biggest reservoirs half empty.

The recent drought "is not without precedence," researchers wrote. "Overall these analyses demonstrate that severe, sustained droughts are a defining feature" of the Upper Colorado, which supplies most of the river's water.

The study echoes other research showing that, when the Colorado water was divided among seven Western states in 1922, it was an unusually wet period, resulting in overly optimistic water allocations.

"There's not a limitless amount of water for growth in the Southwest," said David Meko, study coauthor and an associate research professor at the University of Arizona Laboratory of Tree-Ring Research. "If everybody uses what they're entitled to, there's a shortage. On top of this, we have these climatic episodes."

Meko and coauthors Connie Woodhouse of the National Climatic Data Center and Stephen Gray of the U.S. Geological Survey Desert Laboratory concluded: "The long-term perspective provided by tree ring reconstructions points to looming conflict between water demand and supply."

About 1,400 miles long, the Colorado supplies water to 25 million people and several million acres of farmland from Denver to Southern California.

The basin drought that began in 2000 — the driest since record keeping began in 1906 — has eased somewhat. But Lake Powell and Lake Mead, the river's two major reservoirs that supply Arizona, Las Vegas and the Los Angeles region, remain low.

Powell is 49% full and Mead is at 56% capacity. Moreover, last year's bountiful flows into Powell from the upper basin are expected to fall below normal again this year.

Federal water managers are drawing up guidelines that would determine how a shortage would be handled if one were officially declared on the river, which is governed by complex water laws involving the states and the federal government.

In the meantime, the states have recommended some changes in how Powell and Mead are operated. And they want to explore ways of boosting the river's flow — for instance, through cloud seeding that would increase snowfall in mountains that drain into the basin.

The tree ring study, published in the May issue of Water Resources Research, built on a similar 1976 effort. That earlier study found the highest sustained flows over a more than 400-year period, from 1520 to 1961, occurred in the early 1900s, when farmers were pouring into the region and states were dividing up the Colorado's liquid riches.

The 1976 tree ring study also detected a drought that persisted for about 20 years in the late 1500s.

The new research relies on different ring samples, drawn from both living trees and dead ones, as well as data from the past three decades.

Of the eight droughts the study chronicled since the Columbus era, one was worse than the recent drought. Researchers said two, in the early 1500s and early 1600s, have a 25% chance of being as severe. The others have a 10% chance of being drier than 1999-2004.

The reconstruction also found that below-average flows lasted for longer periods, in one case, 11 consecutive years.

In one bit of good news, the researchers concluded the Colorado's average annual long-term flows are not as low as estimated in the 1976 study.

That research suggested an average flow of 13.5 million acre-feet a year, compared to the new estimate of 14.6 million. (An acre foot is enough to supply two Southern California families for a year.)

Still, that is less water than has been legally allocated to the states and Mexico.

Eric Kuhn, general manager of the Colorado River Water Conservation District in Colorado, said politicians are still relying too much on the unusually wet records of the 1900s in shaping water policy.

"The study gives you a good indication that the past wasn't exactly like the 1900s, and they ought to be very, very cautious" in predicting future water levels, Kuhn said.