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Past droughts geographically widespread in the West, according to tree-ring data

August 19, 2005

When it's dry, it's dry all over, according to a new analysis of more than 400 years of annual streamflow in the Upper Colorado and Salt and Verde river basins.

By using data from tree rings, University of Arizona researchers conclude that water supply for those western rivers fluctuated in synchrony during periods of severe drought. The study goes back almost 800 years in the Salt-Verde basin and covers waterways from the states of Arizona, Colorado, New Mexico, Utah and Wyoming.

The project's overall conclusion is that severe droughts and low-flow conditions in one basin are unlikely to be offset by abundant streamflow in the other basin.

"Prior to the findings from this study, the conventional wisdom was that runoff from the Colorado River would be available to make up for deficits on the Salt and Verde rivers during times of extreme drought," said Charlie Ester, Salt River Project's manager of Water Resource Operations. "The bottom line is that the Upper Colorado Basin and the Salt and Verde basins work together as one entire region."

Tree-ring-based reconstructions of streamflow can peer back into time much further than the records available from stream-flow gauges. Ester said such reconstructions could provide important insights into the hydrologic variability of a river basin over time.

The findings represent just the first phase of a study partnered by SRP and The University of Arizona's Laboratory of Tree-Ring Research. UA scientists Katherine K. Hirschboeck, an associate professor of climatology, and David M. Meko, an associate research professor, conducted the tree-ring analysis.

The study's second phase, which will begin this month, will take a closer look at more recent history when scientists from the tree-ring lab will re-core trees in the Salt and Verde watersheds to gather new data from the last 40 years, a period when the watersheds have experienced both record wet and record dry episodes. The findings of the second phase will likely be available in the summer of 2007.

Key project conclusions from the tree-ring study:

# Extreme events, either low flows or high flows, tended to occur simultaneously in the Upper Colorado and Salt/Verde river basins;

# Such synchronous low-flow and high-flow events tended to cluster in time;

# The longest continuous period when both basins had extreme low-flow years was three years;

# Within any four-year period, having either high flow or low flow for two consecutive years occurred more than 20 percent of the time;

# For the Salt/Verde river basin, the recent drought is similar to that experienced in the 1950s;

# The tree-ring record reveals that between 1200 and 1903, the Salt/Verde river basin had at least eight droughts as severe as the 1950s drought.

Ester said the findings will help devise an assessment tool for implementing the project's results into operational water supply decision-making.

Hirschboeck said the study's findings offered some revelations for the Laboratory of Tree-Ring Research's scientists. "Dave and I were surprised that the two distant basins, the Upper Colorado and the Salt/Verde, were so much in synch during periods of extremely high or extremely low flow." She added, "The recent joint drought, while severe, is not unprecedented when compared to those in the previous five centuries."

The findings of the first phase are being shared with representatives of various federal, state and local agencies such as the National Weather Service, U.S. Bureau of Reclamation, Arizona Department of Water Resources, Central Arizona Project and cities in the Phoenix metropolitan area.

The tree-ring study in partnership with The University of Arizona is the one of many initiatives taken by SRP in response to an ongoing drought that is in its 10th year. Even counting this past winter when 2,017,580 acre-feet of runoff - the first above-normal runoff season since 1998 - filled the reservoirs on the Salt and Verde rivers, stream flows into the Salt and Verde have been below normal for eight of the last 10 January-through-May runoff seasons.

Phoenix-based SRP is the largest provider of water to the greater Phoenix metropolitan area.

The University of Arizona's Laboratory of Tree-Ring Research was established in 1937 by A. E. Douglass, the founder of modern tree-ring science.



University of Arizona