When it's dry, it's dry all over, according to a new analysis of morethan 400 years of annual streamflow in the Upper Colorado and Salt andVerde river basins.
By using data from tree rings, University of Arizona researchersconclude that water supply for those western rivers fluctuated insynchrony during periods of severe drought. The study goes back almost800 years in the Salt-Verde basin and covers waterways from the statesof Arizona, Colorado, New Mexico, Utah and Wyoming.
The project's overall conclusion is that severe droughts andlow-flow conditions in one basin are unlikely to be offset by abundantstreamflow in the other basin.
"Prior to the findings from this study, the conventional wisdomwas that runoff from the Colorado River would be available to make upfor deficits on the Salt and Verde rivers during times of extremedrought," said Charlie Ester, Salt River Project's manager of WaterResource Operations. "The bottom line is that the Upper Colorado Basinand the Salt and Verde basins work together as one entire region."
Tree-ring-based reconstructions of streamflow can peer backinto time much further than the records available from stream-flowgauges. Ester said such reconstructions could provide importantinsights into the hydrologic variability of a river basin over time.
The findings represent just the first phase of a studypartnered by SRP and The University of Arizona's Laboratory ofTree-Ring Research. UA scientists Katherine K. Hirschboeck, anassociate professor of climatology, and David M. Meko, an associateresearch professor, conducted the tree-ring analysis.
The study's second phase, which will begin this month, willtake a closer look at more recent history when scientists from thetree-ring lab will re-core trees in the Salt and Verde watersheds togather new data from the last 40 years, a period when the watershedshave experienced both record wet and record dry episodes. The findingsof the second phase will likely be available in the summer of 2007.
Key project conclusions from the tree-ring study:
Ester said the findings will help devise an assessment tool forimplementing the project's results into operational water supplydecision-making.
Hirschboeck said the study's findings offered some revelationsfor the Laboratory of Tree-Ring Research's scientists. "Dave and I weresurprised that the two distant basins, the Upper Colorado and theSalt/Verde, were so much in synch during periods of extremely high orextremely low flow." She added, "The recent joint drought, whilesevere, is not unprecedented when compared to those in the previousfive centuries."
The findings of the first phase are being shared withrepresentatives of various federal, state and local agencies such asthe National Weather Service, U.S. Bureau of Reclamation, ArizonaDepartment of Water Resources, Central Arizona Project and cities inthe Phoenix metropolitan area.
The tree-ring study in partnership with The University ofArizona is the one of many initiatives taken by SRP in response to anongoing drought that is in its 10th year. Even counting this pastwinter when 2,017,580 acre-feet of runoff -- the first above-normalrunoff season since 1998 -- filled the reservoirs on the Salt and Verderivers, stream flows into the Salt and Verde have been below normal foreight 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 wasestablished in 1937 by A. E. Douglass, the founder of modern tree-ringscience.
More details on the tree-ring report are available at http://www.ltrr.arizona.edu/srp.
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