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`Tree Rings

Rings of Truth

The work of Dr. Florence Hawley, a 30s-era TVA dendrochronologist, gains new relevance as TVA and University of Tennessee scientists draw on her work to assess future weather impacts in the Tennessee Valley.

It’s been a rainy winter following a year of record rain—one that’s had TVA’s River Forecast Center on its toes keeping up with all the water. Flood prevention is top of mind for almost everyone—except lead hydrologist Curt Jawdy. His mind is fixed on drought.

Jawdy is working with Laura G. Smith, a Ph.D. student in the Department of Geography at the University of Tennessee, to resurrect Dr. Florence M. Hawley’s pioneering research from the 1930s. Hawley worked at TVA in its earliest days studying the relationship between rainfall and tree rings. Her goal was to better understand precipitation cycles in the Norris Basin.

And they are looking to the rings to show them a record of the wild weather of the past—the good times and the bad.

Past and Present

Beginning in 1934, Hawley, a dendrochronologist, spent two years in East Tennessee collecting hundreds of tree samples in an effort to look back in time further than had been thought possible. Some of her samples were from red cedars that date back to the 1300s.

Florence Hawley

Florence Hawley hard at work during her time with TVA.

A student of A.E. Douglass of the University of Arizona, who is considered the father of modern dendrochronology, Hawley was uniquely suited to her work. Nevertheless, she faced sexism on the job, and was never allowed to publish the results of her red cedar study. She abandoned the work—and the samples—in the 1940s, but went on to have an influential career in the field of archaeology studying ceramics in the Southwest.

Her red cedar samples would go on to have new life at the University of Tennessee, where Smith has picked up the work Hawley left off. Smith explains: “We’re using her work to develop long-running reconstructions of precipitation and stream flow. What we do is look at the relationship that exists between tree ring width and precipitation.”

Laura Smith

Laura G. Smith (center) studies a core sample while TVA's Suzanne Fisher (left) and Heather Hart (right) look on.

To oversimplify a bit, wide rings correspond with years of ample precipitation; skinny rings with arid years. It’s a bit more complicated than that, especially here in the Valley where moisture from the forest floor must also be taken into consideration, but Smith is running the statistical analyses that will allow her to cut through the clutter.

“Once we establish that relationship, we can look back in time and estimate what the precipitation and stream flow were. While we have only about 100 years of instrumental records, the advantage with tree rings is that we can go back several hundred years to calculate periods of drought.”

The historical samples are being updated with modern cedar tree samples taken from several locations in the Valley. TVA Natural Resources’ Suzanne Fisher and Heather Hart have helped Smith find and sample specimen trees at several TVA natural areas, including sites noted for their unique cedar barren characteristics. Their in-depth knowledge of the regional ecosystem has been a good bridge between dendrochronology and hydrology.

Looking Back to Look Forward

Drought is particularly relevant now, when climate change is a topic of concern for many. And the way forward, Jawdy and Smith agree, is by first looking backward.


This graph represents Valley precipitation between 1380 and 1990. The red line shows actual rainfall, whereas the black line represents the ongoing seasonal average.

‘The goal of the project is essentially to understand what the historic variability of the stream flow is using tree ring data to make better decisions in the future,” Smith says.

Jawdy says, “Looking at the past climate is a noncontroversial way for people to understand that the climate does fluctuate—tree rings are a record of what has already happened. Forecasting the future is very hard, so first we are looking at the past and planning to cover the past extremes. We want to be able to handle the worst we’ve ever seen.”

When Smith’s work is complete, Jawdy will take the analysis and test it within TVA’s river management system to see how the system would react to different precipitation and runoff scenarios.

“We want to take those hundreds of years’ worth of historical data, and run it through our river policy model, look at each intake on our system and see if at any time they would have been dry,” he explains.

What If?

The Valley has faced plenty of dry times—2007, for instance, was a year low on water. But even during the largest drought of record, no intakes within the TVA-managed system went dry.

TVA works closely with the municipalities and industries that draw water from the river system to ensure their water supply stays reliable, mainly by instituting conservation measures when necessary.

Tree Specimen


But Jawdy and Smith are looking for an epic drought—one that might put intakes at jeopardy and call for action. “For those who might have vulnerability to these remote risks, we might work with the states to improve our drought planning,” he says. “We could know on a city-by-city and industry-by-industry basis what the probability is that they might run out of water, as has happened to many intakes in the West.”

If there is a risk, knowing now would ensure there’s time to take action, Jawdy says. “That might look like putting in a new intake lower in the reservoir or even hooking up to a neighbor’s water system to make them more resilient. The key is understanding our drought risk, and being able to plan for it.”

At this point, no results are in, though Smith is working to tell whether the Tennessee Valley was part of a drought experienced elsewhere on the Eastern Seaboard in the mid 1700s.

Jawdy hopes not, and that Smith’s work will verify what he suspects—that there’s no scenario in which a drought is more than the Tennessee River system can handle if we plan well. “We have a better and more reliable water system than just about any place in the country—but we haven’t really quantified that,” he says. “If we can, it will make our region even more attractive to industry.”

In other words, our drought resilience could turn out to be a strong economic development tool. “With this study, we may learn that we need to sit down and do some minor adjustments to our drought plans,” says Jawdy. “And we may be able to go out and say to industries that the Tennessee Valley is one of the safest places to locate your business for a water-secure future.”

Either way, the hard work of Hawley in the 1930s and of women scientists today will shine a brighter light on the future of the Tennessee Valley.

Photo at top of page courtesy of Steven Bridges, Univeristy of Tennessee, Knoxville
Photo of Florence Hawley courtesy of the McClung Museum of Natural History and Culture, Univeristy of Tennessee, Knoxville