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Tree rings, streamflow records indicate parts of Southeast in 1,000-year drought

By: Melissa Schneider / Published: Oct 27,  2017

Two scientists at the University of Alabama have estimated streamflow going back centuries in parts of Alabama, Mississippi and Florida. Through tree ring research and streamflow gauge data, Professors Matthew Therrell and Glenn Tootle found some surprising facts about drought in the Southeast.

Therrell worked with undergraduate and graduate students to collect tree ring data along the Choctawhatchee River in Alabama and Northwest Florida and along the Pascagoula River in Mississippi. Using a boring tool, they collected core samples from bald cypress trees that were more than 1,000 years old.

Scientists used data from streamflow gauges (which measure how much water is flowing in a river or stream) and the tree rings.

Professor Matthew Therrell, of the University of Alabama, teaches students about collecting tree ring samples.
Professor Matthew Therrell, of the University of Alabama, teaches students about collecting tree ring samples.

Streamflow gauges have only been around for about 125 years (the U.S. Geological Survey started the first one in 1889). Studying the tree rings (or classic dendrochronology) helps paint a more comprehensive picture of streamflow over the centuries. Scientists study the overlapping years — the years with both gauge data and tree ring data ­— and determine the relationship between them. They estimate streamflow going back as far as the tree rings go.

“You can create this relationship where ring width of x equals streamflow of y,” Therrell said. “You use the tree rings to estimate streamflow before anyone ever measured streamflow.”

Narrow rings indicate less precipitation. Wide rings indicate more precipitation.

Scientists found that the Southeastern United States has been suffering a hydrological drought since the early 2000s. Along the Choctowhatchee River research site, the drought is the worst one in 1,000 years. The Pascagoula River site, however, has seen worse drought in its history.

Tree ring core samples are analyzed in a lab.
Tree ring core samples are analyzed in a lab.

This information about past streamflow is especially useful for resource managers who make decisions about how much water can be taken out of rivers for agricultural, industrial, municipal and other interests, Therrell said.

“If they base their allotments on not really knowing the worst-case scenario, one day when there is a drought worse than anything we’ve seen before or measured, there won’t be enough water to go around because they didn’t plan for that,” Therrell said. “It boils down to, do you want to make a decision based on 50 years of data or 1,000 years of data?”

Find more info on the study: http://choctawhatcheerivertreeringstudy.weebly.com

El Niño and agricultural drought

Trying to determine what may be affecting the drought, Tootle looked to climate signals. 

El Niño-Southern Oscillation is a climate phenomena that occurs in the tropical Pacific and can affect temperature and precipitation around the world. It has three phases: El Niño, La Niña and neutral. 

El Niño occurs when the sea-surface temperatures are warmer than average in the equatorial Pacific. La Niña occurs when the ocean surface temperatures are cooler than average. When the temperatures are average, ENSO is neutral. 

El Niño typically causes more rain in the Southeast. Tootle analyzed streamflow in relation to known El Niño events and had some surprising findings. 

By studying several past El Niños, their strength (a NOAA measurement) and the associated seasonal streamflow response in coastal Mississippi and Alabama, Tootle discovered similar streamflow patterns in historic El Niño years to the most recent 2015–2016 El Niño. 

“It was shocking how similar the streamflow response to the 2015–2016 El Niño compared to historic El Niño events,” Tootle said. 

He also found that an El Niño year does not necessarily mean “good” precipitation for Southeastern farmers’ crops. Coastal Mississippi and Alabama receive about a 20-percent increase in annual moisture (streamflow) during an El Niño year, he said. While farmers may expect a good harvest because of an increase in moisture, Tootle looked at streamflow by month (and season) and found the timing of El Niño-driven increased moisture may result in potential problems for farmers. The majority of moisture arrived in January, February and March. April and May saw average rainfall. And, coastal Mississippi and Alabama suffered drought in June, July and August. 

“It may be bad for farming because all the moisture comes when you can’t plant, and all the dryness comes when you need the moisture to grow whatever you are trying to grow,” Tootle said. “It’s a double whammy on these folks.”

University of Alabama Scientist Matthew Therrell teaches a student how to collect tree ring core samples.
University of Alabama Scientist Matthew Therrell teaches a student how to collect tree ring core samples.

These findings are particularly useful to farmers who rely on rain to water their crop, he said. Knowing these El Niño-driven droughts may last 2-3 years can help farmers find alternate ways to water their crops. They could consider adding wells or getting a permit to divert water from a nearby stream.

AMO affecting drought

Tootle and his team decided to look at low-frequency climate signals to see if they may be contributing to the 20-year decline in Southeastern moisture.

Studying calendar-year streamflow in Alabama, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Tennessee, they determined that flow has been declining since 1990. 

Scientists also found historic lows for streamflow in 5-year, 10-year and 20-year time periods. 

The Atlantic Multi-decadal Oscillation (AMO) climate signal had already been reconstructed by other scientists using tree rings, and Tootle found that it is associated with the decreased streamflow. 

The AMO has been in a warm period for roughly the same period as the streamflow decline has been observed, Tootle said.

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