Fellow scientists are calling Martin Kelly’s Florida “River Flow Patterns and the Atlantic Multidecadal Oscillation” study one of the more important contributions to hydrologic science in Florida, and perhaps elsewhere, in the past several decades. Kelly is a manager in the District’s Ecologic Evaluation Section.
“The peer review panel’s response to our work was especially gratifying,” said Kelly. “The multidisciplinary panel recognized the importance and implications of this effort not only as it relates to the main emphasis behind the study, but because there were other potential implications for aquatic ecology and water management in general.”
The reason for all the positive feedback is that Kelly’s study examines how sea surface temperature in the Atlantic Ocean and human activities impact the flow patterns of Florida rivers.
While most rivers in the state have higher flows during the summer rainy season, rivers in the north and northwest portion of the state have higher flows in the spring. This is similar to the rest of the southeastern United States. The line for this change seems to extend from the “Big Bend” area around the Suwannee River diagonally to the northeast and the mouth of the St. Marys River north of Jacksonville.
When rivers in the northern part of the state were in a period of generally higher flows, rivers in peninsular Florida generally had lower flows, and vice versa. The Atlantic Multidecadal Oscillation (AMO) appears to be the reason for the change in flow patterns. AMO is a term that describes the long-term oscillations in the sea surface temperature of the North Atlantic Ocean and how it affects climate and rainfall patterns temporally over a multidecade period.
The importance of this study to the District is that staff is currently setting minimum flows and levels for the rivers throughout the District.
“We are setting minimum flows and levels because we are legislatively mandated to do it, and rightly so,” said Kelly. “Since the water management districts permit the consumptive use of water, the setting of minimum flows and levels offers an additional level of protection and ensures that the ecological and resource values of our water bodies are protected from significant harm.”
In order to accomplish this work, staff must first determine a benchmark period, or what a river would be like without human impact for a certain time period. In Florida, scientists usually use the 1940s–1960s because there was relatively little or no development at that time.
When Kelly compared the 1940s–1960s and the 1970s–1990s, he saw a big difference in flow. The next step was to determine why there was such a change. The obvious answer would be that agricultural activities in Florida increased substantially in the 1970s and development soon followed.
However, the most obvious answer is not always the right answer. Kelly cites a study by David Enfield and coworkers that correlated rainfall in the United States with sea surface temperature in the Atlantic Ocean. He noted that the ocean was in a warm phase from 1940 to 1969 and from 1995 until the present. In between, from about 1970 to 1995, the ocean was in a cool phase. The interesting fact about this study is that most of the country received less rainfall when the ocean was in its warm stage and more rainfall when the ocean was in a cool stage. However, rainfall patterns in much of Florida were exactly the opposite.
This is important because rainfall impacts river flow.
Kelly reviewed the flow data for the state’s rivers that had at least 60 years of record. He used 1970 as a midpoint and examined the 30-year period before and after that time.
Rivers in northern Florida tend to have the same flow patterns as most of the southeast. Their flows are highest in the spring and lower in the summer.
Rivers in most of peninsular Florida peak in the summer.
Rivers in the area in between have two peak flows. This is called a bimodal pattern. The first peak resembles the northern pattern, and the second peak resembles the southern pattern.
Differences in river flow volumes before and after 1970 are consistent with Enfield’s explanation for multidecadal differences in sea surface temperature of the Atlantic Ocean and how it affects rainfall.
“It was important that the District be able to account for climate variability in assessing potential river flow volumes. This was necessary to develop an ecologically sound and scientifically defensible minimum flows and levels methodology,” said Kelly.
“Since climatologists believe we entered into another warm period beginning around 1995, we should see a return to a generally wetter period, with more tropical storms and hurricanes, such as existed from 1940 to 1970. From a scientific point of view, this will be an interesting test of the AMO and its affect on rainfall; and from a more pragmatic point of view, could lead to a shift in focus in some of our water management efforts.”