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Florida Lake Management Society Annual Conference, Naples, Florida, June 4 – 7, 2007

CLIMATIC CYCLES AND THEIR EFFECT ON FLORIDA RAINFALL AND RIVER FLOW

Martin Kelly Southwest Florida Water Management District Brooksville, FL

A major step in developing legislatively mandated minimum flows for watercourses in Florida are the selection of an appropriate benchmark or baseline flow period. Although most appreciate that rainfall is the primary driving factor affecting river flows, it has been traditionally

assumed with respect to hydrology that river flows are the consequence of a

independently

and

identically

distributed

random

variables.

Recently

sequence of random climatologists have

suggested a link between multidecadal (the Atlantic Multidecadal Oscillation

periods of warming and cooling of the

  • AMO) and rainfall patterns across

Atlantic much of

Ocean North

America. apparent

Citing Enfield et al. (2001), Basso explanation for observed rainfall

and Schultz (2003) noted that the AMO offered an

deficits

throughout

central

Florida.

The

AMO

suggests that a step trend should be expected in rainfall rainfall. What is particularly interesting to those of us continental United States, Enfield et al. (2001) found for

leading to multidecadal differences in in Florida is that unlike most of the most of Florida a positive (rather than

negative) correlation between rainfall warming (Enfield et al. 2001). While less rainfall over much of the United where rainfall increased.

and prolonged periods of North Atlantic Ocean sea surface periods of warmer ocean temperature generally resulted in States, there are some areas, including peninsular Florida,

Since river flows are largely rainfall dependent, variation in rainfall should result in variations in river flows. While researchers often examine annual variation in stream flow in anticipation of monotonic trends affected by anthropogenic factors, river flow data were tested for a step change as well. Statistical evidence is presented demonstrating a step change in both rainfall (for sites in south-central Florida) and river flows that is consistent with a step in warming and cooling phases of the AMO. While this work does not suggest that monotonic trends and human induced changes in flow have not occurred; it is suggested that many of the observed decreasing or increasing flow trends reported for rivers in Florida are consistent with a step trend.

To be consistent with Enfield et al.'s (2001) conclusions regarding the AMO, it was reasoned that in Florida flows would be highest at stream flow gage sites when sea surface temperatures in the North Atlantic are in a warm period (i.e., positively correlated). At the same time most of the continental United States would be expected to be in a period of lower flows. Conversely, the majority of continental gage sites would be expected to exhibit higher flows during AMO cool periods and much of peninsular Florida would be expected to be in a period of low flows.

Based on these hypotheses, Kelly (2004) examined flow records for multidecadal periods corresponding to warming and cooling phases of the AMO for numerous gage sites within the District, the state, and the southeastern United States to discern if increases and decreases in river

Session 9 – Page 7

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