Alafia River Minimum Flows and Levels - Southwest Florida Water ...

occurred during the spring were similar to differences noted for northern river
flows while differences in summer flows were similar to flow changes that
occurred in southern rivers.
2.3.1.2 Flow Trends – testing for a monotonic trend and a step trend
While several authors have examined temporal stream flow patterns in
anticipation of monotonic trends, the AMO suggests that a step trend rather than
a monotonic trend should be expected in rainfall and river flows. Kelly (2004)
demonstrated that many of the observed decreasing flow trends reported for
rivers in the SWFWMD are consistent with a step trend. He found that, while a
Kendall's tau test of pre and post periods (1940 to 1969 and 1970 to 1999)
suggested no trends in flow in many cases, a Mann-Whitney test of the two
periods indicated a significant difference in flows between the two periods.
Supporting results specific to the Alafia River are presented later in this chapter.
It should be noted that prior to any consideration of the AMO as a causal
mechanism for a step change in rainfall and consequently river flows, several
authors (see especially Hickey 1998) suggested that a rather abrupt change in
rainfall patterns had occurred.
While most workers realize that there can be extreme annual variation in flow,
most also believe that this variation is more or less random. As a result when
flow declines occur, the tendency has been to look for an anthropogenic
explanation for these flow trends. Olsen et al. (1999) in examining flood
frequency estimation for the upper Mississippi and lower Missouri Rivers
observed that, "the annual maximum peak floods are considered to be a sample
of random, independent and identically distributed (iid) events Thus one implicitly
assumes that climatic trends or cycles are not affecting the distribution of flood
flows in a significant way." Olsen et al. (1999) eventually conclude that "current
interest in climate change and its potential impacts on hydrology in general and
on floods in particular calls into question the iid assumption." Although Olsen et
al. (1999) were interested in flood flow, their comments are applicable to flow
variation in general. Further, in the words of McCabe and Wolock (2002), "the
identification of an abrupt increase [or decrease] in streamflow rather than a
gradual increasing [decreasing] trend is important because the implications of a
step change are different from those of a gradual trend. The interpretation of a
gradual trend is that the trend is likely to continue into the future, whereas the
interpretation of a step change is that the climate system has shifted to a new
regime that will likely remain relatively constant until a new shift or step change
occurs."
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