Weeki Wachee River System Recommended Minimum Flows and ...

the regression equation used to predict the location of an isohaline zone has a
reasonable value of 0.66. Nevertheless, a major concern with the salinity
regression equation is that the salinity at the river mouth is not included as an
independent variable. Although the analysis described in the District’s report
concluded that flow from Weeki Wachee Spring did not have a significant impact
on salinity at the river’s mouth near Bayport, the salinity level in the river system
is obviously dependent on the salinity level in the nearshore Gulf waters. This is
an important boundary condition in the numerical model, described below, which
was applied to assess the impact of flow reductions on the thermal regime. The
regression equation for the location of salinity isohalines is used extensively in
determining what flows result in a 15% reduction of various biological resources.
It seems clear to the Panel that boundary salinity at the river mouth should have
been included as an independent variable in the predictive equations.
Initially the District considered including a boundary salinity in the flow/salinity
regression but chose not to do so because at that time the District fully expected salinity
at the mouth to be dependent on the river flow. In other words, boundary salinity would
not qualify as an independent variable since it was assumed that it would be collinearly
related to another independent term, namely flow.
For this reason, the District instead attempted to include a surrogate term
(Withlacoochee River flow) that was independent of the Weeki Wachee flow, but one
that might reasonably impact salinity in the near-Gulf boundary waters. The
Withlacoochee River is large surface drainage system (1,170 mi 2 compared to 38 mi 2
Weeki Wachee that is expected to influence the near-Gulf salinities along the coast. The
District experimented with including a Withlacoochee (same-day and lagged flows -
transformed and untransformed) flow term in the regression, but these largely turned
out to be insignificant or improved the regression only marginally. However, in
retrospect, once the salinity at Bayport was shown to be independent of river flows,
another form of the regression including Bayport salinity as an independent term should
have been evaluated at the time the report was written.
In response to the panel's comments, the District evaluated the impact of adding a
boundary salinity term to the existing regression model. For each sampling date, the
furthest off-shore (minimum offshore distance of -0.41 Rkm) bottom salinity was
assigned as the boundary term. The distance varied some with date but virtually all
were between -0.41 and -0.91 Rkm. Boundary salinity was assigned to a total of 36
sample dates and the multiple parameter regression of the form below was evaluated.
Rkm isohaline = β o + β 1 *(1/Flow) + _ β 2 *Salinity + β 3 *Boundary_Salinity
This form resulted in a slight improvement over the original model which did not include
the boundary term. The original form exhibited an r 2 adj = 0.66 (n= 632, SE estimate
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