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

To avoid problems associated with analysis of CWM rainfall data sets, it may be
appropriate to examine individual gage sites and look for trends/patterns at long-term
NOAA sites as was done by Basso and Schultz (2003). Another alternative, if a
composite number is preferred, is to make sure that estimates are generated using the
same gages each year; however, if older estimates (pre-1970) are needed (and they
would be necessary for examining the climatic signature) and one wants to increase the
sample size of gage sites, then predictive relationships between gages would need to
be developed to extend the record back in time. However, looking for statistical
relationships in this type of synthesized data may not be appropriate and would not
seem to be preferable to analysis of empirical data. Establishment of highly predictive
equations for extrapolating historic conditions becomes especially problematic, since
the relationships between individual gages are not especially strong for high rainfall
months (very weak in many cases as shown in Figure 1). This, of course, speaks to the
highly localized nature of the convective type storms encountered in the summer
months in contrast to the large frontal storms that bring rain in the other months.
Staff did, however, generate (using the 20 sites employed by Kelly, 2004) a composite
District-wide (rather than CWM basin) mean rainfall record just to see if the result would
be statistically significant or not if a composite rather than individual sites were
analyzed. The District-wide yearly means were generated using a straight mean (sites
were not weighted using Thiessen polygons) for this analysis. The results are shown in
Table 1. A one-tailed test was used and is appropriate for this analysis (although this
appears to be questioned by EPCHC), since what is being tested is the idea that rainfall
in peninsular Florida increases (decreases) during AMO warm (cool) periods as
proposed by Enfield et al. (2001). Please note that depending on whether you are
looking at means or medians the difference between the two periods is 4.6 to 5 inches,
and while this amounts to about a 10 percent difference in rainfall between the two
periods this could lead to considerable differences in flow, since most of the rivers in the
water management district typically discharge on average 6 to16 inches of water per
year.
In consideration of the water budget for upper Alafia River, the EPCHC questions the
assertion that declines in low flows are related to removal of phosphate mining related
discharges or are confused by District statements related to flow declines being related
to either changes in water management practices by the phosphate industry or climate.
We maintain, and perhaps it is not clear in the report, that two types of decline have
occurred relative to flows in the upper Alafia River basin. The greatest flow declines as
discussed in considerable detail in Kelly (2004) between AMO warm and cool periods
are related to rainfall differences that occur as a "step change" between these two
periods. We concur with Hickey (1998) and in an analogous manner with McCabe and
Wolock (2002) that an abrupt change in river flows occurred around 1970, and believe
that this change is related to the AMO and rainfall as proposed by Enfield et al. (2001).
However, the hydrology of the Alafia River is somewhat unique (but not totally unlike the
upper Peace River; see SWFWMD 2002) in that during part of the record, low flows
were essentially augmented by mine related discharges. These discharges were
Appendix D vi