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

Although Hickey (1998) concluded that climate was largely responsible for the
decreasing trend, he did note that at the Alafia River at Lithia stream flow
decreased about 44 cfs in the period between January 1962 to December 1981.
He speculated that these flow declines were the result of mining, but were related
to a substantial decrease in water being discharged rather than landscape
changes that resulted in hydrologic alterations. Decreases in discharge were
accomplished through increased water use efficiency and a decrease in ground
water usage. Inspection of water quality data for the river suggests that Hickey
(1998) is correct, and flow declines are related to improved efficiency rather than
a diminishment of flows resulting from landscape alterations due to mining. In
developing a relationship for the expressed purpose of predicting the impact of
increasing area of phosphate mined land on stream flow, SDI (2003) assumed
that the trend of decreasing stream flow in the South Prong of the Alafia River
was related to increasing area of land mined. This assumption was made
because mining was essentially the only land use that changed during the time
interval investigated.
While SDI (2003) assumed that the flow decline was attributable to increases in
land area mined for phosphate, this is not the case. Using logic similar to SDI
(2003), there should be a steady monotonic decreasing trend in flow with
increasing mined area. While mined area in the South Prong above the USGS
gage increased substantially between 1972 and 1999 (from 9 to 72%) based on
land use maps for this time period (see Figure 2-21), flow remained fairly stable
(Kendall's tau was run on mean annual flows for the period 1970 to 1999, and the
slope of the Thiel line was –0.1918 with a p value of 0.8028; indicating no trend;
see Figure 2-22).
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