The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
5.5.8 Time series of water temperatures in surface and bottom waters
Time series plots of hourly outputs of water temperature in surface and bottom waters
for the different scenarios are presented for the coldest period simulations in Figures 5-
26 and 5-27. In these simulations water temperatures in the surface are warmer and
much more variable than the bottom layers. Bottom water temperatures fluctuated near
12 o C for much of the period and never exceeded 15 o C, the suggested threshold for the
lower end of a thermal refuge. In situations such as this, it is expected manatees would
stay closer to the water surface where the spring flow exerts more of a thermal effect. It
is also noted that there were virtually no differences in bottom temperatures between
any of the scenarios, as the diversion of spring water mainly affected water temperature
in the surface layer.
There were apparent differences in surface water temperatures between the four
modeled springflow scenarios. Surface water temperatures for the historic scenario
dipped below 15 o C for about one-sixth of the days in these coldest period simulations.
Differences in surface water temperatures between the baseline and the flow scenarios
increased as increasing springflow was removed, being least for scenario A and
greatest for scenario C. The mean difference in surface temperature between the
baseline and scenario A for the entire simulation period was 1.6 o C. The mean surface
temperature difference between the baseline and scenario B was 2.3 o C, and 2.7 o C for
scenario C. Both of these differences exceed the 2 o C threshold that was recommended
by the FMRI. That recommendation, which was for mean temperatures in the entire
river cell, was made before the model results were examined. Given the results plotted
in Figure 5-26, it is suggested here that the 2 o C temperature change threshold should
apply to where a thermal refuge is present, which is in the surface waters for the coldest
period simulations. It is reiterated, however, these simulations are for the coldest water
temperatures on record for the river dating back to the mid-1970s.
Water temperatures simulated for the thermal refuge period were considerably warmer.
Bottom temperatures remained above 20 o C and were warmer than surface
temperatures for nearly all of the simulation period. Such a switch in temperature
differences between surface and bottom temperatures is supported by data from the
recorders in the river near the spring, which show that differences between surface and
bottom temperatures (surface – bottom) can fluctuate between positive and negative
values during the winter (Figure 3-32). This is apparently due to the effects of both
short-term and long-term cold periods on surface and bottom temperatures in the river.
Regardless of which is warmer, both the model and the USGS recorders (Figure 3-33)
indicate that short-term variations of water temperatures are much greater for surface
layers, as density stratification in the river largely isolates bottom waters from the effects
of short-term changes in air temperatures.
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