The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
the intent of that condition is to ensure that Sulphur Spring is used only as a back-up
water supply source that is used during times of impending water shortages. The
condition specifies that withdrawals from Sulphur Springs cannot occur until water levels
in the reservoir fall below 20 feet for the months from March through June, or below 18
feet for the remaining eight months of the year.
The minimum flows recommended by this report will require that pumpage rates from
the spring be considerably less than the 20 mgd rate the City has used in the past. As
previously discussed, the City now has the capability to manage withdrawals in
increments which will allow much smaller withdrawal rates to be possible. Since the
withdrawal rates will have to be reduced to meet minimum flows, it is recommended that
linking spring withdrawals to water levels in the reservoir be dropped from the City's
water use permit as long as minimum flows for Sulphur Springs are met. This will allow
the City to withdraw water from Sulphur Springs much sooner, albeit at smaller
withdrawal rates, to meet water supply needs in the dry season. It is reiterated the
minimum flow will be linked to reservoir levels, so that slightly higher withdrawal rates
will be possible if water supplies in the reservoir become low.
5.5 Criterion 3 - Maintain a thermal refuge for manatees
5.5.1 Purpose
Changes in water temperatures that would result from reducing or re-routing flows from
Sulphur Springs were investigated to assess any adverse potential impacts to the
biological communities in the spring run and lower river due to alterations of the spring's
flow regime. As described earlier, the lower river and the spring run are utilized by the
Florida manatee (Trichechus manatus latritostris), a federally listed endangered
species. The water temperature of flow from Sulphur Springs remains near 25 o C
throughout the year, while the waters of the lower river are more influenced by seasonal
air temperatures and can be considerably cooler. During cold winter periods, the
warmer water from Sulphur Springs provides a thermal refuge for manatees in the
spring run and the lower river near the mouth of the spring run. Flows from the spring
also benefit other species that are sensitive to cold, such as the snook, a highly valued
saltwater game fish that is found in the lower river.
The objective of the analysis presented below was to evaluate the effect of reducing or
re-routing flow from Sulphur Springs on water temperatures in the lower river during
winter months. It was reasoned that water temperature in the spring run should be less
sensitive to flow alterations than temperature in the nearby river. Therefore, if the
requirements of a thermal refuge in the lower river near the spring were met, the
thermal requirements of a refuge in the spring run would be met as well. Analyses were
conducted to determine if the minimum flows that are recommended to meet salinity
requirements of the spring run and lower river are sufficient to meet the temperature
requirements of a thermal refuge for manatees in the lower river.
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