The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
The biological data collected for this project found pronounced changes in the
invertebrate fauna of the spring corresponding to major reductions in flows from the
spring pool during the 2000-2001 drought. Salinity in the run rose dramatically during
the drought, when pumping from the pool reduced spring flows to zero or very low rates
of flow. Despite the relative severity of these physico-chemical alterations, a recovery of
the species richness of the spring run accompanied a return to normal flows. The
determination of minimum flows can consider that periodic impacts to ecosystems can
be tolerated if they are infrequent or of short duration and do not result in a permanent
change in ecological characteristics. Such an approach could be considered for Sulphur
Springs to allow the city additional water use during dry years when water supplies from
the reservoir are low.
In this regard, a lower minimum flow could be considered for infrequent use if it is
assured that such a minimum flow will not result in permanent changes or impacts to
the ecosystem. In the case of Sulphur Springs, a minimum flow of 13 cfs could be used
periodically and likely not result in longstanding harm to the ecology of the run and
lower river. Maintaining a flow of 13 cfs would maintain markedly better conditions than
what the spring run experienced during the 2000-2001. Salinity incursions at a
springflow rate of 13 cfs appear to last no more than several hours during any day, and
many of the species in the spring run would suffer no negative effects. If any species
were lost, they would likely recolonize the spring when higher flows return.
Reiterating that 18 cfs is the preferred minimum flow for routine use, a minimum flow of
13 cfs could be acceptable with a return frequency that does not cause permanent harm
to the ecology of the upper spring run. In unimpounded Florida rivers, the upstream
movement of brackish waters into what are normally tidal freshwater zones happen
during very dry years. It is suggested here that salinity incursions associated with a 13
cfs minimum flow would be acceptable if they occurred only every 2 – 3 years. With this
return interval, the spring run would typically have a period of year or more to recover
after period of salinity incursions. This would allow for recolonization by invertebrate
species that have seasonal life cycles with reproduction in dry season.
The City's need for back-up water supplies from the Sulphur Springs has largely been
during dry years. In that regard, the switch to a possible 13 cfs minimum flow could be
based on water levels in the City's reservoir. This would allow increased use of the
spring for water supply when the reservoir becomes low, but would occur only on an
infrequent basis. A plot of average daily water levels for the period of 1988-2003 is
presented in Figure 5-22. This time period was evaluated as withdrawals from the
Hillsborough River reservoir have been near the current rate of water use (60- 66 mgd),
thus these levels are indicative of current water level fluctuations in the reservoir. The
number of days that water levels fell below 19 feet and the minimum stage each year
are also listed in Table 5-4.
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