The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
4.3 Consideration of seasonal variations of springflow and relationships to
flows from the Hillsborough River reservoir
Other District efforts to establish minimum flows for flowing water bodies have
emphasized the maintenance of complete flow regimes, which include natural seasonal
variations of low, medium, and high flows that reflect the climatic and watershed
characteristics of a particular river system (SWFWD, 2001). District assessments of
freshwater inflows to estuaries have similarly emphasized maintaining natural flow
regimes by limiting diversions to a percentage of streamflow at the time of withdrawal
(Flannery et al. 2002).
The maintenance of a flow regime with natural seasonal variations was not applied to
the determination of minimum flows for Sulphur Springs for a number of reasons. First,
the flows from Sulphur Springs show subdued seasonal variations, generally fluctuating
between 25 and 41 cfs under normal flow conditions. Due to this fairly constant rate of
flow and the physical characteristics of the spring run, seasonal variations of physical
and water quality variables in the upper spring run are small under normal flow
conditions. However, the data collected for this project demonstrate that the ecology of
the spring run can be strongly affected by withdrawals and corresponding flow
reductions that allow the incursion of high salinity water from the Lower Hillsborough
River into the spring run. Therefore, rather than focusing on the seasonal range of
flows necessary to maintain the spring run, the District's analysis focused on flow rates
that prevent or minimized salinity incursions.
The percentage of inflow to the Lower Hillsborough River comprised by Sulphur Springs
typically has a strong seasonal component, but this is primarily due to the effects of
withdrawals from the Hillsborough River reservoir on flows from the dam. On the
average, there have been no flows from the reservoir to the lower river for about half of
the days in recent decades. During these periods, Sulphur Springs is the principal
source of inflow of low salinity water to the lower river. When there are flows from the
reservoir, the percent of inflow contributed by Sulphur Springs declines greatly. In
general, the ecological functions of flows from the spring to the lower river are most
critical when there is no flow to the lower river from the reservoir.
As described in Section 2.5.3, the discharge from Sulphur Springs is fairly mineralized
and exceeds potable water standards for some constituents. As a result, there has
historically been little emphasis to withdraw water from the spring for water supply,
except during water shortages when water levels in the Hillsborough River reservoir
become low. Since 2001, a portion of the flow from Sulphur Springs has been routed to
the base of the Hillsborough River dam to provide minimum flows to the lower river
during periods of no flow from the reservoir. An important objective of this minimum
flows report is to determine if such diversions can occur and still meet the minimum flow
requirements of Sulphur Springs. If they cannot, then minimum flows to the spring will
take precedence and other sources of water will have to be used to provide minimum
flows at the base of the dam.
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