The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
Water levels dropped below 19 feet during half of the sixteen years from 1988-2003.
However, these occurrences did not happen at regular intervals. Water levels did not fall
below 19 feet for a six-year period from 1993-1998. However, water levels fell below 19
feet for five years in a row from 1988-1992 and four years in a row from 1999-2002.
Water levels were below 19 feet for only 5 to 9 days during each of the last three years
of the 1988-2002 period and slightly over five weeks during the first two years. Although
low water occurrences happened during successive years, the relatively short period
that a 13 cfs minimum flow would be in effect would probably allow sufficient period for
recolonization by any species that were extirpated from the spring run by salinity
incursions. Lower water levels were more prolonged during the 1999-2002 period,
ranging from 37 to 59 days for those years. These occurrences happened during one of
the most severe and prolonged drought the region has experienced, and should not be
considered typical events.
These results show that simply linking a minimum flow of 13 cfs to water levels in the
reservoir does not provide a perfect mechanism for providing a regular recurrence
interval for implementing a reduced minimum flow. However, the duration of the
reduced minimum flow each year is relatively short, and should allow for periods of
recolonization during the dry seasons of most years. Also, the salinity incursions that
will happen with a 13 cfs minimum flow should not exceed several hours each day, and
the impacts to the invertebrate fauna of the spring should be must less dramatic than
the changes observed during this study. Future biological monitoring of the spring run
could be conducted during dry years to determine the effect of a 13 cfs minimum flow
on the invertebrate fauna of spring run. The continuation of that 13 cfs minimum flow
during low water levels, or possible use of a 13 cfs on a more frequent basis, could be
linked to the results of such monitoring.
5.4.1 Application of minimum flows based on criteria 1 and 2
In summary, the minimum flow for Sulphur Springs based on criteria 1 and 2
(maintaining low salinity habitats in the spring run and river) is 18 cfs. This minimum
flow may be reduced to 10 cfs during low tide stages in the lower Hillsborough River if it
does not result in the movement of brackish waters from the river into the upper spring
run. A minimum flow of 13 cfs can be implemented when water levels in the
Hillsborough River reservoir fall below 19 feet NGVD. This minimum flow can also
switch to 10 cfs at low tide stages in the lower river if it does not result in salinity
incursions into the upper spring run. Evidence of a salinity incursion could be a salinity
value at the upper spring run recorder that is more than 1 ppt greater than the
concurrent salinity value in the spring pool.
Water use from the spring by the City of Tampa is regulated under a special condition in
their water use permit for withdrawals from the reservoir. Because of the important
functions that springflow has with regard to the ecology of the spring run and lower river,
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