The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
Sulphur Springs < 33 cfs,
Hillsborough < 3 cfs
Figure 5-5. Box and whisker plot of 15 minute salinity
measurements at the data recorder in the upper spring run. Data
restricted to periods when there was no flow to the lower river from
the Hillsborough River reservoir.
There is a conspicuous break in the median and 75 th percentile values between 6 and 7
cfs, with a smaller secondary drop in the 75 th percentile values between 7 and 9 cfs.
Above 9 cfs, the median and 75 th percentile values are relatively flat across the
remaining higher flows. Similarly, there is a noticeable drop in the 95 th percentile values
between 7 and 8 cfs. There were a considerable number of days at 7 cfs (24 days),
while there were 17 combined days at 8 and 9 cfs (Figure 5-4). Thus, these breaks in
the data are supported by a reasonable number of observations. A secondary drop in
the 95 th percentile values occurs between 12 and 13 cfs, which respectively had 23 and
26 days of record. At 13 cfs, the 95 th percentile value is 3.3 ppt. A second general drop
in the 95 th percentile values occurs near 16 or 17 cfs, above which most of the 95th
percentile values are near or below 2 ppt.
A conspicuous drop in salinity values above the 95 th percentile occurs between 17 and
18 cfs. Above 18 cfs, incursions of brackish water appear to be very infrequent.
Assuming the salinity values presented in Figure 5-5 are representative of longer
periods within those flow ranges, 18 cfs flow may represent a threshold flow to prevent
brackish incursions into the spring run except for very high tides.
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