The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
Other breakpoints in the data are apparent and could be used for the determination of
minimum flows, depending on the management goals for the spring run. The 75 th
percentile salinity values showed a breakpoint between 6 and 7 cfs. At 6 cfs, the median
value is 2.1 ppt. However, the 75 th percentile value is 7.8 ppt, with a 90 th percentile
value of 13.5 ppt. The median value for 7 cfs was 1.5 ppt, which is equal to median
salinity of the spring run under normal flows. At 7cfs the 75 th percentile drops to 2.8 ppt,
while the 90 th percentile value drops to 6.7 ppt. There were three times as many days of
record with flows of 7 cfs (24 days) than there was for 6 cfs (8 days), so inferences
about salinity for 7 cfs are probably more valid.
The daily values listed in Appendix I can be used to determine on what days salinity
incursions occurred for each flow rate. Differences in minimum and maximum salinity
values in excess of 1.5 ppt can be used to indicate the movement of river water
upstream of the weir on a sampling day. Using this criterion, salinity incursions occurred
on 22 of the 26 days (85%) when flows were 7 cfs. Daily maximum salinity values in
excess of 10 ppt occurred on six of the days that had incursions. As previously
discussed, the 95 th percentile salinity values show a breakpoint between 12 and 13 cfs,
dropping from 7.3 to about 3.3 cfs. There were 26 total days of record with flows of 13
cfs. Salinity incursions occurred on nearly half (46%) of those days, with maximum
salinity values in excess of 10 ppt occurring on four of the incursions.
The number of days there were salinity incursions equal to or greater than 5 or 10 ppt
are summarized in Table 5-2 for flow classes greater than 5 cfs, along with statistics for
the durations of these incursions. The percentage of the 15 minute observations that
exceeded 5 or 10 ppt are also listed in Table 5-2 and illustrated in Figure 5-6. Similar to
the results shown in the box and whisker plots, there are breakpoints in the percent of
observations over 5 ppt at 7, 13, and 18 ppt. As discussed earlier, the occurrence of
high salinity values at flows above 20 cfs are due to large changes in flows during those
days that are masked by the average flow value, or are due to unusually high tides
during a flow of 26 cfs. Excluding these outliers, the percent of observations above 5 ppt
never exceeded 4% at flows above 13 cfs, and were zero at flows above 18 cfs.
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