Not surprisingly, estimates of total freshwater inflow to Tampa 
Bay contain terms which involve a considerable level of uncertainty. For 
instance, streamflow is not measured for most of the minor tributaries to 
the bay and gauging sites on major rivers are upstream of significant 
portions of their respective drainage basins. Linear extrapolation using 
drainage basin areas can be used to estimate flow from ungauged areas, 
but differences in runoff coefficients may differ and thereby introduce 
sources of error. Even in areas where streamflow is measured, 
differences in length of record can introduce bias into estimates of 
average flows. 
Despite these sources of error, estimates of total tributary flow 
to Tampa Bay have been presented by several authors. Dooris and Dooris 
(1985) estimated average total flow from seven gauged streams at 1,792 
cfs. Goodwin (1987) estimated average total flow to the bay from 
tributaries at 1,904 cfs, but this also did not include estimates of flow 
from ungauged streams. Hutchinson (1983) estimated flow from ungauged 
areas to be 344 cfs, giving a total freshwater inflow of 2,229 cfs to the 
bay. In this report, I have re-estimated average inflow to the bay by 
using streamflow data up to 1986 and employing a factor of 81.5 percent 
for total flow contributed by the four major rivers. This factor is the 
average of the percentages indicated by Hutchinson (1983) and Lewis and 
Estevez (1988). Using this formula, my estimate for total tributary flow 
to Tampa Bay is 2,011 cfs. This estimate accounts for withdrawals made 
from the bay’s tributaries, but does not account for any effluents which 
enter these streams downstream of gauging stations. 
Streamflow Reductions 
As shown in Figure 7, withdrawals are taken from the Hillsborough, 
Manatee, Alafia and Little Manatee Rivers. The Hillsborough and Manatee 
Rivers are impounded by instream reservoirs and withdrawals are made for 
municipal water supply. Included in the values for the Manatee River are 
municipal withdrawals from the Braden River, an impounded tributary to 
the Manatee which enters the main river 8 miles from the bay. In 
contrast to these three instream reservoirs, withdrawals from the Little 
Manatee River are diverted to an offstream reservoir and used for power 
plant cooling water. Withdrawals shown for the Alafia River are actually 
taken from artesian springs which flow into the river. 
Using values from 1987, average daily withdrawals from these four 
streams were 93 cfs for the Hillsborough, 50 cfs for the Manatee, 7 cfs 
for the Braden, 8 cfs for the Alafia and 19 cfs from the Little Manatee. 
Collectively, these withdrawals are equivalent to 8.8% of the estimated 
average streamflow to Tampa Bay, suggesting that the impact of these flow 
reductions may be limited when viewed on a net annual basis. The effects 
of these withdrawals, however, can be very important seasonally. Also, 
the refilling of reservoir storage can markedly increase flow reductions 
during recovery after low flow periods. 
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