The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
400
1940 1950 1960 1970 1980 1990 2000
Dpy_lt3cfs.grf
400
Days/yr With < 3 cfs Flow
300
200
100
300
200
100
0
0
1940 1950 1960 1970 1980 1990 2000
Year
Figure 3-9. Number of no-flow days per year from the
Hillsborough River Reservoir .
A principal effect of these flow reductions has been a dramatic increase in the
occurrence of no-flow days at the reservoir spillway, or days when there is no discharge
from the reservoir other than leakage and seepage near the dam. A value of 3 cfs is
chosen to represent no-flow days since daily flows values up to 3 cfs appear in the
USGS records to denote leakage from the dam. The occurrence of no flow days at the
spillway began to first regularly appear in the late 1960s (Figure 3-9). The number of noflow
days per year increased rapidly in the 1970s, when water use from the reservoir
first reached average yearly quantities of about 77 cfs (50 mgd). Since that time, it is not
uncommon for no flow days to exceed 200 days in dry years, reaching a maximum
quantity of 320 days in the year 2000 at the height of the recent drought.
A flow duration curve of discharges from the Hillsborough River dam for the period 1988
– 2002 is shown in Figure 3-10. This period was chosen as it represents a period when
withdrawals by the City were near their present use. No flow conditions at the dam
occurred on about half of the days during this period. However, when there is flow from
the dam it can be substantial. For example, flows of 303 cfs were exceeded 20% of the
time during 1988-2002, while flows of 1,500 cfs were exceeded 5% of the days.
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