FLORIDA GEOLOGICAL SURVEY


 The flow-duration curves indicate the percentage of time that
specified discharges were equaled or exceeded during the period
of record. These may be considered probability curves used to
estimate the percent of time a specified discharge will be equaled
or exceeded in the future. The use of flow-duration curves to
indicate the future pattern of flow from a basin is valid only if the
climatic conditions remain the same and the amount and
distribution of runoff from the basin is not significantly changed
by man.
 The flow-duration curve for Withlacoochee River at Trilby
(station 42) may be only an approximate representation of duration
of future low flows because of the progressive increases in
ground-water inflow by pumpage above the gaging station.
However, the flow-duration curves for the other four stations
shown in figure 10 may be considered probability curves and used
to estimate the percent of time that a specified discharge will be
equaled or exceeded in the future.
 During a period of extremely low flow on May 23-25, 1961,
streamflow was measured and water samples for chemical analysis
were collected at several sites on the Withlacoochee River. The
results of this low-flow investigation are shown on the map in
figure 11.
 The base flow of the Withlacoochee River near Dade City
 (station 40) represents the natural drainage from 390 square miles
 because no surface flow is diverted to the Hillsborough River basin
 through the overflow channel, C-9. Since about 1941 or 1942, the
 effluent from citrus processing plants at Dade City has been drained
 into the Withlacoochee River by way of the Pasco Packing Company
 canal. The water used by these plants is pumped from deep wells.
 Measurements at station 41 of the effluent from the Pasco Packing
 Company canal during 1958-62, ranged from 5 cfs, when the plant
 was at minimum operation, to about 76 cfs at peak operation during
 the citrus packing season. Inflow to the river from this plant and
 others at Dade City produces higher discharge below station 40
 east of Dade City than would be derived from the natural yield of
 the basin. During dry periods, the effluent at Dade City greatly
 exceeds the base flow of the Withlacoochee River (see fig. 11).
 The drainage area above Trilby (station 42) comprises
two-thirds of the Green Swamp area and the record collected at this
station is a good index of the long-term variations of surface runoff
from the entire area except at low flow.