REPORT OF INVESTIGATIONS No. 42


 The discharge at the Trilby gaging station does not represent
the natural runoff from the Withlacoochee River basin because of
the high-water flow diverted from the basin to the Hillsborough
River by the Withlacoochee-Hilisborough overflow channel (C-9)
and the effluent into the river from the citrus concentrate plants
at Dade City. When the Withlacoochee River reaches a stage of
about 78.5 feet above msl at the overflow channel, part of its flow
is diverted into the Hillsborough River. At high stages more than
a fourth of the flow from the upper Withlacoochee River is diverted
through this channel. Computations of basin runoff for either the
Withlacoochee or the Hillsborough Rivers must be adjusted for the
amount of discharge from one basin to the other. Percentagewise,
the plant effluent into the basin is small except when the discharge
in the Withlacoochee River is extremely low and the plant is at peak
operation.
 The annual and mean monthly discharges at the Trilby gaging
station are shown by the bar graphs in figure 12. The general
relation between rainfall and streamflow is evident from figures 6
and 12. During the wet years of 1959 and 1960, annual rainfall over
the Green Swamp area was 70.9 inches and 69.5 inches, respectively.
The annual mean discharge at the Trilby gaging station was 1,157
cfs for 1959 and 1,209 cfs for 1960. The higher runoff for 1960 was
probably the result of a carry-over from the high rainfall of 1959.
 The maximum discharge of record at the Trilby station was
 8,840 cfs on June 21, 1934. Flood-frequency studies by Pride (1958)
 indicate that the recurrence interval of a flood at this magnitude
 is more than 100 years. The peak discharge of the flood of March
 1960 was 6,920 cfs and was the third highest flood of record. The
 recurrence interval of a flood of this magnitude is about 40 years.
 The drought of 1954-56 was the most severe dry period of
 record, considering its 3-year duration and yearly deficiencies.
 Annual rainfall on the basin above the Trilby station for 1954-56
 was 39.9, 40.2, and 46.2 inches per year, respectively. The prolonged
 period of low rainfall resulted in low discharges at the Trilby
 station during each of the 3 years. The lowest annual mean
 discharge at the Trilby station was 75.4 cfs for 1932, a year in
 which the total rainfall amounted to 39.6 inches. The total annual
 rainfall on the basin in 1961 amounted to only 35.2 inches and was
 the minimum for any year of record. Effluent from citrus
 concentrate plants, derived from ground-water sources, accounted
 for the higher annual mean discharges for 1954-56 and 1961 than
 that for 1932.