REPORT OF INVESTIGATIONS No. 42


developments have been made. Long-term records of rainfall and
streamflow in the upper Palatlakaha Creek (stations 11 and 12)
and Withlacoochee River (station 42) have been used to detect
changes or trends in the pattern of discharge from the upper
Palatlakaha Creek basin since 1946.
 Double-mass curves of cumulative measured runoff and
cumulative computed runoff have been plotted to provide a means
of examining the records of streamflow from the area of
investigation to detect changes that may have occurred (Searcy
and Hardison, 1960). The variables used in preparing the curves
shown in figure 19 are the values of cumulative computed runoff,
taken from the precipitation-runoff relations in the figures on
pages 97 and 99 and cumulative measured runoff at each of the
two gaging stations.
 The rainfall pattern is not affected by the progressive changes
 in the drainage system in the Green Swamp area. The theoretical
 or computed runoff based on rainfall is taken from an average curve
 for several years of record. Any change in slope in the double-mass
 curves of figure 19 would reflect progressive man-made changes in
 runoff.
 Figure 19a is the double-mass curve for the Withlacoochee
 River basin above the Trilby gaging station. Straight lines are
 drawn to average several points that show definite trends. These
 lines change in slope between 1942 and 1943, between 1945 and
 1946, and between 1953 and 1954. The two changes in slope in the
 1940's indicate changes in the runoff pattern but the authors have
 no knowledge of the causes of such changes. Minor deviations of
 the plotted yearly values of runoff are probably caused by variations
 of rainfall distribution and intensity during the year and are not
 necessarily indications of changes in the long-term trends. Yearly
 values of runoff for 1954-61 define an average line with less slope
 than that for any previous period. This change in slope indicates
 that a higher rate of runoff from the basin occurred during 1954-61
 than that indicated from the same rainfall pattern of previous
 years.
 Figure 19b is the double-mass curve for the upper Palatlakaha
 Creek basin. The figures of annual runoff were adjusted for changes
 in storage in lakes. For the period 1946-49, the curve takes the
 general direction .as shown by the straight line. However, after
 1949, a definite break occurs in the slope of the average line
 indicating, less runoff from the area.