WATER RESOURCES OF ORANGE COUNTY 63
station 18, it was 7,840 cfs, both in March 1960. The minimum flow at station 19 was 6.7 cfs in June 1945. The river flow ceases at station 18 in most dry years. The average flow at station 19 was 275 cfs or 1.06 cfs per square mile for the period 1936 to 1963.
The estimated average flow at station 18 is 88 cfs or 0.74 cfs per square mile. Runoff from the part of the basin above this station is estimated to be 10 inches per year. Average runoff from the entire basin above station 19 is 14.4 inches per year. Runoff from the area above station 26 on Little Econlockhatchee River is estimated to be 10 inches per year. Prorating the 10 inches of runoff from the 146 square miles above stations 18 and 26 with the 14.4 inches from the 260 square miles above station 19 gives an average yearly runoff from the intervening 114 square miles of 20 inches. About 11/ inches (11 cfs) of the 10-inch increase in runoff from the lower basin over runoff from the upper basin is accounted for by the effluent from Orlando's sewage plant. The remaining 8.5 inches is accounted for by higher base flow resulting from ground-water seepage into the more deeply incised channel in the lower basin and possibly upward seepage of artesian water. Curve 3 (fig. 14) is the flow-duration curve for station 19 and curve 1 (fig. 15) is the estimated flow-duration curve for station 18. Note the similarity in the shape of the curves up to 50 percent duration when direct runoff is the main source of flow. Above 50 percent the curve for station 18 falls off rapidly to no flow at 75 percent reflecting the absence of base flow whereas the curve for station 19 continues on at about the same slope reflecting the base flow supplied by the sewage effluent and ground-water seepage. Low-flow characteristics of Econlockhatchee River at station 19 are shown by figure 28. The streamflow indicated by these curves is somewhat more than occurs within Orange County.
A continuous record of conductivity from October 1959 to June 1962 and analyses of water collected periodically to July 1963 from the Econlockhatchee River near Bithlo (station 18), show the water to be high in color, soft, and low in mineral content. Table 8 gives the ranges of mineral constituents from October 1959 to July 1963. The color is always greatest during the early part of high-flow periods. The pH of the water was as low as 5.7 during high-flow periods which indicates that the water would be slightly corrosive. The high percentage of calcium bicarbonate detected during low-flow periods indicates that ground-water inflow may occur.
Figure 29 shows a cumulative frequency curve of specific conductance of the water in the Econlockhatchee River near Bithlo