WATER RESOURCES OF ORANGE COUNTY 77
negative (rainfall less than evaporation), the lake levels fall, and when the difference is positive (rainfall more than evaporation), the lake levels rise. The levels of lakes with surface outlets normally decline in October despite a slight excess in rainfall over evaporation because of large surface outflow when the lakes are high. Other factors such as manipulation of control structures may account for inconsistencies like the behavior of Lake Maitland in December and January and Lake Apopka in June.
RANGE IN LAKE-LEVEL FLUCTUATIONS
All of the lakes in Orange County receive about the same number of inches of rainfall on their surfaces and lose about the same number of inches of water by evaporation from their surfaces; yet the range in fluctuation of their levels varies widely from lake to lake. Differences in the physiographic features of the individual lakes and in some cases, control and use of the lake water account for this wide variation in range of fluctuation. The physiography of a lake determines how each of four processes (surface inflow, surface outflow, underground inflow, and underground outflow) will affect its level. The relationships of these processes to lake levels is extremely complex. The degree of imbalance between inflow and outflow determines the range in fluctuation. A specific change in a lake's level can be brought about either by a small imbalance occurring over a long period or by a large imbalance occurring over a short period. To further complicate matters, the same lake may be affected by different combinations of factors at different times.
In Orange County the lakes having the greatest range in fluctuation are those effectively connected to the artesian aquifer in areas where the piezometric surface has a large range in fluctuation. Lake Sherwood, whose range in stage is the greatest observed in Orange County (22.4 feet) is an example. The surface drainage from about 2,400 acres and ground-water seepage from the water-table aquifer in the surrounding sand hills enter Lake Sherwood from which the only escape is by evaporation and downward seepage into the underlying artesian aquifer. The rate at which the downward seepage occurs depends on how high the lake level is above the piezometric surface. In September 1960, the piezometric surface at Lake Sherwood rose to 85 feet and inflow was so great that the lake level rose to above 88 feet before equilibrium between gains and losses was achieved. By June 1963,