REPORT OF INVESTIGATIONS No. 42


coefficient) of the aquifer for a given period of time. Separate
analyses were made for both the nonartesian and the Floridan
aquifers.
 Evapotranspiration (ET) cannot be measured directly for
large areas such as the Green Swamp, but may be approximated
by balancing the water-budget equation and determining the
residual quantity. The other quantities of water loss are small in
proportion to the quantity lost by evapotranspiration. Therefore,
the residual quantity in the equation reasonably represents the
evapotranspiration loss.

 EVAPORATION AND WATER BUDGET OF LAKE HELENE
 Most of the precipitation that falls on an area is dissipated
through the natural processes of evaporation and transpiration.
Because evaporation plays such an important role in the hydrologic
cycle, much effort was expended to measure it directly at Lake
Helene instead of calculating it as a residual in the storage equation.
 Harbeck (1962) describes a practical method for measuring the
evaporation from an open-water surface utilizing the mass-transfer
theory. A report by the U. S. Geological Survey (1954), and one
by Harbeck and others (1958), may be consulted for additional
information on the method.
 The mass-transfer method provides a technique for measuring
the evaporation and for determining the seepage from a lake, two
factors of the water budget that are generally determined indirectly
or estimated. The change in volume resulting from evaporation
on the water surface of a lake is computed by use of an empirical
equation based on measurements of the evaporative capacity of the
air. The water-budget equation is then balanced to account for
volume changes from rainfall, surface inflow and outflow,
evaporation, seepage, and other consumptive losses.
 Evaporation computed by applying a coefficient to measured
 evaporation from a pan may be subject to considerable error,
 particularly for periods of less than a year. The annual average
 Class-A pan coefficient for central Florida has been estimated by
 Kohler and others (1959, pl. 3) to be about 77 percent. The monthly
 evaporation-pan coefficients vary more widely and with a greater
 range of probable error than the annual coefficients. There is,
 therefore, the need to supplement pan records with direct
 measurements of evaporation.
 Evaporation from Lake Helene was measured in 1962 by use
 of the mass-transfer method. Lake Helene is located about 1 mile


101