Lake Hefner was chosen for the evaporation research conducted in 1950-51 and for the evaporation suppression tests in 1958 primarily because it is one of the few reservoirs in the United States where evaporation can be determined accurately by the water-budget method. In the present study, measurement of the actual evaporation from Lake Hefner was not sufficient; of equal importance was the determination of the evaporation that would have occurred if a monomolecular film had not been applied.