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reflected solar (short wave) radiation, and incoming and outgoing long-wave radiation. The method does not give a measure of the results of evapotranspiration as does the atmospheric flux method but rather the energy available for evapotranspiration. For a body of water this is not a problem, but for vegetative cover the rate of evapotranspira- tion depends not only on the energy available but also upon the require- ments of the plant. During the early stage of growth, transpiring surfaces of the plant are small, and so use only a portion of the energy available, with the remaining portion converted to sensible heat in the soil. At full development the plant may use all of the available energy, and when mature, limits transpiration to its needs, so that some energy may convert to sensible heat. The difficulty with the method lies in the ability to measure the sensible soil heat. Unless this is done, evapotranspiration computed during the early and late stages of plant development may be larger than actual. A further complication is advected heat that may be transferred from an adjacent area. If warmer air is advected over a vegetated cover, relatively more of the net radiation is converted to evapotranspira- tion. If the air is cooler, the reverse occurs. Feasibility of the energy-balance method depends upon the accuracy with which the para- meters are measured, particularly sensible soil heat and advected heat. The method requires extensive instrumentation and an elaborate data processing by a computator. Empirical Formulas Several empirical formulas have been developed for expressing the rela- tionship between evaporation, evapotranspiration, climatic factors and coefficients for a particular vegetative species. These formulas assume that water is nonlimiting, and that the vegetation shades the ground completely--putting the calculated values in the potential rather than the actual realm. The elements of climate that enter into the several formulas include temperature, humidity, wind movement, percent of day- time hours, and duration of bright sunshine. Temperature is the only element common to all formulas. The three most widely used formulas--the Blaney-Criddle, Thornthwaite, and Penman--appear well suited for the geographical location for which they were developed, Blaney-Criddle in the arid western United States, Thornthwaite in the humid seaboard of the eastern United States, and Penman in cool England. The formula, developed by H. F Blaney (1962) and W. D. Criddle, is based on the assumption that water being ample, evapotranspiration is directly proportional to the climatic factor, and that all other factors are constant for a particular species. 12