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001720 is limited to a valley in which ground-water inflow is relatively con- stant. The method involves measurements of streamflow at successive points along the channel--in the winter months when evapotranspiration is essentially zero, and again in the growing season. Evapotranspira- tion is taken as the difference between the ground-water inflow during the nongrowing season in the winter and the net gain or loss of surface water entering or leaving the reach during the growing season. The relation of evaporation, as measured by a Weather Bureau Class A evaporation pan or a Livingston atmometer, to evapotranspiration has received a great deal of attention in recent years especially in California. The relation, a simple ratio, has been expressed as a pan coefficient or an atmometer coefficient. So far, the coefficients developed have been largely for irrigated crops or pasture grasses. No coefficients have been developed for woody phreatophytes. Coefficients for a particular species were found to vary from one environment to another and from month to month. Preliminary evaluation of this method indicates the coefficients should be limited to the same species or species association under the same climatological environment. The evapotranspiration tent method determines evapotranspiration as the difference in water content of the air entering and leaving a ven- tilated polyvinyl tent enclosing a plant or grass plot. The method is still in the experimental stage, but is continually being improved and refined. The method is simple and the instruments are not expensive. It is, however, limited to a single plant or plot of grass. In addition, there has been considerable difficulty with a temperature build-up in the tent, that, with prolonged confinement may injure or even kill the plant. Injury appears least where water is nonlimiting as with phreato- phytes. Aerodynamic and Energy-balance Methods Although the atmospheric flux and energy-balance methods have been used successfully in computing evaporation from a water body, their use in computing evapotranspiration has been largely experimental. The atmos- pheric flux method requires the measurement of fluctuations in water vapor concentration, wind speed and direction at a specified height above the vegetation. The chief difficulty lies in lack of reliable, fast response, sensing and recording instruments. Also, the amount of data is so prodigious as to require processing by a computer. Within the last year an instrument called the IIEvapotronll has been developed in Australia, which according to the designer, senses the fundamental parameters and automatically calculates the evapotrans- piration rate. However, it is not commercially available, and there are no plans to make it so. The energy-balance method is an accounting of the incoming and outgoing thermal energy for a vegetated surface. It requires the measurement of wind speed, humidity, air temperature, soil temperature, incoming and 11