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Potential evapotranspiration, as defined by Thornthwaite, is "the amount of water which will be lost from a surface, completely covered with vegetation if there is sufficient water in the soil at all times for the use of the vegetation." A further requirement is that the test site be surrounded by an unlimited area of actively transpiring vegeta- tion adequately supplied with water. This requirement limits the formula to extensive areas of vegetation as meadows, alfalfa fields, and dense stands of phreatophytes. For row crops, it would apply only to that part of the growing season when the land surface is com- pletely shaded. Generally, it is not suited for calculating evapotranspiration by phreatophyte stands in the West because of the requirement for unlimited boundary conditions. Most stands of phreatophytes occur in the flood plain of streams; and although they may be many miles in length, the width may vary from a fraction of a mile to only a few miles in width. In a comparison of measured evapotranspiration from irrigated alfalfa fields in southern California, Nixon, MacGillivray, and Lawless (1963) with the Blaney-Criddle and Thornthwaite formulas, found the Thornthwaite formula performed poorly. Nixon ascribes shortcomings in both the Blaney-Criddle and Thornthwaite formulas to their dependence on temper- ature rather than solar radiation. In this connection, he states that "no single climatic index of those investigated (solar radiation, temperature, vapor pressure deficit, wind, Weather Bureau pan evapora- tion, and evaporation from black and white spherical atmometers), will universally predict evapotranspiration rates." Penman's formula (1956, p. 44 and 1963, p. 40) is more complex and requires more meteorological data than either of the other two. The data required are duration of bright sunshine, temperature, humidity and wind speed. Calculation of evapotranspiration involves a two-stage process: first, a value for potential evaporation for a hypothetical open water surface exposed to the measured weather, and then, conversion to potential evapotranspiration by a coefficient based on an experimental factor. Thus: Et = f (l1Ho 11 E + y a) +Y in which f = a coefficient ranging from 0.6 to 0.8 for the year for southeast England ~ = a temperature-dependent constant which is the slope of the saturation vapor-pressure curve at mean temperature Ho = the heat budget for "open water" Jr = the constant of the equation for a wet and dry-bulb psychrometer Ea = an expression involving wind speed and saturation deficit Et = potential evapotranspiration 14