Potential evapotranspiration estimates by the approximate energy balance method of Penman

The energy balance approximation of Penman, used to estimate potential evapotranspiration, was tested for daily estimates on 37 days during 1956 and 1957. Though the Penman estimates were highly correlated with those obtained from detailed energy balance measurements, the absolute values of the Penman estimates were much too small. In order to obtain suitable daily estimates of evapotranspiration with the Penman method, a wind function that accounts for surface roughness and direct measurements of radiation energy were found necessary. The stomatal and daylength factors proposed by Penman and Schofield appeared to have little basis for general application.