Using the Penman-Monteith approach to extrapolate soybean evapotranspiration

Soybean evapotranspiration (ET) measured by the Bowen ratio energy balance method is used as the basis for the comparison of a commonly used semi-empirical (combination equation) and a pure statistical approach. Surface conductance within the combination equation was fitted to measured conductance by the Bowen ratio energy balance method. A multiple linear regression to vapor pressure deficit, solar radiation and wind speed was found. The same parameters were used to predict soybean evapotranspiration through a direct multiple linear regression. Despite a statistically rather poor forecast of surface conductance during 1987 (64% of variance explained) both evapotranspiration estimates agreed well with measured evapotranspiration (r² = 0.94). However, for a second year (1991) both, the combination equation and the pure statistical approach based on 1987 statistical relationships yielded only 70% of actual evapotranspiration. Actual ET was 70% and 80% of potential ET in 1987 and 1991, respectively. These similar relationships to potential ET and the statistically weak prediction of surface conductance indicate poor physiological control of soybean ET or inadequate parametrization.