Using the Penman-Monteith approach to extrapolate soybean evapotranspiration

Summary

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 (r ² = 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. ActualET was 70% and 80% of potentialET in 1987 and 1991, respectively. These similar relationships to potentialET and the statistically weak prediction of surface conductance indicate poor physiological control of soybeanET or inadequate parametrization.