Evapotranspiration from Soybean and Sorghum Fileds1

Abstract The evapotranspiration model, presented by Monteith (1965) and later tested by Black et al. (1970) on a loosely structured snap bean (Phaseolus vulgaris) canopy, was used to separate soil evaporation and transpiration on soybean Glycine max (L.) Calland and sorghum Sorghum bicolor (L.) Pioneer #846 canopies during the growing season. Soil evaporation was estimated as the net radiation below the crop canopy minus the soil heat flux. Stomatal resistance was determined with the diffusion porometer (Kanemasu et al. 1969). Potential evapotranspiration, soil evaporation, and transpiration rates were determined on an hourly basis. The sum of transpiration and soil evaporation was compared to the lysimetric estimates of evapotranspiration. The model and the lysimeters were in agreement, except under conditions of high atmospheric demand when the model underestimated actual evapotranspiration. The model showed the proportion of water lost as transpiration was closely correlated to leaf area index (LAI) with transpiration at approximately 50% of the total evapotranspiration at a LAI of 2 and as much as 95% at a LAI of 4.