Evapotranspiration Model for Developing Crops

Crop water use is a complex function of climatic conditions, stage of crop development and soil water content. Using principles of heat and mass balance, a comprehensive evapotranspiration model was developed to predict water use, soil evaporation, transpiration and microclimate of a crop where the soil may go through cycles ofwetting and drying. This model accounts for the sensible and latent energy exchanges and feedback effects within the canopy air space, and among the canopy, soil, and ambient air above the canopy. Using an electrical analog, equations for energy fluxes in the soil-plant-atmosphere continuum were developed. Solutions were developed that allow practical applications and evaluation of the model using measured weather, plant, and soil data as inputs. The model describes the influence of soil evaporation on plant transpiration and the changes in plant temperature under changing vapor pressure deficit within the canopy. Bragg soybeans were grown in 330, 660, and 990 ppm CO2 concentrations under well irrigated conditions in outdoor growth chambers to obtain a data base to evaluate the ability of the model to predict water use during canopy development. The model accurately predicted diurnal water use patterns as well as the reduction in canopy ET under the higher CO2 levels. Average daily differences between measured and simulated evapotranspiration were less than 4% and these differences were not significant at the 95% level of confidence..