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A modified surface energy balance (SEB) model based on the Shuttleworth-Wallace and Choudhury-Monteith methods was developed to estimate evaporation from soil covered by crop residue, and transpiration from crop canopies. The model describes the energy balance and flux resistances for partially-vegetated and residue-covered surfaces. Physical and biochemical energy storage terms and lateral fluxes are neglected in the model. Net radiation is one of the inputs in the SEB model and provides the energy needed for soil evaporation, crop transpiration and heat transfer through the canopy, soil/residue surfaces and the atmosphere.
A sensitivity analysis of the SEB model parameters showed that simulated evapotranspiration was most sensitive to changes in canopy surface resistance, soil surface resistance, and residue surface resistance. Comparisons between estimated ET and measurements from three eddy covariance systems located in soybean and maize fields provided support for the validity of the model. The SEB model accurately simulated hourly and annual amounts of evapotranspiration during periods with a wide range of crop canopy cover.
As in the Penman Monteith (P-M) approach, canopy surface resistance can be back-calculated with the SEB model if latent heat fluxes and other environmental variables are measured. Since the SEB model has the ability to separate ET into canopy transpiration and soil evaporation, canopy surface resistance estimated with the SEB model should be less affected by soil evaporation than with the P-M approach. The SEB model and the P-M model were used to estimate canopy surface resistance for maize under irrigated and rainfed conditions. Canopy resistance estimated with the P-M and the SEB models followed the same pattern during the growing season and during the day but with different magnitudes. As was expected, canopy resistance estimated with the SEB model was higher than calculated with the P-M equation. Differences were more important under low leaf area index conditions. Results suggest that soil evaporation considerably affects the canopy surface resistance obtained with the P-M equation.