Technical Note: Simulating the Surface Energy Balance in a Soybean Canopy with the SHAW and RZ-SHAW Models

Abstract

Correct simulation of surface energy balance in a crop canopy is critical for better understanding of soil water balance, canopy and soil temperature, plant water stress, and plant growth. One existing effort is to incorporate the surface energy balance in the Simultaneous Heat and Water (SHAW) model into the Root Zone Water Quality Model (RZWQM). In this study, an improved version of the RZ-SHAW (RZWQM-SHAW) hybrid model was tested for energy balance components, canopy and soil temperature, evapotranspiration (ET), and soil water content against eddy covariance data measured in a soybean canopy and against predictions of the original SHAW and RZWQM models. The experiment was first used previously to test the SHAW model for radiation energy fluxes within the canopy without examining the energy balance components, soil water balance, and soil temperature. The same parameters from that study were used in both the SHAW model and RZ-SHAW hybrid model without any modification in this study. In terms of root mean squared error (RMSE), both RZ-SHAW and SHAW simulated net radiation, sensible heat, and latent heat well. However, the ground heat flux simulated by RZ-SHAW was less accurate, with RMSE of 28.9 W m-2 compared to 22.6 W m-2 with SHAW, which could be due to differences in simulated soil evaporation. Simulated soil temperature at both 1.5 cm and 4.5 cm depths with RZ-SHAW was comparable to that of SHAW, with RMSE of 2.18C and 2.23C, respectively, compared to 2.13C and 2.20C with SHAW. Similarly, simulated canopy temperature was essentially the same, with RMSE values of 1.77C with RZ-SHAW and 1.69C with SHAW. Simulated surface soil water content was reasonable for both models. Simulated ET had an RMSE of 0.069 cm d-1 with RZ-SHAW and 0.074 cm d-1 with SHAW. The new RZ-SHAW model was an improvement over the original RZWQM model in simulating soil temperature and moisture, in addition to its ability to provide complete energy balance and canopy temperature.