Simulation of within-canopy radiation exchange

Radiation exchange at the surface plays a critical role in the surface energy balance, plant microclimate, and plant growth. The ability to simulate the surface energy balance and the microclimate within the plant canopy is contingent upon simulation of the surface radiation exchange. A validation and modification exercise of the Simultaneous Heat and Water (SHAW) model was conducted for simulating the surface short-wave and long-wave radiation exchange over and within wheat, maize and soya bean plant canopies using data collected at Yucheng in the North China Plain and near Ames, Iowa. Whereas model testing was limited to monocultures and mixed canopies of green and senesced leaves, methodologies were developed for simulating short-wave and long-wave radiation fluxes applicable to a multi-species, multi-layer plant canopy. Although the original SHAW model slightly underpredicted reflected solar radiation with a mean bias error (MBE) of −5 to −10 W m−2, one would conclude that the simulations were quite reasonable if within-canopy measurements were not available. However, within-canopy short-wave radiation was considerably underestimated (MBE of approximately −20 W m−2) by the original SHAW model. Additionally, leaf temperatures tended to be overpredicted (MBE = +0.76 °C) near the top of the canopy and underpredicted near the bottom (MBE = −1.12 °C). Modification to the SHAW model reduced MBE of above canopy reflected radiation to −1 to −6 W m−2 and within-canopy radiation simulations to approximately −6 W m−2; bias in leaf temperature was reduced to less than 0.4 °C. Model modifications resulted in essentially no change in simulated evapotranspiration for wheat, 4.5% lower for maize and 1% higher for soya bean. Alternative approaches for simulating canopy transmissivity to diffuse radiation were tested in the modified version and had a minor influence on simulated short-wave radiation, but made almost no difference in simulated long-wave radiation or evapotranspiration. Modifications to the model should lead to more accurate plant microclimate simulation; further work is needed to evaluate their influence.