Modeling of long-wave and net radiation energy distribution within a homogeneous plant canopy via multiple scattering processes

摘要

This paper presents a newly developed multiple-layer, long-wave radiation scattering model for use in homogeneous vegetation canopies. The model is able to simulate the radiation distribution within and the outgoing radiation above the canopy. This model differs from the shortwave model developed earlier by the authors owing to the complexity introduced by the fact that leaves within and soil below the canopy emit long-wave radiation in accordance with their surface temperature. Combined, the short- and long-wave models are able to simulate net radiation distribution above and within the canopy sublayers and at the soil surface. The model represents multiple scatterings of radiation reflected, transmitted, and emitted from leaf surfaces and penetrating through gaps as infinite series equations, which are reduced analytically to simple forms. In stand-alone mode this model has the limitation that it requires canopy temperature profile data as input in order to simulate outgoing long-wave radiation and net radiation. However, once we or other users couple this model to a turbulent transfer model, canopy temperature profiles will be produced as model output, making this a useful tool for remote sensing data assimilation. The model was tested against measurements collected in a wheat field in 2002. Satisfactory agreement was obtained between the modeled outgoing long-wave radiation above a wheat canopy and observed long-wave radiation measured with an Eppley precision infrared radiometer (PIR), both for daily total values and diurnal variation of 20-min averages. The root-mean-square error (RMSE) of daily total values of outgoing long-wave radiation, with respect to measurements, was only 1.1% of the mean of the measurements. Comparison of the modeled net radiation with two independent measurements produced RMSE values equal to 3.7% of the mean measured daily total net radiation.