A simplified approach for predicting shortwave radiation transfer through boreal forest canopies

Transmission of shortwave radiation through forest canopies is the primary determinant of the energy inputs to the understory or the ground surface. In the context of land surface change studies and distributed hydrological modeling, there is a need for models that represent the radiative transfer in a simple yet realistic manner, only employing input parameters that are routinely measured. A simple canopy radiation model is presented, which simulates the below-canopy shortwave radiation as a function of measured above-canopy radiation. The model uses a Beer's law approach and accounts separately for the transmission of diffuse and direct shortwave radiation. The model accounts for multiple scattering in the canopy in the near-infrared as well as the effects of multiple reflections between the understory and the overstory. The only required input parameter is the effective leaf area index, although additional information about canopy structural and optical properties can easily be included where available. The model was tested using above- and below-canopy radiation measurements at a mature jack pine site and a black spruce site in the BOREAS southern study area near Prince Albert, Saskatchewan. Without calibration the model performed well at the mature jack pine site. However, at the mature black spruce site, the below-canopy radiation was overpredicted, although some of the apparent overprediction appeared to be due to sampling error.