Radiation fluxes and canopy transmittance: Models and measurements inside a willow canopy

Upward and downward fluxes of photosynthetically active and broadband solar radiation, and the transmittance of direct solar radiation measured at different heights inside a willow (Salix viminalis) canopy are compared with modeling results. Two approaches are used for modeling the radiation field inside the canopy: solving the radiative transfer equation using the exact kernel discrete ordinates method and a Monte Carlo model. The measured and modeled canopies include a 1-year-old willow coppice with leaf area index (LAI) = 5 and a 4-year-old coppice with LAI = 8. Biometrical data used in the models are taken from detailed measurements of canopy structure; measured fluxes are used as boundary conditions. An attempt is made to include the effect of clumping on the penetration of direct solar radiation into the radiative transfer model. All models agreed well with measurement results; enhancement of modeled fluxes due to inclusion of clumping depended on the height inside the canopy while the distribution of leaf inclination angle had a minimal effect on the radiation regime of the stand.