HPEval: A canopy shortwave radiation transmission model using high-resolution hemispherical images

Abstract

Forested environments feature a highly complex radiation regime. The three-dimensional canopy structure masks solar radiation as it penetrates through the forest. This entails sub-canopy radiation to vary in space, over time, season, and with weather conditions, which challenges both measurements and modelling. Here we combine the experimental acquisition of forest structure information with radiation transfer modelling to achieve efficient and accurate estimation of sub-canopy shortwave radiation at very high temporal resolution. For this purpose we have developed a Matlab-based software tool to analyse digital hemispherical imagery for directional canopy transmissivity which yields, in combination with measured or estimated above-canopy shortwave radiation, respective below-canopy radiation. We demonstrate the utility of the tool to accurately model shortwave radiation in comparison to pyranometer measurements at both the point scale and along a transect under heterogeneous canopy cover. Optimized for computational efficiency the software even enables fully-distributed simulations. Without necessitating high performance computing or complex ray-tracing, this tool enables easy access to detailed sub-canopy radiation information. These should be valuable for validating site-scale radiation transfer models, as input to eco-hydrological and snowmelt models, and for researching light-dependant bio-physiological processes.