Diffuse PAR irradiance under clear skies in complex alpine terrain

The diffuse component of photosynthetically active radiation (PAR) is important for models that calculate carbon dioxide uptake in photosynthesis by vegetation. Few PAR measurements are usually available, especially in complex terrain. Thus, extrapolation based on models becomes the primary means for deriving diffuse PAR in complex terrain. Most models treat the diffuse PAR sky distribution isotropically to avoid complex calculations. However, this leads to errors in diffuse PAR estimation due to the anisotropic nature of PAR sky radiance distribution. This paper describes a model for estimating diffuse PAR at any given point within complex terrain from measurements at a reference site, by taking into account both the anisotropic nature of sky radiance distribution and topographic effects. The model has been validated using two different sites, Kederbichl, and

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