Development of a semi-parametric PAR (Photosynthetically Active Radiation) partitioning model for the United States, version 1.0

A semi-parametric PAR diffuse radiation model was developed using commonly measured climatic variables from 108 site-years of data from 17 AmeriFlux sites. The model has a logistic form and improves upon previous efforts using a larger data set and physically viable climate variables as predictors, including relative humidity, clearness index, surface albedo and solar elevation angle. Model performance was evaluated by comparison with a simple cubic polynomial model developed for the PAR spectral range. The logistic model outperformed the polynomial model with an improved coefficient of determination and slope relative to measured data (logistic: R-2 = 0.76; slope = 0.76; cubic: R-2 = 0.73; slope = 0.72), making this the most robust PAR-partitioning model for the United States currently available.