Estimating the photosynthetically active radiation under clear skies by means of a new approach

Abstract. The k-distribution method and the correlated-k approximation of Kato et al. (1999) is a computationally efficient approach originally designed for calculations of the broadband solar radiation by dividing the solar spectrum in 32 specific spectral bands from 240 to 4606 nm. This paper describes a technique for an accurate assessment of the photosynthetically active radiation (PAR) from 400 to 700 nm at ground level, under clear-sky conditions using twelve of these spectral bands. It is validated against detailed spectral calculations of the PAR made by the radiative transfer model libRadtran. For the direct and global PAR irradiance, the bias is −0.4 W m⁻² (−0.2%) and −4 W m⁻² (−1.3%) and the root mean square error is 1.8 W m⁻² (0.7%) and 4.5 W m⁻² (1.5%). For the direct and global Photosynthetic Photon Flux Density, the biases are of about +10.3 μmol m⁻² s⁻¹ (+0.8%) and 1.9 μmol m⁻² s⁻¹ (−0.1%) respectively, and the root mean square error is 11.4 μmol m⁻² s⁻¹ (0.9%) and 4.0 μmol m⁻² s⁻¹ (0.3%). The correlation coefficient is greater than 0.99. This technique provides much better results than two state-of-the-art empirical methods computing the daily mean of PAR from the daily mean of broadband irradiance.