Availability of Solar Photosynthetically Active Radiation

Abstract

ABSTRACT THE ability to predict the amount of photosyntheti-cally active radiation (PAR) from the sun for field crops or within a particular greenhouse design is useful. PAR is a portion (0.4-0.7 micrometer wavelength) of the total radiation spectrum directly related to plant photosynthetic productivity. While total solar radiation data are readily available for many locations around the world, the published information on PAR is lacking. Experiments have been conducted to determine the correlations between the available hourly and daily PAR and total solar radiation. PAR was measured in the photon flux density unit of mol s^ m^ and total radiation was in the radiant flux density of W m^. Regression equations are presented in this paper for two purposes: (a) to show the predictability of PAR using available total solar radiation data, and (b) to provide empirical equations for estimating direct and diffuse PAR based on total radiation values.