Separating the diffuse and direct component of global radiation and its implications for modeling canopy photosynthesis Part I. Components of incoming radiation

In modeling canopy photosynthesis, it is important to discriminate between the direct and diffuse components of incoming, global radiation. An equation is presented to estimate the share of both components from the measured daily global irradiance only. In this equation the share of the diffuse component is related to the ratio between global and extra-terrestrial radiation. This relation is based on a summary of literature data and of radiation measurements in The Netherlands. The diurnal trends of global, direct and diffuse radiation were derived from a sinusoid with a correction depending on solar angle. The random variation around this sine wave is characterized. For clear skies about 15% of the diffuse flux comes predominantly from directions near the sun and this circumsolar component has to be added to the direct flux. On the other hand, for clear skies the diffuse fraction in the photosynthetically active wave bands is about 40% larger than that for the total global radiation. In the past, the partitioning between direct and diffuse radiation was tackled by assuming that short periods of either fully clear or overcast conditions alternate within the day. That approach severely underestimated the share of the diffuse component in the total global radiation. The method presented in this paper is particularly useful for application in crop growth models.