ABSTRACT
The characteristics of the incident radiation above the canopy are very important in determining the radiant energy available for interception by individual leaves. Considerable information is available on this topic in the book by Kondratyev, 12 the reviews by Davies and Idsou and Norman,' and the discussions by Monteith5 and Ross.• Approximately one half of the solar radiation is in the visible and thus contributes to photosynthesis; most of the remaining radiation is in the near-IR, with only a few percent in the ultraviolet. Considering the relatively unpredictable effects that clouds, haze, water vapor, pollution, etc. have on visible and solar radiation, it is desirable to measure beam and diffuse radiation in the visible and solar wavelengths separately throughout the day. It is particularly important to measure visible radiation separately from solar radiation because many plant responses are specific to visible wavelengths. For careful radiation work, either the thermal sky radiation or net thermal radiation should be measured; however, it is more easily predicted than solar radiation. 13 For use in radiation models where beam and diffuse radiation are attenuated differently, the sun angle is calculated from astronomical equations, 14 and the diffuse angular distribution is assumed to be isotropic.
