Ultraviolet irradiance of inclined planes at the top of plant canopies

The potential increase in ultraviolet-B (UV-B) irradiance and potential decrease in productivity of agricultural crops due to stratospheric ozone loss requires knowledge of the characteristics of UV irradiance above and within crops. Measurements of UV irradiance at the top of two crops were made during the growing seasons of 1990 and 1491. Maximum levels of irradiance relative to the horizontal (I-s) did not occur at slopes equal to the solar elevation angle, but typically occurred at slopes closer to the horizontal due to the high diffuse fraction in the UV. In general, I-s for the UV tends to be smaller than that for the total short wave solar radiation (SW) as a result of the greater diffuse fraction in the UV over that for the SW. Results also showed that the UV I-s over the maize and winter wheat canopies are similar. The measured I-s was compared against inclined plane I-s models incorporating either an isotropic or anisotropic sky radiance model. The anisotropic sky model was more accurate than the isotropic model for predicting the measured I-s for planes inclined at any angle. The isotropic model was, however, found adequate to describe I-s for azimuthally-invariate distributions of inclined planes typical of many canopy radiation models. Corrections for the anisotropy of the sky radiation were developed to be applied to the diffuse sky radiation term in the isotropic model to estimate the relative irradiance for specific azimuths. Using the anisotropy correction factors in a simple analytic model of irradiance improved the isotropic I-s estimates by 7% (accounting for 97% of the measured I-s variance). A set of functions were developed to provide analytic solutions for the anisotropy factor. The irradiance model can be used to predict the influence of orientation of plant, animal, or human surfaces on the received global UV-B irradiance above most plant canopies under clear skies. (C) 1998 Published by Elsevier Science B.V.