ULTRAVIOLET IRRADIANCE OF INCLINED PLANES AT THE TOP OF PLANT CANOPIES

The potential increase in ultraviolet-B (UV-B) irradiance and potentia l 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 tw o crops were made during the growing seasons of 1990 and 1491. Maximum levels of irradiance relative to the horizontal (I-s) did not occur a t slopes equal to the solar elevation angle, but typically occurred at slopes closer to the horizontal due to the high diffuse fraction in t he UV. In general, I-s for the UV tends to be smaller than that for th e total short wave solar radiation (SW) as a result of the greater dif fuse 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 incorporat ing either an isotropic or anisotropic sky radiance model. The anisotr opic sky model was more accurate than the isotropic model for predicti ng the measured I-s for planes inclined at any angle. The isotropic mo del was, however, found adequate to describe I-s for azimuthally-invar iate distributions of inclined planes typical of many canopy radiation models. Corrections for the anisotropy of the sky radiation were deve loped to be applied to the diffuse sky radiation term in the isotropic model to estimate the relative irradiance for specific azimuths. Usin g the anisotropy correction factors in a simple analytic model of irra diance improved the isotropic I-s estimates by 7% (accounting for 97% of the measured I-s variance). A set of functions were developed to pr ovide analytic solutions for the anisotropy factor. The irradiance mod el can be used to predict the influence of orientation of plant, anima l, or human surfaces on the received global UV-B irradiance above most plant canopies under clear skies. (C) 1998 Published by Elsevier Scie nce B.V.