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MODELING

A Geometric Ultraviolet-B Radiation Transfer Model Applied to Vegetation Canopies

^a USDA UV-B Radiation Monitoring and Res. Progr., Coop. Inst. for Res. in the Atmos., Fort Collins, CO 80523
^b USDA UV-B Radiation Monitoring and Research Progr., Nat. Resour. Ecol. Lab., Colorado State Univ., Fort Collins, CO 80523
^c Dep. of Agron., Purdue Univ., West Lafayette, IN 47906
^d Northeastern Res. Stn., USDA Forest Serv., Syracuse, NY 13210

* Corresponding author (wgao{at}uvb.nrel.colostate.edu)

Received for publication August 21, 2000. The decrease in stratospheric ozone (O₃) has prompted continued efforts to assess the potential damage to plant and animal life due to enhanced levels of solar ultraviolet (UV)-B (280–320 nm) radiation. The objective of this study was to develop and evaluate an analytical model to simulate the UV-B irradiance loading on horizontal below-canopy surfaces, as influenced by vegetation. The UV-B irradiance above canopy and transmitted to below-canopy points was measured in a widely spaced orchard and in a closely spaced maize (Zea mays L.) crop during cloud-free days, with solar zenith angle ranging from 20° to 80°. The sky view fraction was typically 0.59 for the orchard and 0.28 for the maize canopy. Transmitted irradiance fractions were simulated and compared to measured fractions. Measured and simulated values of UV-B canopy transmittance generally agreed well both for points in locations shaded by plant crowns and for points below the top of the canopy that were not shaded. The model had mean bias errors of 0.04 and 0.03 for the orchard and maize canopies respectively, and the root mean squared error of the model was 0.08 for orchard and 0.06 for maize. The model can serve as a much-needed tool to examine UV-B irradiance loading of organisms below tree canopies and of sensitive plant surfaces in and below tree and vegetation canopies.

Abbreviations: 1-D, one dimensional • 3-D, three dimensional • MBE, mean bias error • RMSE, root mean squared error • T_canopy, canopy transmittance • UV, ultraviolet • UVRT, ultraviolet radiation transfer

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