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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

View larger version (238K): [in a new window]   Fig. 1. Hemispherical photograph of a measurement site in the (left) orchard and (right) maize canopy. The centers of the photographs represent the zenith. Distance from the center toward the edge is linearly related to the zenith angle.

View larger version (13K): [in a new window]   Fig. 2. Variability in ultraviolet (UV)-B canopy transmittance (Tcanopy) for different measurement locations in the maize canopy. A represents the probability distribution of individual Tcanopy measurements over the course of a measurement period for a location without sunflecks while B and C represent the probability distributions for individual Tcanopy measurements at locations with sunflecks. The intervals of Tcanopy were 0.001 for A and 0.01 for B and C.

View larger version (15K): [in a new window]   Fig. 3. Accuracy of three-dimensional (3-D) ultraviolet radiation transfer (UVRT) canopy transmittance (Tcanopy) model. The simulated Tcanopy values for the orchard (open circles) and maize canopy (open squares) are indicated. The solid line has a slope of 1, and the dotted line is a linear regression of simulated Tcanopy on measured Tcanopy.

View larger version (16K): [in a new window]   Fig. 4. Errors of simulated to measured canopy transmittance (Tcanopy) with solar zenith angle in orchard measurement area.

View larger version (15K): [in a new window]   Fig. 5. Comparison of simulated canopy transmittance (Tcanopy) with different sky radiance distribution and measured Tcanopy with solar zenith angle in sunlit locations in the orchard canopy. The simulated Tcanopy, assuming an isotropic (open triangles) and anisotropic (open circles) sky and measured Tcanopy (open squares).

View larger version (13K): [in a new window]   Fig. 6. Effect of sky radiance assumptions on the distribution of sky radiance along the vertical plane between the sensor and the sun. The effect of sky obstruction on simulated isotropic and anisotropic irradiance differences at sunlit locations (sky zenith angles away from the sun location), where the isotropic sky radiance is greater than the anisotropic sky radiance, is opposite that of sky obstruction at shaded locations (obstruction of the solar disk) where the anisotropic sky radiance exceeds the isotropic radiance. For this example, the sun is located at 30° zenith angle. All values of sky radiance have been normalized to an isotropic sky radiance values so that the value of 1 corresponds to the radiance of the isotropic sky (dashed line).

View larger version (15K): [in a new window]   Fig. 7. Difference in simulated canopy transmittance (Tcanopy) between assumed isotropic and anisotropic sky radiance distribution with sky view fraction. The sunlit and shaded measurement locations are indicated by the open squares and open circles, respectively.