AN EVALUATION OF THE SOLAR RADIANT ENVIRONMENT IN THE SHADE OF DECIDUOUS TREES
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abstract
A method of estimating solar radiant energy incident (irradiance) in the shade of deciduous trees using photographs was tested against measured values. Photographs of trees were computer digitized to determine optical porosity of crowns. Simultaneous measurements of solar irradiance under the test trees and in the open were recorded. The study compared the optical porosity of a tree crown to the percentage of ambient solar irradiance under the tree. The photographic method used in the study underestimated the solar irradiance in both winter and summer. This appears to be due to differences in the measurement instruments used. Photographs record the visible portion of the solar spectrum coming from one direction (i.e. towards the camera). Pyranometers record virtually the complete solar spectrum from the whole atmospheric hemisphere. The underestimation in winter canopies could be accounted for by considering the diffuse solar radiation coming from the atmospheric hemisphere outside the photograph (called the Unphotographed Diffuse Radiation or UDR in this study). The underestimation in summer canopies could be explained by including both the UDR and the Near Infrared Radiation (NIR) transmitted through the leaves (as distinct from through openings in the canopy). Neither the UDR nor the NIR is recorded in a normal photograph but is sensed by Pyranometers (and elements in the landscape) as radiant energy. Landscape architects must consider radiant energy rather than light in virtually every application in the design and analysis of outdoor environments. Taylor & Francis Group, LLC.
