MEASURING RADIOMETRIC PROPERTIES OF SCREENS USED AS CROP COVERS

This paper reviews our work on measuring and modeling radiometric properties of agricultural screens. Methodology for determining radiometric properties in the photosynthetic, total solar, near infra-red (NIR) and far infra-red wavebands is described. Outdoor measurements were made using arrays of sensors and 3×3 m screens mounted above a roof. Theoretical models of non-scattered transmittance of direct and diffuse radiation based on screen dimensions and solidity were developed. Free Open Area of the screens was determined from the radiation balances, models, and scanner images. Differences between scanner analyses and transmittance of direct radiation were up to 15% and averaged 8%. Although transmissivity of direct and diffuse radiation can be predicted, significant downward scattering of direct radiation by the screens increases transmittance to total radiation, so models underestimate actual transmittance. Screen orientation can have a significant influence on transmission, with clear implications for screen deployment above agricultural crops. And since screens do not transmit radiation at low angles, sloped roofs can increase transmittance in the morning and evening. The need for better characterization of screen radiometric properties and standardization is discussed.