Transmission of short-wave radiation by agricultural screens

Transmittance of short wave radiation components by seven different agricultural screens made of white, off-white, or clear threads was investigated. Theoretical models of non-scattered transmittance of direct and diffuse radiation based on screen dimensions and solidity were developed. Transmission of photosynthetic, global, and near infra-red (NIR) radiation components by the screens was measured outdoors using an array of sensors and 3 × 3 m screens mounted above a roof. Free Open Area (F, the complement of solidity) of the screens, determined from transmittance of direct radiation, ranged from 36 to 74%. F from analyses of screen geometry or scanner images differed from that by less than 13%. Direct transmittance declined with solar elevation angle and became zero below a cutoff angle depending on inter-thread spacing and thread width. Although transmissivity of direct and diffuse radiation was predicted adequately by the models, significant downward scattering of direct radiation by the screens increased transmittance to total radiation, so models underestimated actual transmittance. Empirical trigonometric equations fitted results well, and can be used for prediction. Orientation of screens with rectangular holes has a significant influence on transmission, with clear implications for screen deployment above agricultural crops. Measurements in a screenhouse covering a banana plantation showed that transmission decreased linearly with time by about 0.1% day−1 during the rainless summer due to dust accumulation on the screen, but recovered after rain. The implications of the findings for agriculture are discussed.