Effects of cover diffusive properties on the components of greenhouse solar radiation

This study deals with the characterization and analysis of the components of solar radiation under a highly diffusing cover material. Measurements of outside and inside global (Go and Gi, respectively) and diffuse (Do and Di, respectively) radiation were carried out over the course of several years (2000–2004) under a plastic greenhouse. It was shown that the cover diffusive properties can be characterized in situ through sound physically-based parameters. We suggested the use of two specific greenhouse transmittances (direct-to-direct, τb-b, and direct-to-diffuse, τb-d, transmittances), or alternatively, the use of greenhouse diffuse ratio (ρ = Di/Do) and its derivate, the enrichment coefficient (ɛ). The transmittances were primarily dependent on the diffusive properties of the cover, with a seasonal modulation due to the influence of the beam incidence angle. We found that τb-d was significantly less sensitive to the incidence angle than τb-b. Our results also evidenced that extrinsic factors, such as condensation events and dust deposition, might have significant non-permanent effects on both τb-b and τb-d during some periods of the year. Finally, we proposed a straightforward means to obtain a quantitative estimate of the amount of inside diffuse radiation through the relationship (power function) between ρ and the outside diffuse-to-global fraction, fo. Importantly, the value of the exponent of the power function could be used to characterize and quantify the diffusive power of greenhouse cover materials. Overall, the results stressed the significant impact of the cover diffusive properties on inside solar radiation partitioning, as well as the need to account for these changes in models aimed to estimate the direct and diffuse components, canopy radiation interception and yield in greenhouse agrosystems.