Responses of plastic shading nets to global and diffuse PAR transfer: Optical properties and evaluation

To cover agricultural structures for protecting plants from high solar radiation in hot and sunny regions different types of shading nets are used. In summer, the amount of photosynthetically active radiation (PAR) transmitted through these nets usually fulfils plant growth requirements. However, their applicability in winter is still unknown. So far, the choice of nets has mostly been empirical, because technical data such as the radiometric properties that are used to characterize the different types of nets are not available. This paper presents a simple method for measuring the radiometric properties related to global and diffuse PAR of seven types of shading nets including nets with colours and shading rates that are commonly used in hot regions. A shading system was developed to simulate the diffuse PAR under cloudy conditions. The results show that under clear sunny and cloudy conditions the radiometric properties depended on both net porosity and colour. Net reflectance strongly depended on net colour. Net transmittance and absorptance depended primarily on colour and secondarily on porosity. The radiometric properties of nets under global and diffuse PAR conditions tended to be similar to those of translucent materials so that a plastic net can be considered as translucent material when investigating the equivalent optical parameters (refractive index and absorption coefficient). This can help to predict the radiometric properties of the net without the need of measurements. Under the sunny conditions of winter seasons, the structures covered with the tested nets are suitable for growing crops that can grow at PAR less than 150 W m−2. However, under cloudy conditions very few plants will grow that can grow at PAR less than 30 W m−2.