Shortwave radiation transfer through a plant canopy covered by single and double layers of plastic

Highlights

• A model to predict radiation transfer of plastic covered canopies is proposed.

• Plastic transmittance, reflectance and haze are related to wetness, age and angle.

• An upscaling procedure from plane to rough plastic surfaces is derived.

• Kohlrabi canopies exhibit overdispersion at early vegetative stages.

• Simulations are most sensitive to angular plastic properties and canopy processes.

Abstract

A model that predicts radiation transfer through single and double layers of plastic covering over kohlrabi canopies is developed, parameterised and tested. This model will be the foundation of an energy balance and growth module for covered kohlrabi crops that can be used in cover management. Radiation transfer through covers is based on their laboratory-measured angular-resolved transmittances, which are upscaled to non-plane covers in the field. The upscaling procedure accounts for distributed facet slopes according to the Beckmann distribution and visibility, as well as interception preference according to the cosine of the facet-ray incidence angle. Additional measured and upscaled quantities include absorptance and the degree of haze at several angles. The effects of plastic ageing and wetting are measured and implemented into the model using simple empirical approaches. Radiation transfer through the canopy is described by a thoroughly tested 1D canopy model, which accounts effectively for multiple reflections between leaves and the soil. A reanalysis of combined gap fraction and leaf area data from a previous study revealed a tendency of kohlrabi canopies to overdisperse at early growth stages, when only minor leaf area overlapping occurs. Using hourly measurements of photosynthetic active radiation flux densities at the soil level over two growth seasons at one site, the overall model performed reasonably well for a non-woven fabric-based cover (n = 1067, R² = 0.96, RMSE = 6.62 W m⁻²) and a combination of a low-density polyethylene perforated plastic on top of a non-woven fabric (n = 1112, R² = 0.97, RMSE = 5.11 W m⁻²). Simulations showed rather low degree of model sensitivity to the specification of cover roughness, but a high level of sensitivity to a proper parameterisation of angular optical properties of covers and of canopy radiation transfer in the NIR spectral range.