Two-dimensional solar radiation interception model for hedgerow fruit trees

A two-dimensional, hourly or daily time step model was developed, which takes canopy characteristics and row orientation into account to simulate solar radiation interception in hedgerow orchards. In order to determine the spatial and temporal distribution of soil irradiance across the tree row, the canopy path length through which the radiation must travel to reach a certain point on the soil surface is calculated. The model assumes leaves to be uniformly distributed within an ellipsoid, and radiation penetrating the canopy is attenuated according to Beer’s law. Beam or direct radiation and diffuse radiation for the PAR (photosynthetically active radiation) and NIR (near-infrared radiation) wavebands are calculated separately, as they interact differently with the canopy. The attenuation of beam radiation by the canopy is strongly dependent on canopy dimensions and architecture, zenith and azimuth angle, as well as row orientation. Radiation can penetrate neighbouring rows, so two rows on either side of the simulated row are considered. Validation of the model was carried out for a wide range of conditions (crops, row orientation, canopy density, tree size and shape). Field measurements included solar radiation, soil irradiance at different distances from the tree row with tube solarimeters, leaf area density, as well as canopy size and row orientation. Model predictions of soil irradiance were excellent in orchards with symmetrical and elliptical canopies having a uniform leaf distribution. In orchards where the canopy was non-symmetric and/or had non-uniform leaf distribution, errors in predictions of solar radiation transmittance occurred. As a result of these discrepancies, the overall MAE was 40% of the average measured value of radiant transmittance over the whole day.