Radiation Model for Row Crops: I. Geometric View Factors and Parameter Optimization

Row crops with partial cover result in different radiation partitioning to the soil and canopy compared with full cover; however, methods to account for partial cover have not been adequately investigated. The objectives of this study were to: (i) develop geometric view factors to account for the spatial distribution of row crop vegetation; (ii) combine view factors with a widely used vegetation radiation balance model; and (iii) optimize three parameters required by the model that describe leaf angle, visible leaf absorption, and near-infrared leaf absorption. Measurements of transmitted and reflected shortwave irradiance for corn (Zea mays L.), grain sorghum [Sorghum bicolor (L.) Moench], and cotton (Gossypium hirsutum L.) were used to optimize parameters and evaluate the model. View factors were derived by modeling the crop rows as elliptical hedgerows. The optimized ellipsoid leaf angle parameter, visible leaf absorption, and near-infrared leaf absorption were 1.0, 0.85, and 0.20 for corn; 1.5, 0.82, and 0.20 for grain sorghum; and 3.0, 0.83, and 0.14 for cotton, respectively. Visible leaf absorption was similar for all crops. Near-infrared leaf absorption was the same for corn and grain sorghum but less for cotton. The only parameter that changed for each crop species was leaf angle. The optimized parameters for corn and grain sorghum were within the range of values recommended in previous studies, and the leaf angle parameter for cotton agreed with a previous study of cotton leaf angles. All parameters were distinctly identifiable, and no parameter correlation was observed.