Validation of a three-dimensional solar radiation interception model for tree crops

Solar irradiance is fundamental in estimating evapotranspiration and dry matter production from agricultural fields. In general, testing of radiation interception models is limited by the number of measurement points both in space and time and the few crop species used. We considered many nodes (points of measurement), more than in any other published work, more data points (days of measurements), and numerous tree crop species. A three-dimensional, hourly time-step tree-canopy radiation interception model is validated for different tree crops including Jatropha curcas (Jatropha), Leucaena leucocephela (Leucaena), Macadamia integrifolia (Macadamia) and Acacia mearnsii (Black Wattle). Planting patterns varied from isolated trees to hedgerows to trees arranged in tramline mode. Measurements were made using tube solarimeters 0.5 m apart from the base of a tree trunk in four directions, along and perpendicular to the row to mid-way between trees and rows. For validation, tube solarimeters ranging from five to 22 in number were used for each experiment. Model validation was for each node using a range of input parameters. Model estimations of radiant transmittance were in agreement with measurements: overall coefficient of determination 0.91; index of agreement 0.96; general absolute standard deviation 17.66%; slope = 0.97; intercept = 18.10 W m−2. Agreement was influenced by distance and direction of the node from the tree trunk, sky conditions, canopy symmetry, stand uniformity and leaf distribution.