Modelling and measurement of radiation interception by olive canopies

We present the formulation, calibration and validation of a model to estimate photosynthetically active radiation (PAR) intercepted by olive (Olea europaea L,) canopies, The model calculates the PAR transmittance at any point located within the four central trees of the orchard. The spatial and time integration of this process allows calculation of PAR transmitted to the ground and, thus, the PAR intercepted at any given time. Direct-beam radiation, diffuse radiation and scattering are separately taken into account. Model parameters are the G-function, leaf reflectance and transmittance. Leaf inclination distributions, measured in the field, were used to derive the G-function. A planophile distribution was found. Leaf reflectances measured in three olive cultivars were, on average, 0.06 for adaxial and 0.12 for abaxial surfaces, while leaf transmittance was below 0.01. The model was barely sensitive to the parameters. Predictions of diffuse and total transmittance were tested on clear and overcast days in winter, fall and summer at Cordoba, Spain, in seven olive orchards of quite different characteristics. The validation showed that when estimating average canopy values, local errors compensate and the predictions of intercepted PAR seem correct for all canopy types, sun position and heights within the stand. The procedure seems applicable to any olive tree variety and does not present important systematic errors. (C) 2000 Elsevier Science B.V. All rights reserved.