SIMULATION OF CITRUS ORCHARD REFLECTANCE BY MEANS OF A GEOMETRICAL CANOPY MODEL

Computer simulation of the reflectance for citrus crops, by using a geometrical canopy model, has been carried out to analyse and interpret the reflectance values from Landsat-5 Thematic Mapper (TM) images in two different periods: summer and winter. As well as the tree cover of the crop, the plantation framework and orientation of the citrus parcels have also been analysed. These architectural parameters determine, together with the sun zenith angle, the proportion of shadows in the scene. In summer, the main influence of shadows occurs in the near-infrared band (TM4). Shadow is such an important effect in this spectral region that it may cause an overlap between spectral responses of canopies with different tree cover. In winter, the high proportion of shadows produces a saturation effect on the reflectance values when the tree cover of the parcels is increased. A threshold value of tree cover (approximate to 40 per cent) has been observed experimentally and explained theoretically. Above this threshold, all the citrus parcels present approximately similar reflectance values.