Measurement of canopy interception of solar radiation by stands of trees in sparsely wooded savanna

Abstract

A three-dimensional tree canopy model of scenes which consist of a discontinuous tree layer and a continuous field layer is described. ‘Scene’ or spatial average tree canopy layer transmissions are defined for direct solar radiation, diffuse sky radiation and emission of reflected radiation from the ground and field layers. Hemispherical canopy photographs are used to measure these three types of transmission and the measurements provide the parameters for the model which can be inverted to give estimates of the spatial average properties of a scene. The model requires only the frequency distribution of tree canopy diameters to be specified and these can be measured on the ground or on aerial photographs. The effect of each canopy is additive and so the model can be applied to any pattern and density of trees. Hemispherical photographs can be reinterpreted for any solar azimuth and zenith angle and are therefore applicable for long periods of time. Measurements in two areas of savanna woodland in Kordofan, Sudan, showed that canopy cover measured in the traditional way, which treats the canopy as opaque, overestimated interception by approximately half. Comparisons between Acacia Senegal and other species indicate that there are some significant differences in transmission characteristics between savanna tree species. The relevance of these observations to remote sensing of savanna field layer primary production is discussed.