APPLICATION OF AGGREGATION MODELS TO SURFACE HEAT-FLUX FROM THE SAHELIAN TIGER BUSH

Areas of natural open forest in the Sahel, locally called tiger bush, typically consist of patches of vegetation and bare soil on a scale of approximately 50 m. The ratio of vegetation height to the patch size is roughly 1/10. It is not clear whether, with this height to patch ratio, the surface can be treated as a sparse canopy or as a patchwork terrain. Two aggregation models are applied to measurements of temperature and heat flux from an area of tiger bush in the Sahel: a patchwork model, which has a separate energy balance for the soil and bush component of the surface, and a sparse canopy, or coupled model, which has a mechanism to allow some horizontal heat flux between the soil and the bush. The coupled model is found to fit the data better than the patchwork model, presumably because there is a considerable flux of heat from the soil to the bush which is not taken into account by the patchwork model. One of the overall objectives of studying heterogeneous terrain is to identify which ratios of vegetation height to patch size can be treated as sparse canopy and which ratios can be treated as a patchwork terrain. The model results give a useful datum point on that scale.