Using a simple SVAT scheme to describe the effect of scale on aggregation

Avaporation is enhanced at a dry-to-wet transition but not equally depressed at a wet-to-dry transition. Therefore, the more dry-wet edges there are per unit area, the higher the evaporation. This behaviour is predicted by the blending height; the smaller the length scale of variation, the smaller the blending height and the higher the evaporation. The blending height principle can be modelled simply for a mixture of two surfaces using a model with three resistances and applying the energy combination theory developed for sparse canopies. Results from numberical model simulations are shown to agree with the simple analytical model. The blending height is found to be too large to model correctly observations from small-scale heterogeneous terrain in the Sahel. This is assumed to be due to edge effects.The results show that a modification of the sparse canopy model can be made to represent heterogeneity at all scales.SVAT is Surface Vegetation Atmosphere Transfer.