Modeling the exchange of water and energy over natural land surfaces = Het modelleren van de uitwisseling van water en energie over natuurlijke landschappen

This thesis deals with the modeling of the surface energy balance and the atmospheric boundary layer over natural land surfaces, on scales of the grid cell of large-scale atmospheric models. In the first part, a model to calculate the canopy conductance as a function of environmental variables evaluated at leaf level is developed and validated. The parameter values of this approach are retained from plant-physiological theory. For a C ₄ prairie grass in Kansas and a C ₃ soybean crop in southern France, the plant-physiological approach gives better estimates of the canopy conductance, compared to a traditional Jarvis-Stewart approach which relates the canopy conductance to environmental variables at a reference level, using empirical-statistical functions. For a C ₃ grassland in the Netherlands, both the plant-physiological approach and the Jarvis-Stewart approach give comparable estimates of the latent heat flux density. In the second part, two approaches to calculate the impact of soil moisture stress on the surface flux densities over natural, heterogeneous areas are compared: a bulk approach where the soil moisture content is assumed to be uniform in a grid cell, and a distributed approach which takes account of the spatial variation of the soil moisture content. In wet conditions, the bulk approach gives larger predictions of the latent heat flux density than the distributed approach. In dry conditions the bulk approach gives lower predictions than the distributed approach. Especially for dry climates the bulk approach predicts during the dry season a severe suppression of the latent heat flux density. In the third part, using three cases that occur frequently in nature it is shown that only a tiling approach can provide estimates of the averaged surface flux densities that are consistent with the averaged temperature difference over the surface layer, the layer of air adjacent to the surface, in all situations.