Surface energy balance of shrub vegetation in the Sahel

Recently, the development and use of Global Circulation Models, employed for climate change prediction, has taken off. These models provide us with the current and future status of the surface, expressed by the surface energy and water balances. In order to obtain reliable climate-predictions there is an urgent need for reliable input data from all major biomes covering our Globe. The Sahel has been one of the most important areas of application for GCMs, in order to understand the reasons for its declining rainfall, a process that started in the early seventies. It is therefore of paramount importance to monitor the surface conditions of this vulnerable area, where drought and land use interact to create surface conditions able of altering the regional climate.This thesis describes the surface energy balance of Sahelian shrub vegetation (fallow savannah and tiger-bush) by using micrometeorological data obtained in the framework of the HAPEX-Sahel and SEBEX project. The surface energy balance defines how net radiation is partitioned over the atmospheric fluxes (sensible and latent heat flux) and the soil heat flux. The energy balance is studied on a diurnal, seasonal and interannual scale. In addition the CO 2 flux of savannah is described. The fallow savannah is part of a rotation scheme (usually millet) and consists of scattered shrubs (Guiera senegalensis) interspersed by a sparse undergrowth of grasses and herb species. The tiger-bush takes its name, when viewed from the sky, from the resemblance to a tiger skin. It is composed of elongated vegetation patches, mainly consisting of Guiera senegalensis with an occasional tree, growing on bare, crusted, practically impenetrable soil.To study the exchange between the (Sahelian) land surface and the atmosphere, various surface parameterizations have been developed. These soil-vegetation-atmosphere-transfer schemes (SVATs, in their simplified form being the basis of the large-scale models) require detailed input of surface parameters (e.g. albedo, roughness length for momentum, or soil thermal conductivity) and the resistances encountered by the fluxes. This thesis provides the various input parameters and surface and aerodynamic resistances of the savannah and tigerbush and their possible temporal variability. Various SVATs have been tested and it can be concluded that simple parameterizations of soil heat flux give results comparable to elaborate soil models. Of major importance are the surface resistances and the aerodynamic resistance describing the transport within the understorey. Simple relationships, in which the roughness length and the displacement height are characterized by canopy height only, do not work satisfactorily for these sparse canopies. They require a relatively detailed drag partition model, applying roughness density.If simple, one-layer, bulk transfer models are used (e.g. for remote sensing) the excess resistance has to be taken into account. It appears that the resistance for this shrub-dominated vegetation is much larger than the usually applied value