Knowledge of evaporation on local scale is a prerequisite for the prediction of drought. The Surface Energy Balance System (SEBS) provides the means to do this. Input data of SEBS are satellite data and a limited set of ground measurements. By using the dual-looking viewing capabilities of the ATSR sensor on board the ERS-II it is possible to determine vegetation and soil temperature separately. These two temperatures can be used in SEBS - the dual source SEBS - in order to gain more insight into theevaporation process and to improve the physical basis of the algorithm. Testing and validation of single and dual source SEBS has been carried out with the use of existing data sets of the Netherlands and Spain. The algorithm sensitivity towards input variables is established. Air and surface temperatures and wind speed have a large impact on the results. Two input variables proof to be very hard or impossible to measure, i.e. the roughness length for momentum and the roughness length for heat transfer.The existing methods and models do not give satisfactory results. Validation is carried out with ground fluxes, measured by eddy devices and scintillometers.