The AIMWATER project, supported by the Environmental and Climate program of the European Community, deals with important aspects of multipurpose reservoir operation under two contrasting European climates, humid temperate and semi-arid Mediterranean. AIMWATER attempts to access a dependable forecasting tool to assist them in making decisions based on the assumption that soil moisture state is unanimously recognized as a key hydrological variable among the hydrological community. Recent studies have shown that information derived from Earth Observation (EO) data could reliably provide spatialized and temporal soil moisture information. The integration of this information into catchment scale hydrological models could significantly improve the reservoir operation decisions. The purpose of the project is then to set up an approach to derive catchment soil moisture from Earth Observation data using microwave space-borne Synthetic Aperture Radar (SAR) images from ERS satellites and to study the improvements brought about by an assimilation of this information into hydrological models. The aim of this paper is to introduce the approach carried out to comply with the objectives of the AIMWATER project, which will be presented in companion papers.