The objective of this paper is to present a procedure for parameter updating that can be combined with any conceptual rainfall-runoff model for flood forecasting purposes, and to study the improvements in flood forecasting induced by the assimilation of soil moisture information into rainfall-runoff models. The main feature of this method is that it does not carry out updating by reference to only recent streamflow observations, as classic procedures do, but also to soil moisture measurements which can be either retrieved from TDR probes or from satellite remote sensing systems. The aim of the research was to assess the usefulness of this additional soil moisture information. To this end, an approach has been suggested that introduces gradually the additional information and can detect a threshold above which this information benefits to the whole flood forecasting procedure. This methodology was put forward for use in the European AIMWATER project on four catchments within the Seine River basin upstream Paris (France) and on the Arade catchment in Southern Portugal.