Improved regional hydrologic modelling by assimilation of streamflow data into a regional hydrologic model

River streamflow is closely related to the surface runoff over a river basin. It is a quantity which can be measured with relatively high accuracy. In regional hydrologic models, surface runoff is usually calculated as the residuum of the surface water balance equation and is one of the most uncertain model outputs, especially for mountainous areas. In this paper, we propose a simple method within the framework of a regional hydrologic modelling system (RHMS), which integrates a hydrologic model and a land surface model, to constrain the estimates of surface runoff through progressive assimilation of observed streamflow data based on the channel flow equation. The proposed method is applied to a sub-basin of the Huaihe River Basin in East China, and numerical experiments are carried out to test the effectiveness of successively (from upstream to downstream) assimilating the streamflow data collected at three hydrological stations to RHMS. It is found that with the assimilation, the timing and magnitudes of the modelled flood peaks are much improved and the correction of surface runoff is substantial. The numerical results demonstrate the usefulness of the new method in improving the RHMS runoff-streamflow simulation as well as providing better initial conditions (e.g. channel water heights) for flood forecasting.