Flood forecasting is usually based on the simulation of rainfall-runoff models. In this study, based on the saturation excess runoff mechanism, a distributed three-source runoff model was developed for rainfall-runoff simulation in mountainous areas. Firstly, a watershed was delineated into several hillslope units and a digital terrain analysis was conducted to extract the topographic features from the DEM dataset, the discretization of the parameters was realized by the digitization of the watershed at the same time and providing the necessary support for the application of the model. Secondly, the runoff yield in the slope surface unit was calculated by using the flow continuity equation and Darcy formula. Taking the simplified kinematic wave equation of Saint-Venant equations being as the governing equation, the approximate partial differential equation of slope surface flow was derived, and the confluence of each slope surface unit was calculated with characteristic line method. Storm runoff can then be simulated from the unsaturated lateral flow to the saturated overland flow on the hillslope unit of the proposed distributed runoff model. Moreover, outflows from the hillslope units were confluent and routed by using an improved Muskingum-Cunge method in the main channel. To test the applicability of the model, the simulations by using the hydrological records of Goodwin Creek experimental watershed in USA were assessed by the deterministic coefficient, correlation coefficient. The results showed that the proposed distributed runoff model can afford precise rainfall-runoff simulations and the accuracy can meet the requirement of forecasting. In general, the developed model can extend the applicability for rainfall-runoff routing from small hillslopes to mid-size or larger watersheds.
