Distributed Three-source Runoff Model For Small Mountain Watersheds

The development of hydrological models is one of the important issues for water resource planning and flood forecasting.Due to the significant progresses in Geographic Information System,Remote Sensing,and Digital Elevation Model,hydrological simulation has been moved from the lumped modeling to distributed modeling.The distributed hydrological model based on DEM can completely reflect spatial heterogeneity of the watershed underlying surface;therefore,the hydrological model simulation is closer to the watershed physical process.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.Considering the accessibility of the hydro-meteorological data and spatial heterogeneity of underlying surface,a subwatershed in the upstream of the Goodwin Creek Experimental Watershed in U.S.was adopted as the test site to verify the capability of the proposed distributed runoff model.The main contents and achievements are as follows:Firstly,a digital terrain analysis 'was conducted to extract the topographic features from the DEM dataset,which included flow direction analysis,river network extraction,and catchment discretization.The discretization of the catchment was conducted based on the Representative Elemental Area method and the Aggregated Simulation Area considering stream network structure and hillslope topography.To meet the requirement of simulation scale,the hillslope was further divided into sunny and shady slopes.The discretization of the parameters was realized by the digitization of study watershed at the same time and providing the necessary support for the application of the model.Secondly,the existing of overland flow on the hillslope was determined according to the analysis of continuity equation and Darcy equation.A simplified form of Saint-Venant equations was introduced to deduce the approximate partial differential equation of overland flow.Each slope unit was approximated by different quadratic polynomials and the method of characteristics was used to simulate the overland flow process.As a whole,this module took into account the effect of unsaturated lateral flow on the generation and development of overland flow so that would be significant for the purpose of constructing the rainfall-runoff model with rigorous physical mechanism.Consequently,outflows from the hillslope units were confluent and routing by using an improved Muskingum-Cunge method in the main channel,thus a complete distributed three-source runoff model was performed.To test the applicability of the model,the simulations were assessed by the deterministic coefficient,correlation coefficient,and the relative error of the flood discharge,flood peak and peak time.In conclusion,the model developed in this study has been extended the applicability for rainfall-runoff routing from small hillslopes to mid-size or larger watersheds.Moreover,detailed formulas were derived to clarify the limitation of the original model using on quadratic polynomials of the hillslope.In general,the proposed distributed runoff model can afford precise rainfall-runoff simulations which can be a valuable reference for flood control design work.