Abstract: A water tracing method is proposed based on an urban rainfall-runoff-drainage-inundation 1D-2D coupled model,to illustrate the source and contribution for urban inundation in flood simulations of complex urban underlying surfaces.It introduces a net rainfall tracer in the 2D surface numerical model to represent the runoff source areas within each computational grid of the study area.After rainfall and infiltration calculations,the flux is tracked to reflect the variation and movement of net rain in the source areas.Dynamic link library (DLL) technology is used to couple the 2D surface model with the 1D sewer and water quality modules of the SWMM model.In the sewer model,pollutants are used as tracers to track the composition of runoff,achieving bidirectional dynamic coupling between the 1D and 2D models for both water volume and tracer tracking.The proposed method is validated using theoretical example.For a practical case study,the method was applied to an urban area in Changzhi,Shanxi,where a 1D-2D tracing model was constructed.The comparison between simulated inundated data and observed measurements shows a good agreement in terms of inundation locations and inundated water depths,with depth errors smaller than 15%.Additionally,the contributions of different source areas and the composition of drainage overflows at these inundation points were quantified.The results demonstrate that the proposed tracing method is reasonable and feasible.The developed 1D-2D tracing model provides reliable insights into urban hydrological and hydrodynamic processes,offering a solid theoretical foundation and technical support for the refined management of urban flooding.