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Urban wind environments are closely related to air pollution and outdoor human comfort. The urban natural ventilation potential (NVP) is an important factor in urban planning and design. However, for ventilation studies on urban scales, neither macroscale numerical simulations (i.e., WRF, MM5, etc.) nor microscale computational fluid dynamics (CFD) simulations can conduct efficient analyses. Based on the similarity between water flows and airflows, an efficient approach is proposed in this paper to map the urban NVP. Through integrating the urban terrain model, urban form model, and prevailing wind pressure model, an airflow digital elevation model (AF-DEM), which represents the resistance to airflow and can be used for a hydrological simulation, is generated and applied to evaluate the urban airflow patterns under different terrain, urban form and ambient wind conditions. The objective was to develop a simulation platform that can efficiently predict the distribution of natural ventilation corridor and NVP. The stream network calculated through the simulation is regarded as potential ventilation corridors within the city, and an index calculated from the coverage rate of wind corridors (CRW) is proposed for evaluating the relative NVP. Taking Nanjing as a case study, 8 AF-DEMs based on different wind directions and wind speed conditions are generated, and their corresponding ventilation corridor maps are constructed. The results are in good agreement with the empirical evidence, indicating that the hydrological model, though a rudimentary approximation of the actual airflows, was effective in revealing the natural ventilation corridor and characterize the relative NVP. Moreover, the implementation of this novel method is simple and convenient, and it has great application potential and value in urban design and management.