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Flood risk is one of the most critical disasters that threatens sustainability of human-dominated ecosystems.Fuzhou,a typical coastal megacity in southeastern China,is subject to huge calamities and economic loss annually due to the typhoon storm floods during rainy seasons.According to time series analysis of local weather dataset,the number of heavy rain has been rising annually since the 1990s.At the same time,urbanization process changed urban land use status and the impervious surface ratio.Both of these had significant influences on the change in return periods flood.Thus,it is very important to conduct a series of studies on the change in return periods flood,aiming to provide practicable guidance for flood control and flood disaster reduction given the ongoing trend of urban expansion. Impervious surface ratio(ISR) is the ratio of impervious surface area to total land area,and it is an important indicator of the urban hydrology.This study focuses on the relationship between the impervious surface ratio and flood return periods.The Bayi Reservoir watershed,situated in Jin′an district of Fuzhou city,was taken as an example.By using remote sensing technology,the land use information were extracted from the TM/ETM+ image of studies in 1989,1994,1999,2001,2002,2003,and 2007.In accordance with the land use data,the watershed impervious surface ratio of selected years were calculated.The results showed that with the urbanization process,the whole watershed impervious surface ratio during the 19 years continued to grow,with a change interval of 5%—10%. Additionally,original dataset,including digital elevation model(DEM) data,soil type data,meteorological and hydrological data,were collected to provide model information in the study.The HEC-HMS hydrological model of Bayi Reservoir watershed was well established after the pretreatment of partial data using WMS 7.1 software.Furthermore,the process of single rainfall-runoff in the Bayi Reservoir watershed was simulated.The results were consistent with the in-situ observation.According to the simulation results,designed to combine the rainfall conditions with land use case to calculate the discharge,and took impervious surface ratio and return periods as the variables,the graphs of return period change were drawn.Finally,according to these graphs,in sub-basin A the return periods of 100,50,and 25 years were ahead of designed time by 20,8,and 3 years,respectively,with the growth of impervious surface ratio.In sub-basin B the return periods of 100,50,and 25 years were ahead of designed time by 25,10,and 4 years,respectively,with the growth of impervious surface ratio. This study provided precious experiences for study on flood return periods.The results are beneficial to make the planning for urban flood control and flood risk reduction.Furthermore,the results supply both practicable and theoretical basis for land use planning in urban watershed as well as scientific prevention and treatment of non-point pollution.