This research investigation constructeda unit hydrograph(UH) for the purpose of simulating flash floods in small catchments using long-term rainfall-runoff data.The characteristics of the flash-flood responses were described,particularly from the aspect ofprecipitation.In addition,this study explored how the characteristics of flash-flood responses changed under different environmental conditions.Two small catchments (USGS gauges at the Catawba River and Swatara River) in the United States were investigated in this study.It was found that when the antecedent precipitation indexes (API) were high,the peak UH valueswere higher and the peak current timeshad occurred earlier.The results of the surface runoff simulations using the average UH with different API were considered to be promising,with average Nash efficiency coefficients of 0.846 and 0.940,respectively.The average relative peak errors were determined to be 9.40% and 7.47%,respectively.It was observed that by combining the rainfall and flood-causing precipitation values,the simulation results could be further improved and better represent the characteristics of flash-flood responses.Generally speaking,the method used in this study was considered to have very good potential for real-time flash-flood forecasting processes.Furthermore,further analyses of the interannual variations of UH in the CatawbaRiver Basin indicated intensified flash-flood responses with increasing peak magnitudes and decreasing flood durations.These findingswere found to correspondwell with the increases in both accumulated precipitation amounts and frequency of heavy rainfall observed during the past several years in the study area.