Abstract: The separation of storm hydrographs using stable isotope tracers provides a scientific basis for the study of runoff-formation mechanism and runoff calculation.Many research results show that there is a correlation between the initial watershed state and the event/pre-event water proportion. Hydrograph separations were conducted using stable isotope(δ¹⁸O) to analyze the proportion of event/pre-event water on four rainfall events during 2015-2016 in Hemuqiao watershed and especially the influence of antecedent streamflow and soil moisture. The results indicated that pre-event water dominated streamflow but the event water accounted for more in the peak; the difference in hydrograph separations implied that multiple runoff generation patterns existed in different initial system states. The runoff coefficient and the contribution of event water to streamflow were negatively correlated with antecedent streamflow. The relationship between the contribution of event water and antecedentsoil moisture was complicated and the percentage event water of streamflow would decrease in the condition of high antecedentsoil moisture or large antecedent 5-day rainfall

Key words: stable isotope, hydrograph separation, antecedent streamflow, initial soil moisture, Hemuqiao watershed