Impacts of climate variability and human activities on streamflow in Weihe River catchment were quantified by hydrological process simulation and statistical analysis.Data from 1958to 2008were analyzed by Mann-Kendall method to test trends of meteorological and hydrological elements.A significant decreasing trend in streamflow was found, while rainfall and potential evapotranspiration showed no significant trend, suggesting that changes in streamflow were mainly caused by human activities.The abrupt time point of streamflow change was 1990.SIMHYD rainfall-runoff model was constructed to simulate streamflow from 1958 to 1989 and from 1990to 2008.SIMHYD was calibrated and validated by monthly meteorological and hydrological observation data from 1958-1989(pre-treatment period).The model was then used to simulate streamflow in a testing period from 1990 to 2008, to obtain quantitative assessment of effects of climate variability and human activities.Improved climate elasticity approach was also used to analyze contribution of runoff change.These two methods were compared and verified, similar results were obtained.The contribution of climate variation to decrease in streamflow was found to be 18%-22%, and human activities 78%-82%.