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Quantitative assessment of the effects of climate change and human activities on runoff is very important for regional sustainable water resources utilization. Determining abrupt changes in runoff could enhance identification of the main driving factors for the sudden changes. In this study, the double mass curves analysis combined with field investigation is used to determine abrupt changes in runoff in two sub-catchments of Upper Yongding River Basin(UYRB), while trend analysis via the traditional Mann-Kendall test for the period 1961-2017 is used to identify the basic trend of precipitation, temperature and potential evapotranspiration(E0). The results suggest an insignificant change in precipitation, a significant increase in temperature and a significant decline in E0 in both sub-catchments. For both of the sub-catchments, abrupt changes in runoff occurred in 1982 and 2003. Both Budyko's curve and double mass curves are used to evaluate the potential impacts of climate variability and human activities on mean annual streamflow. The results showed that, from the 1960s to the 1980s, runoff declined by 20.01% and 22.28% for Xiangshuibu and Shixiali, respectively; from the 1980s to the 2000s, runoff declined by 68.23% and 67.77% respectively. In the variation stage I (1983˜2003), human activities contributed 90.6% and 62.7% of the mean annual streamflow change in YRB and SRB, respectively. In the variation stage II(2004˜2017), human activities contributed 99.5% and 93.5% of the change in YRB and SRB, respectively. It is also noted that the first abrupt decline in runoff was actually at the beginning of China's land reform, when the land reform motivated farmers to productively manage their reallocated lands, agricultural water consumption therefore increased. The second abrupt change point occurred in 2003, when "Capital water resources planning" implemented including water conservation projects and irrigation district construction. In general, human activities, including soil and water conservation projects and water consumption, are found to be the dominant factors responsible for the significant decline in the annual streamflow in the UYRB over the last six decades.