Mo, S.; Li, Z.; Gou, K.; Qin, L., and Shen, B., 2018. Quantifying the effects of climate variability and direct human activities on the change in mean annual runoff for the Bahe River (Northwest China).Evaluating the impacts of climate change and human interference on runoff is critically important for regional water security. This study investigated the effects of climate variability and direct human activities on the change in mean annual runoff for the Bahe River in Northwest China. Precipitation, evaporation, and runoff data from 1959 to 2010 were analyzed at the Luolicun and Maduwang hydrologic gauge stations, and their temporal trends were explored using the Mann-Kendall test. A Mann-Kendall-Sneyers test and the double mass curve were applied to detect the statistically significant transition points in the hydrometeorological records. Results indicated no change point in upstream runoff in the Bahe River from 1959 to 2010; however, a clear transition point existed in midstream runoff in 1989, indicating a significant decrease in stream-flow. While climate change could be a major factor leading to runoff variation in the upstream, human activities had significantly contributed to the decline of the midstream runoff after 1989. A decomposition method based on a Budyko equation was used to quantify the relative impacts of climate change and direct human activities on the midstream runoff. Results show that 35.3% of the reduction in annual runoff was caused by human activities and that a 2.2% increase in annual runoff could be attributed to climate change. This finding means that human impact rather than climate exerts the dominant influence on runoff decline in the Bahe River basin.