Abstract: The runoff of some rivers in the world, especially in the arid and semi-arid regions, has decreased remarkably with climate change and intensive human activities of the development of society and economy. This paper analyzes the characteristics of annual mountainous runoff and runoff consumption in the midstream of Shiyang river in the past 50 years. Using Fisher classification method and relativistic consumption rate, a major inflection point has been identified to be the year of 1975, during which human activities started to have significant influences on runoff consumption in the midstream of Shiyang river. The inflection point is clarified to be a point of significant change of runoff consumption caused by human activities, as opposed to the change of natural runoff, which is an improvement to the current research. Based on the types of human activities and the strength of the influences, the trend of runoff consumption in more than 50 years is divided into four stages; the contributions of human activities and natural changes o the influences on the runoff consumption in each stage from 1956-2009 are analyzed quantitatively and qualitatively. The results indicate that, generally speaking, prior to 2000 the contribution of human activities gradually increases; the contribution of natural changes decreases. Natural contribution is significantly correlated with mountainous runoff. Since mid-1970s, the contribution of human activities on the runoff consumption has increased significantly in the midstream of Shiyang river. In 1990s the influences of human activities and natural changes on the runoff consumption reach an equal level. Quantitatively speaking, from 1956 to 1999, the contribution of human activities is equivalent to 0.633 billion m³/a, with 62.31% of water from upstream as directly artificial water consumption, accounting for 71.52% of overall results. After that, the contribution of human activities started to gradually decrease, indicating the sign of relief of the conflicts between water conservation and human activities in the midstream of Shiyang river.

Key words: inflection point, influencing factors, midstream of Shiyang river, relativistic consumption rate, runoff consumption