Abstract: The surface runoff of some rivers in the world has decreased significantly with global or regional climate changes and increasing human activities, which has caused severe eco-environmental problems. The Songhua River is a large river in Asia and its basin is an important bread basket in China. The runoff of the whole river basin and in different sections has experienced dramatic changes over the last few decades. It is necessary to study the trend of the runoff changes and quantify the influencing factors on the changes. Based on annual runoff data at 4 gauging stations in the mainstream of the river and annual precipitation data from 61 meteorological stations in and around the river basin during 1955-2010, a decreasing trend of annual runoff and precipitation for four sections (above Jiangqiao, Jiangqiao-Dalai, Dalai-Haerbin and Haerbin-Jiamusi) was revealed using the linear trend method. The cumulative anomaly was employed to detect the abrupt changes of each runoff series. Three inflexion years for the runoff incremental changes were detected and they divided the entire period into four parts: above Jiangqiao (1963, 1982 and 1998), Jiangqiao-Dalai (1962, 1985 and 1998), Haerbin-Jiamusi (1966, 1980 and 1998), and only one turning year (1988) was found in Dalai-Haerbin section. The first period (T₁) is regarded as the baseline period; the others are measurement periods. Without considering the effect of evapotranspiration, the slope change ratio of cumulative quantity (SCRCQ) was adopted to estimate the impacts of precipitation and human activities on runoff changes for the sections. In above Jiangqiao (T₂: 1964-1982, T₃: 1983-1998), Jiangqiao-Dalai (T₂: 1963-1985, T₃: 1986-1998) and Haerbin-Jiamusi (T₂: 1967-1980, T₃: 1981-1998) sections, the impacts of human activities on runoff incremental decrease were 71%~88%, 94%~97% and 72%~85%, respectively, while those of precipitation were 12%~29%, 3%~6% and 15%~28%, respectively, for the measurement periods of T₂, T₃ and T₄ (1999-2010) compared with T₁. In the measurement period (1989-2010), the impact of human activities on runoff changes in Dalai-Haerbin section was 75%, while that of precipitation was 25%. Obviously, the human activities were the most important factor causing runoff yield reduction in the later three periods for different sections of this study area. In addition, the impacts of human activities on runoff yield changes reached the maximum in the 1980s and 1990s. Although the runoff yield for different sections showed different variation characteristics, the reduction is mainly due to population increase and construction of water conservation projects.

Key words: human activities, inflexion year, precipitation, runoff change, Songhua River Basin