Abstract: The Balager River Basin is a typical inland drainage basin and is located in a grassland plateau in China, where the grassland degeneration and the change of hydrological process are caused by many factors. It is very important to quantitatively analyze the variation of runoff volume, climate change and human activities for the reasonable utilization of water resources and conservation of grassland eco-environment. In this paper, the double cumulative curve, departure accumulation, improved Mann-Kendall trend test method and change rate of cumulative slope were used to quantitatively calculate and analyze the characteristics and main driving factors causing the change of runoff volume at different scales based on the hydrological, meteorological and socioeconomic data in the Balager River Basin during the period from 1959 to 2015. The following results were concluded (1) The shift point of runoff volume occurred in 1998. The runoff volume was changed under the influence of precipitation before 1998 (named as the reference period), while there were the double influences of climate change and human activities on runoff volume after 1998 (named as the variable period); (2) The annual, wet-seasonal and dry-seasonal runoff volumes were holistically decreased, but there was no significant precipitation variation; (3) Although there was no significant variable trend of precipitation (p= 0.428) at annual scale, the runoff volume was deceased significantly (p= 0.042). The runoff volume in the reference period was higher than that in the variable period when the precipitation was similar; (4) Both the runoff volume (p= 0.117) and precipitation (p = 0.077) decreased slightly in wet season, while the runoff volume in dry season (p = 0.003) decreased significantly but the precipitation (p = 0.286) did not. No matter in wet season or dry season, the average runoff volume in the reference period was higher than that in the variable period; (5) The monthly precipitation and runoff volume in the whole time series, reference period and variable period fluctuated similarly. The precipitation in July (107mm) was the highest, but the corresponding runoff volume (1.7mm) was not the highest. The highest runoff volume (2.963mm) occurred in April because of the melting of accumulated snow cover in winter; (6) Compared the climate factors in the reference period with those in the variable period, the variable ranges of temperature, precipitation and wind speed were 65.12%, -16.97% and -13.3% separately, while the fluctuations of other climate factors were less than 5%. So the temperature, precipitation and wind speed were the main climate factors affecting the runoff volume; (7) In the study area, the primary industry, sheep number and population increased by 3.8 times, 12.06% and 9% separately; (8) The contribution rates of runoff volume reduction due to climate change and human activities were 28.86% and 71.14% respectively, so human activities were the main driving forces causing the change of runoff volume. The study results can provide the references for analyzing the ecological hydrological change, water resources utilization and conservation in the process of socio-economic development in grassland watershed in semiarid area of north China.

Key words: inland drainage basin in plateau, typical grassland basin, climate change, human activity, runoff volume, contribution rate, Balager River, Inner Mongolia