The runoff from the Laohahe basin in northern China has been greatly influenced by climate variability and human activities, and it is very important to quantitatively analyze these effects. Three methods, the Mann–Kendall test, ordered clustering and double cumulative curve were used to detect the trend and change points in annual precipitation and runoff for the period from 1964 to 2008, and the runoff series were divided into a ‘natural period’ (1964–1979) and a ‘human-induced period’ (1980–2008). Then the three-layer Variable Infiltration Capacity (VIC-3L) model was used to quantitatively evaluate the effects of climate variability and human activities on runoff. The results show that compared with the ‘natural period’, the annual precipitation varied by −10, 10 and −10% during the periods of 1980–1989, 1990–1999 and 2000–2008, respectively, while the corresponding yearly runoff changed by −57, 17 and −72%, respectively. It was found that the range of yearly runoff percentage change is much larger than the range of annual precipitation variation. Additionally, the computing effects of human activities on runoff decreasing are more evident during the two drier periods of 1980–1989 and 2000–2008 with the contribution of 70 and 78%, respectively, while that of the wetter period of 1990–1999 is only 21%. It is suggested that within the Laohahe basin human activities are the main reason that runoff decreased during the two drier periods of 1980–1989 and 2000–2008, while the increasing of the runoff during the wetter period of 1990–1999 is mainly attributed to the climate variability.
Keywords: climate variability, human activities, hydrologic model, runoff