Runoff change and attribution analysis in a semiarid mountainous basin

Climate change and human activities are the main causes of runoff change. Exploring runoff change over time, in response to climate change and human activities, tests past effects of ecological restoration and provides a solid foundation for future decision-makers. To reduce uncertainty in our attribution analysis, we used two methods, the catchment water-energy balance equation and the geomorphology-based hydrological model (GBHM), to examine runoff change in a semiarid mountainous basin-the Chagan Mulun River Basin. The results are described as follows: (1) In the past 40 years, runoff and leaf area index (LAI) in the study area showed a significant downwards trend. (2) The two methods were consistent in terms of quantification and showed that change in the underlying surface, especially direct water abstraction, was the main reason for the reduction in runoff. Climate change had the greatest impact on runoff, which increased in the Schaake, 1990s. In other periods, climate change reduced runoff. (3) The quantitative interdecadal results of the two methods differed. The GBHM does not accurately simulate processes, such as reservoir regulation, which may cause deviation in an attribution analysis for runoff in arid and semiarid basins affected by human activities.