The runoff in the upper reaches of the Heihe River has been continuously abundant for more than a decade, and this has not happened previously in history. Quantitative analysis of runoff variation and its influencing factors are of great significance for the ecological protection of the basin. In this paper, the soil and water assessment tool model was used to simulate runoff in the study area, and the method of scenario simulation was used to quantitatively analyze the runoff response with respect to land use and climate change. According to the abruptness of the runoff sequence, the years before 2004 are categorized as belonging to the reference period, and after 2004 is categorized as the interference period. According to the analysis, compared with the reference period, the contribution rate of climate change is 87.15%, while the contribution rate of land use change is only 12.85%. The climate change scenario simulation analysis shows that the change in runoff is positively correlated with the change in precipitation. The relationship with the change in temperature is more complicated, but the influence of precipitation change is stronger than the change in temperature. According to the land use scenario simulation analysis, under the economic development scenario, the runoff decreased, whereas under the historical trend and ecological protection scenario, the runoff increased. Additionally, the runoff increased more under the ecological protection scenario.

The study of water resource carrying capacity (WRCC) is significant for rational water resource utilization and promotion of the coordinated development of a regional economy, society, and ecology, especially in arid regions. In this paper, using different scenarios, a fuzzy comprehensive evaluation model based on water resource allocation is constructed to obtain the WRCC in the middle reaches of the Heihe River. The results show that the current development of water resources has a certain scale, and the carrying capacity is relatively low. Compared with the current water resource scheme, various scenario schemes have higher evaluation indexes. Among the schemes, scheme 7 is the optimal plan for the recent planning year, and scheme 13 is the best for the long-term planning year. Based on a subsystems analysis, the social subsystem has the highest score, which is followed by the economic subsystem, water resource subsystem, and ecological subsystem, and the evaluation index of the economic subsystem shows the largest increase. The main factors affecting the WRCC are the water-saving level and crop irrigation quota. Therefore, the WRCC should be improved by raising the level of agricultural water use, restricting the irrigation area, and adjusting the local industrial structure.