Many previous studies have quantified the impacts of climate change and human activities on annual river runoff neglected the contribution of human activities to seasonal runoff variation. This paper describes an extended decomposition method based on the Budyko hypothesis and distinction between three seasons: irrigation season, wet season, and dry season. We have applied this method to calculation of the climate change and human activities impacts on the river runoff of the Fengman sub-basin II in each of the seasons, and evaluated the contribution of water storage projects to runoff variation in wet season (from July through September). The results show that runoff decreasing depends on the integrated effect of climate change and human activities and their individual effects are different in form and order of magnitude. Generally, human activities cause a decrease in runoff in all the seasons, while climate change causes a decrease in wet and dry seasons but an increase in irrigation season.