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Climate change and human activities have induced alterations to hydrological processes. The quantification of their impacts on streamflow is a challenge, especially at the seasonal scale due to seasonality of climate variables and human activities. In this study, the decomposition method based on Budyko equation is extended to the seasonal scale for quantifying the climate and direct human impacts on annual and seasonal streamflow changes in Huifa River basin by defining prechange period (1953-1974) and postchange period (1975-2005). The results are further verified by the monthly abcd model. Both climate change and direct human activities are found to induce a decrease in streamflow at the annual scale, with 68% of the change contributed by direct human activities. At the seasonal scale, the direct human-induced declines in streamflow account for 67% and 39% of the total changes for energy-limited and water-limited seasons, respectively; whereas, the impact of direct human activities is more pronounced during the irrigation season due to water withdrawal for irrigation. In addition, the decomposition results are analyzed for each month in the energy-limited season to reveal the effects of precipitation and operation rules of ponds and reservoirs during the flood season.