<p>This paper proposes a framework for quantifying the impacts of climate variability and anthropogenic interventions on streamflow changes at both annual and seasonal time scales in transboundary river basins. Those quantified impacts are determined using a physics-based hydrological model and an Extended Impact Factor Formula newly developed by the authors. The attribution results are also exposed to empirical evidence to ensure the validity of the outcomes. The key advantage of the new framework lies in its high interpretability, reliability, and capability to take the model errors into account at both seasonal and annual time scales while saving time and computational resources. This allows for all magnitudes, influential directions, and relative contributions of human and climatic impacts to streamflow variation to be separately specified. The framework was successfully tested with a real case study in the upper Da River basin where most data were achieved from various open data sources. The outcomes of this application revealed that the human activities upstream have lessened the annual streamflow to the downstream since 2009. Overall, human activities have reduced and enhanced the streamflow during flood and dry seasons, respectively. Although human activities upstream of the study area have significantly influenced streamflow, the impact of climate variabilities appears to dominate over human activities during the first impacted period. Furthermore, the results show that during extreme drought and wet years, the upstream reservoirs have played an insignificant role in mitigating those extreme event impacts.</p>