Water is essential for life and plays a vital role in both natural ecosystems and human society. However, growing global populations, resource scarcity, and environmental degradation have intensified water shortages in many regions. To address this critical challenge, scientific assessment and effective management of water resources are crucial. Recent water crises are largely attributed to global warming and human activities. Understanding the relative influence of these drivers is essential for developing effective mitigation and adaptation strategies. This study investigates the separate contributions of climate change and human activities to runoff changes in the Ahar Chay Basin, located in northwestern Iran, over the period 1989 to 2020. The Budyko framework was employed to quantify these effects. The Mann–Kendall and Pettitt tests were applied to detect long-term trends and identify significant change points in the hydroclimatic time series. Based on the detected breakpoints, the study period was divided into pre- and post-change sub-periods to isolate the effects of climate variability and anthropogenic interventions. Results from the Budyko-based analysis revealed that the reduction in upstream runoff was primarily driven by climate change (62–68%), while downstream runoff decline was mostly attributed to human activities (59.7–82.5%). The Sattarkhan Dam was found to play a major role in differentiating between climatic and anthropogenic impacts in upstream versus downstream areas.