Research findings concerning the main processes influencing water resources differ substantially, and so the topic remains controversial. Recent studies indicate that the changes in water yield, expressed through the n-parameter of Budyko framework, are associated with vegetation coverage changes. Here, we use runoff measurements and outputs from 13 dynamic global vegetation models, to investigate the underlying drivers of the n-parameter changes. Unlike previous studies, we instead find that climate change is the primary driver of adjustments on water resources. Changing climatic characteristics, particularly the intensity and seasonality of rainfall, modulates the runoff generation process. Indirect effects of climate change occur through altering vegetation properties, which in turn also impact river flow. We also find that in the arid and sparse vegetation regions, water yield is more sensitive to changes in n-parameter. Thus, the Budyko framework provides a reliable parameter-sparse representation of runoff changes, and reveals that terrestrial water cycle is changing substantially under climate change. This climate forcing requires on-going investigation to generate more refined and reliable projections of future water availability.