Study region: Nine representative watersheds across northern China, encompassing mountain forest, steppe, and sandy coarse sand watersheds. Study focus: To quantify climate change and human activities' impacts on runoff dynamics in arid/semi-arid regions, using the Mann-Kendall test and an ABCD-snowmelt hydrological model to disentangle their relative influences. New hydrological insights for the region: With the exception of a transient increase in dry-season runoff in the Miandu, all watersheds exhibited significant declines in annual, wet-season, and dry-season runoff. At the monthly scale, runoff dynamics in March and April were strongly affected by spring snowmelt, with climate change emerging as the dominant driver in steppe and sandy coarse sand watersheds. Seasonal analysis indicated that spring and summer runoff variations were largely controlled by climate change, while winter runoff reflected comparable contributions from climate change and human activities. At the annual scale, runoff changes in mountain forest watersheds were primarily attributed to climate change (73-86 %), whereas steppe (43-65 %) and sandy coarse sand watersheds (62-80 %) were more strongly influenced by human activities. This study identifies the spatiotemporal heterogeneity of runoff responses to climate change and human activities across watersheds with different underlying surface conditions. The findings provide a clear scientific basis for developing targeted ecological restoration measures and sustainable water-resource management strategies.