There is convincing evidence that climatic variation and human activities are key factors impacting the hydrological behavior of a watershed. But little attention has been paid to such impacts on mesoscale and microscale forest basins, and key details remain unresolved. In this study, a small forest watershed in north China was selected as our study object. Separate evaluation and statistical methods were used to assess the impacts of climatic variation and human activities on changes in river discharge. The Mann-Kendal test indicated that annual rainfall did not vary significantly, whereas the annual river discharge decreased significantly at 99% confidence interval during 1981-2011. Through separation evaluation, it was determined that climatic variation was the strongest contributor to changes in annual river discharge, with the average contribution reaching 66.8%, while the remaining contributor was total human activities (33.2%). The influence of various types of human activities on river discharge reduction is quite different. Forest recovery has a more positive impact on river discharge decrease, which accounts for about 89.24% of the discharge decrease caused by total human activities. Compared to the impact of climatic variation, it is inferred that large-scale forestation in this region may not be the main reason for the reductions in river discharge. Results provide important benchmarks for assessing the significance of afforestation on regional water resources, and also provide an objective assessment for improving future forestry development strategies in north China.