In the discussion of climate impacts, 1.5 and 2°C have become iconic values. This study examines the impacts of 1.5 and 2°C global warming on water availability, runoff seasonality, and extreme monthly and daily runoff in two catchments, using the semidistributed hydrological model Hydrologiska Byråns Vattenbalansavdelning-D, based on a combination of five global climate models (GCMs) and four representative concentration pathways (RCPs). Subsequently, quantitative assessments were made for projection uncertainties from GCMs and RCPs. The two catchments are the Yiluo River catchment (YLC) in northern China and the Beijiang River catchment (BJC) in southern China. The results indicate wetter flood seasons for YLC and warmer mean annual temperatures, drier springs, and more severe floods over long return periods (25 and 50 years) for both catchments. The change magnitude of most indicators is expected to be larger in YLC than in BJC. Mean annual temperatures in both catchments are expected to have smaller changes under the 1.5°C scenario than under the 2.0°C scenario. However, the change magnitude of the other hydrological variables is projected to be approximately equivalent for both catchments under both scenarios. Uncertainties of projected impacts from GCMs are generally larger than those from RCP scenarios, for both catchments and warming scenarios, with the exception of mean annual temperature of BJC. These findings indicate that effective measures are required to Climatic Change (2017) 145:145-158 DOI 10.1007/s10584-017-2072 Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10584-017-2072-3) contains supplementary material, which is available to authorized users.