Understanding the responses of the hydrological cycle and extreme events to climate change is essential for basin water security. This study systematically assessed climate change impacts on runoff and floods in the Fujiang River basin, which is the main tributary of the upper Yangtze River, China, using the Soil Water Assessment Tool (SWAT) driven by the latest climate simulation of 14 General Circulation Models (GCMs) under two Shared Socioeconomic Pathways (SSP2-4.5 and SSP5-8.5). This study indicates a generally warmer and wetter climate projected in the Fujiang River basin, and correspondingly an overall increase in projected ensemble annual mean runoff, monthly runoff, monthly high flow (Q05), and monthly low flow (Q95) in the periods of 2021–2060 and 2061–2100, with the long-term period being more substantial than that of the near future, especially for SSP5-8.5. However, the projected changes in monthly runoff show a large spread across GCMs, with greater increases mainly occurring in the early rainy season. Most of the GCMs show that projections of Q95 will substantially increase compared to Q05. The intensity and frequency of floods with a 30-year return period are likely to increase, especially under SSP5-8.5. Despite the uncertainties in projected future changes in runoff, these findings highlight the complexity of runoff response to climate change, promoting the need for adaptive water resource management.

China is committed to achieving carbon neutrality before 2060. This study projected the changes in climate and runoff in the source region of the Yellow River Basin for 2021–2060 under lower carbon emission pathways (SSP1–2.6 and SSP2–4.5) using a statistically downscaled climate dataset and the SWAT hydrological model. Results showed that the climate will become warmer and wetter from 2021–2060. In comparison with the baseline period (1995–2014), in terms of the ensemble mean, annual mean air temperature, annual precipitation, and annual runoff will increase by 1.3 °C and 1.6 °C, by 11.1% and 11.2%, and by 12.8% and 11.9% under SSP1–2.6 and SSP2–4.5 scenarios, respectively. Moreover, the seasonal pattern of runoff was projected to change. The proportion of monthly runoff to the annual total will decrease by 0.6–1.0% in summer but increase by 0.1–1.0% during the period from January to April and September to December. The multimodel ensemble mean (MEM) of extremely high monthly flow (Q10) will increase by 3.5–13.4% in the flood season (June to August) and water storage season (September to December). The MEM of extremely low monthly flow (Q90) will increase by 19.4–26.2% from February to April but decrease by 5.0–8.9% in January, May, and December. Thus, the warmer and wetter climate from 2021–2060 will likely cause flatter seasonal distribution of runoff, lower risk of water scarcity at the annual scale and of drought from February to April, but higher risk both of flood in the flood season and of drought in December, January, and May. Generally, the flatter pattern of runoff would likely alleviate water scarcity in the dry and water storage seasons to some degree, and the increase in monthly runoff in the water storage season will benefit hydroelectric power generation and agriculture and animal husbandry production. However, in some years, the increase in Q10 in the flood season will likely increase flood prevention pressure, and the decrease in Q90 in May will likely obstruct grass revival.