Abstract: Accurately depicting the spatial and temporal distribution characteristics of base flow in the upper Yangtze River is crucial for the rational development and utilization of water resources, particularly as it serves as a significant recharge source for river runoff during dry season. To achieve this, we combined the digital filtering method and the distributed hydrological model SWAT to analyze the time characteristics of base flow. Additionally, we employed a spatial interpolation method to describe the spatial distribution of base flow in the upper Yangtze River. The results demonstrate that the value and modulus of average annual base flow in the upper Yangtze River ranged from 5 to 9 500 m³/s and 0.36 to 28.00 L/(km²·s) during the period from 1990 to 2000. Furthermore, we found a clear positive correlation of annual runoff and base flow with annual precipitation, whereas the base flow index (BFI) shows a distinct negative correlation with annual precipitation. The BFI exhibits relatively stable inter-annual variation, with a variation within the year showing a higher base flow during the dry season compared to the wet season. This indicates the crucial role of base flow in supplying rivers. In terms of seasonal variation, summer sees the largest base flow, followed by autumn, winter, and spring in sequential order, reflecting that base flow in wet season is larger than that in dry season. Moreover, the base flow in the upper Yangtze River basin increases progressively with an increase in runoff generation and concentration area.

Key words: baseflow, dry season, digital filter method, SWAT model, the upper Yangtze river