Long-term variations and regional disparities in baseflow during 1960–2021 across China

Baseflow (Q_b), the slow-release component of streamflow, plays a vital role in sustaining river ecosystems and water supply during dry periods. However, its long-term spatial and temporal dynamics remain poorly understood across China. This study presents the first nation-scale investigation of baseflow variations across 288 Chinese catchments from 1960 to 2021. We initially evaluated seven widely used baseflow separation methods and identified the Eckhardt method performs best in most (91 %) catchments. Results reveal a distinct south-to-north decline in mean annual Q_b with averaging 317 mm in the south and 54 mm in the north. The proportion of baseflow to streamflow (BFI) increases from southeast to northwest (0.10–0.83), while the proportion of baseflow to precipitation (BFC) shows an increasing gradient from northeast to southwest (0–0.80). Significant intensification of spatial heterogeneity along the 1200 mm isohyet is observed, with northern catchments experiencing a decrease in Q_b and southern catchments showing an increase. Intra-annual Q_b distribution becomes more uniform in 59.0 % catchments. Meanwhile, BFI increases in 68.8 % catchments, highlighting growing baseflow contributions, while BFC declines in 60.0 % catchments, indicating reduced groundwater recharge and potential depletion risks. The key controls on Q_b include precipitation, soil and vegetation in humid catchments, and snow fraction and temperature in arid catchments; and rainfall regimes and topographic factors play significant roles in both partitions (BFI and BFC). These findings provide a comprehensive understanding of the spatial distribution and long-term trends of baseflow, and offer insights for evaluating climate change impacts on groundwater-surface water interactions, as well as guiding basin-specific water management strategies across China.