Contrasting roles of climate change and land-use change on runoff dynamics in a semi-humid monsoon watershed of China

This study examines the separate and combined effects of climate variability and land-use change on historical and future runoff dynamics in the Dongwan Watershed, the largest sub-basin of the Yihe River, China. Runoff was simulated using the Soil and Water Assessment Tool (SWAT), future land-use change was projected with a Cellular Automata-Markov (CA-Markov) model, and climate forcing was derived from five CMIP6 GCMs under SSP126, SSP370, and SSP585. The SWAT model performed well during calibration and validation, with Nash–Sutcliffe Efficiency values of 0.87 and 0.85. Attribution results show climate variability accounted for 121.1% of the historical annual runoff increase between 1971–1990 and 1991–2010, with land-use change exerting a compensating effect (−25.4%). Annual runoff increased by only 3.0%, but seasonal shifts were pronounced: summer runoff rose 21.7% while winter (−32.2%), spring (−8.1%), and autumn (−8.9%) all declined. This pattern reflects higher flood risk in wet months alongside increasing water scarcity in dry ones. Future projections indicate further amplification of these seasonal contrasts, with increasing summer runoff dominance and reduced dry-season buffering capacity. Continued forest expansion and the near loss of grassland and shrubland moderately dampen climate-driven runoff increases but cannot offset intensified climatic forcing. Overall, climate change emerges as the primary control on runoff dynamics, while projected land-use change under business-as-usual trajectories plays a secondary but moderating role. These findings offer insights into seasonal water management and climate adaptation in semi-humid watersheds of China.