Natural and anthropogenic imprints on seasonal river water quality trends across China

Climate change and human activities have redefined seasonal river water quality patterns, yet their respective impacts remain unclear. Here, we propose a novel trend-based metric, the T-NM index, to isolate asymmetric human amplification and suppression effects across 195 natural and 1540 managed watersheds in China (2006–2020). Consistent trends in 52–89% of watersheds suggest climatic dominance, while anthropogenic drivers intensified or attenuated trends by 22–158% and 14–56%, especially in summer. Four independent multivariable models simulated seasonal COD and DO concentrations. Attribution analysis showed that seasonal factors explained 47.08% of the variation, while rainfall (25.37%) and slope (17.40%) accounted for COD and DO changes in natural watersheds; in contrast, Shannon Diversity Index (11.58%) and Largest Patch Index (10.66%) dominated in managed watersheds. This study establishes a generalizable framework for distinguishing natural and anthropogenic influences, offering key insights for adaptive water quality management under future climatic and socio-economic transitions.