Hydrological seasonality drives a predictable reversal in anthropogenic pollution dominance in mountain watersheds.

Seasonal hydrology can fundamentally reorganize the drivers of ecosystem function. Here, we uncover a seasonal reversal of pollution dominance (SRPD) mechanism in six monsoonal watersheds of China's Taihang Mountains. By coupling the water quality index (WQI) with principal-component and redundancy analyses, we develop a diagnostic framework that decodes this dynamic. Human-intensive basins show significantly greater seasonal parameter volatility than forested ones. Cropland explains 90.9% of constrained variance in dry seasons, whereas impervious surfaces emerge as the key driver in wet seasons (75.1%). The derived SRPD axis explains 54.5%-91% of pollution variance. Spatial hotspots of seasonal degradation, identified via a ΔWQI (difference between wet-season and dry-season WQI) model, highlight the ZH and HTH watersheds as most vulnerable. Our framework transforms seasonal monitoring data into a quantifiable signal of shifting anthropogenic forcing, offering a tool for dynamic ecosystem assessment and seasonally adaptive management in monsoon-affected regions globally.