Differentiated effects of watershed land use patterns on water quality: a multi-buffer scale and seasonal coupling analysis

• Examined basins, showing land use-water quality links across seasons and scales • Found forests improve water quality, while croplands and urban areas degrade it • Identified seasonal and spatial impacts of land use on water quality • Proposed tailored management recommendations for each basin Changes in land use over space and time are key drivers of water pollution. However, current studies on land-use–water-quality relationships in small watersheds are insufficient to support regional development. Comparative research across large-scale watersheds can better inform water environmental protection, yet such studies remain limited. This study analyzes data from 100 sampling sites across four major watersheds in Zhejiang Province. Using multivariate statistical methods and redundancy analysis, it investigates the effects of land use patterns on water quality across seasons and spatial scales. Results reveal pronounced spatial and temporal heterogeneity among watersheds. In the Qiantang River Basin, pH remains relatively stable, while other indicators vary considerably. Reduced downstream flow, particularly during the dry season, promotes the accumulation of pollutants. During the wet season, water quality in the Feiyun and Ou River Basins is more strongly influenced by geogenic processes related to land use. The Feiyun River Basin, dominated by forests and grasslands, is susceptible to rainfall-induced erosion. In the Ou River Basin, land reclamation alters hydrodynamics and salinity, and precipitation intensification further intensifies land-use impacts. At the spatial scale, the 2000-m buffer exerts the strongest influence on water quality in the Feiyun, Ou, Yong, and Jiao River Basins, likely due to longer runoff pathways integrating multiple pollution sources. In contrast, in the Qiantang River Basin, the 500-m buffer is more influential during the dry season, while larger buffers dominate in the wet season. Overall, this study provides a scientific basis for watershed-specific land-use planning and water-quality protection, emphasizing policies tailored to distinct spatiotemporal dynamics.