Multi-scale landscapes influence seasonal water quality variations in a semi-arid basin: Mechanistic insights and...

River water quality degradation poses critical challenges in semi-arid regions due to intensifying human activities and limited water resources, making an understanding of scale-dependent landscape-water quality relationships essential for effective watershed management. To address this, our study quantified multi-scale relationships between landscape patterns and seasonal water quality variations in the Dahei River Basin, northern China, identifying critical spatial scales and landscape thresholds that trigger abrupt water quality changes. Utilizing water quality data from 17 monitoring stations (2021-2022) and land use datasets, we applied redundancy analysis, generalized additive models, and structural equation modeling to examine these relationships across multiple spatial scales (0.5-10 km buffers) and seasons. The results revealed pronounced spatiotemporal variations, with water quality in hilly area significantly better than that in urban and irrigation areas. The 5-km buffer emerged as the optimal spatial scale, explaining 61.15% of water quality variation. Evident seasonal differences were also observed: grassland proportion dominated wet season quality (41.9% explanation), while cropland proportion controlled dry seasons (45.8% explanation). Critical thresholds occurred at 40-60% cropland proportion, 30-50% grassland proportion, and 99.2-99.5% landscape connectivity. Furthermore, path analysis identified impervious surfaces and croplands as pollution sources, whereas grasslands and forests functioned as purification sinks. Configuration metrics exhibited indirect effects by mediating land use-water body coupling. Ultimately, these findings demonstrate that landscape effects on water quality are highly scale- and season-dependent in semi-arid watersheds. The identified thresholds provide actionable targets for precision-based management, suggesting the implementation of season-differentiated strategies, such as protecting grasslands during wet seasons and controlling agricultural expansion during dry periods, thereby offering quantitative guidance for similar regions.