Asynchronous responses of river–lake water quality following water diversion: A long-term assessment of the Niulanjiang-to-Dianchi project

Interbasin water diversion projects are widely implemented to alleviate water scarcity and improve aquatic environments, yet their long-term integrated responses remain insufficiently understood. In this study, we assessed water quality dynamics in the Niulanjiang–Dianchi Water Diversion Project (NDWDP), China, using a main quarterly monitoring dataset from 15 fixed sites during 2011 – 2021, with additional 2000 – 2010 records from two River Panlong stations used only as pre-operation riverine context. Key water quality parameters were analyzed using system-specific Water Quality Index (WQI) to characterize within-system monitored water-quality trajectories before and after water diversion. Overall, water quality improved following the implementation of the NDWDP. The WQI in the River Panlong increased by 57.60%, while that in Lake Dianchi increased by 27.11%, indicating improved monitored water-quality conditions within both subsystems. Distinct temporal response patterns were observed between riverine and lacustrine systems, with an earlier turning point in the River Panlong (2014) than in Lake Dianchi (2019), indicating an asynchronous integrated response between the river and lake systems. Seasonal analysis revealed consistently lower water quality during the rainy season than during the dry season, indicating that runoff-driven inputs and weaker rainy-season diversion reduced the apparent seasonal benefit of water diversion. Spatially, higher WQI values were observed in the source area of the River Panlong and the water-receiving region of Lake Dianchi compared with urban and non-receiving regions. The high chemical oxygen demand and nitrogen levels in the SA of the River Panlong, high TP, NH 4 + -N in the UA of the River Panlong, and high Chl-a during the rainy season in the River Panlong-Lake Dianchi corridor should be prioritized in future management. Collectively, our results demonstrate that long-term monitoring coupled with system-specific WQI formulations can effectively summarize decadal spatiotemporal water-quality patterns, identify asynchronous integrated responses, and support management-oriented evaluation in large interbasin water diversion systems.