Urbanization and climate extremes amplify upstream-downstream water quality disparities across Chinese urban...

Sustaining water quality in urban watersheds is critical for ecosystem health and human water security, yet the spatiotemporal dynamics of upstream-downstream water quality disparities under rapid urbanization and climate change are lacking. Here, we use high-frequency monitoring data (2021-2024) for six key water quality parameters (water temperature-WT, permanganate index-COD_Mn, total nitrogen-TN, total phosphorus-TP, turbidity-TUR and electrical conductivity-EC) to assess multi-year and seasonal upstream-downstream water quality disparities (Δ) and their responses to heatwaves and extreme precipitation events across 65 urban watersheds in China. We find that over 92% of watersheds exhibit statistically significant upstream-downstream disparities for each parameter, with downstream water quality generally poorer than upstream conditions. These disparities display pronounced seasonal variability, with ΔWT, ΔCOD_Mn, and ΔTP peaking in summer and ΔTN and ΔEC being higher in winter. We further identify urbanization as a primary anthropogenic factor contributing to increasing upstream-downstream disparities and show that larger urban water-body areas and higher intra-urban vegetation cover can partially mitigate downstream water quality degradation. In addition, watershed characteristics, including slope, hydrological connectivity, and shape complexity, govern the magnitude of multi-year and seasonal disparities across urban watersheds. Extreme climatic events further amplify upstream-downstream disparities, with heatwaves increasing ΔTN, ΔTUR, and ΔEC, while extreme precipitation affects all parameters and exerts a greater impact on ΔTUR than heatwaves. Our study underscores the necessity of integrating anthropogenic pressures, watershed characteristics, and climate extremes into urban water quality management strategies, providing a basis for targeted interventions under ongoing urbanization.