Significant differences in optimal riparian buffer zone on water quality between different segments within the same river

Determining the optimal riparian buffer zone based on the relationship between landscape metrics and water quality is a widely used method for water quality management. However, failing to account for the differences between various segments of the same river can lead to inaccurate identification of riparian buffer zones, thereby affecting the effectiveness of water quality improvement. Here, based on water quality monitoring data from January 2023 to December 2023 in different segments (JuRong segment and XieXi segment) of a typical traditional-intensive agricultural watershed (Qinhuai River watershed), we identified the differences in the optimal riparian buffer zone on water quality between different segments within the same river through redundancy analysis (RDA) and variance partitioning analysis (VPA). Subsequently, utilizing the nonparametric change-point analysis (nCPA), we further identified the critical landscape thresholds causing abrupt changes in water quality within the optimal riparian buffer zone. Results showed that in the JuRong segment, the optimal width for riparian buffer zones was 100 m, with landscape metrics explaining 96.7% of the water quality variation. In the XieXi segment, the optimal riparian buffer zone width was 600 m, with landscape metrics explaining 82.6% of the water quality variation. Interspersion and Juxtaposition index of water (IJI_Water) and Interspersion and Juxtaposition index of Garden (IJI_Garden) were found to be the most influential landscape metrics on water quality in the JuRong and XieXi segments, respectively. Furthermore, the landscape thresholds of IJI_Garden and IJI_Water resulting in abrupt changes in water quality were 68.50 and 39.88 in the JuRong segment, and 76.07 and 56.39 in the XieXi segment, respectively. This study highlights the importance of considering the varying effects in optimal riparian buffer zones and developing distinctive water quality management strategies between different segments within the same river.