Analysis of Spatio-Temporal Characteristics of Water Quality for Ecological Water Replenishment in the Gorge Section of Yongding River

As one of the key water bodies in the Beijing region, the water quality of the Yongding River directly impacts the ecological security and residents’ lives in China’s capital. The national government places high importance on the ecological environment of the Yongding River and initiated ecological water replenishment efforts in 2019. This study combines ecological water replenishment with flood season water volume segmentation, employing correlation analysis, principal component analysis, and cluster analysis to conduct an in-depth investigation of water quality monitoring data (2019–2023) from the mountain gorge section of the Yongding River. Results indicate that, temporally, the permanganate index, chemical oxygen demand, ammonia nitrogen, and total phosphorus in 2023 decreased by 7.67%, 11.75%, 38.05%, and 18.23%, respectively, compared to 2019. Spatially, water quality at the outlet of the mountain gorge section showed significant improvement compared to the inlet. Statistical analysis by period revealed that water quality during non-flood seasons without water replenishment was superior to that during non-flood seasons with replenishment, which in turn was better than during flood seasons without replenishment. This indicates relatively good original water quality in the Yongding River. Overall, the river’s water quality meets the Class III surface water environmental quality standards.