[Water Quality Evolution in the Guishui River Section of Guanting Reservoir in Recent Years]

Based on manual and automatic monitoring data of water quality at the Gujiaying section of the upstream of Guanting Reservoir （Guishui River section） from 2017 to 2023， this study comprehensive applied the Mann Kendall statistical test， STL trend decomposition， grey prediction model， and emission inventory statistics and other analysis methods， combined with literature research， and systematically revealed the water quality evolutions， influencing factors， and the impact of epidemic control and rainstorm events on water quality in the upper reaches of Guanting Reservoir. The results showed that： ① The period from 2017 to 2023 was the best time for the upstream inflow water quality of Guanting Reservoir in 50 years. Since Guanting Reservoir was restored as a backup water source in 2007， the inflow water quality was significantly improved， rising from Class Ⅳ to Class Ⅲ， reaching Class II in some months. The most significant decrease in NH₄⁺-N concentration from 2017 to 2023 may be related to the strengthened control of agricultural non-point sources and domestic sources in Yanqing District in recent years. ② During the lockdown period from March to May 2022， the concentrations of the main water quality indicators at the inlet section of Guanting Reservoir were significantly decreased year-on-year and month-on-month， and the epidemic control measures significantly improved the water quality. During the "23·7" rainstorm， the water quality at the inlet section of Guanting Reservoir deteriorated sharply， and turbidity， TP， and permanganate index responded fastest to the sudden change of upstream flow. ③ Agricultural non-point sources and domestic sewage were the main emissions affecting the water quality of the upstream section of Guanting Reservoir， with both contributing over 99%. The predicted water quality by the end of the 14th Five Year Plan period would be generally stable in Class Ⅱ-Ⅲ. The next step should focus on the variations in water quality at the inlet section of Guanting Reservoir during flood and dry seasons. This study provides technical means for the follow-up pollution control and sustainable improvement of river ecological environment.