Water Quality Trend Analysis of the Sanshenggong Section of the Yellow River from 2011 to 2024 Based on Mann-Kendall Test and Fuzzy Comprehensive Evaluation

• • Mann-Kendall test identified 2013 and 2019 as abrupt change points; water quality showed non-significant improvement from 2013 to 2015, but significant improvement after 2016, indicating a delayed but sustained positive trend. • • Single-factor evaluation revealed that total phosphorus (TP) was the primary pollutant in 2011–2012; its concentration decline drove the water quality upgrade from Class III to Class II in 2013, demonstrating the effectiveness of TP control measures. • • CRITIC-entropy weight combination assigned the highest weight (28.96%) to permanganate index, highlighting it as the primary pollutant; its concentration reduction was the main driving force for water quality improvement, underscoring the need for targeted organic pollution management. • • The improved fuzzy comprehensive evaluation showed that the membership degree of Class III water dropped to zero in 2013, indicating stable improvement, but periodic rebounds in Class II membership suggested potential degradation risks, emphasizing the need for continuous monitoring.