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
The Sanshenggong section of the Yellow River is a critical hydrological monitoring and control point, whose water quality directly affects the ecological safety and sustainable water resource utilization of the middle and lower reaches. This study analyzed water quality monitoring data from 2011 to 2024 using the Mann-Kendall test to identify abrupt change years, combined with single-factor evaluation and a fuzzy comprehensive evaluation method improved by CRITIC-entropy weight combination to systematically assess water quality evolution. The Mann-Kendall test identified 2013 and 2019 as abrupt change points, with non-significant improvement from 2013 to 2015 and significant improvement after 2016. Single-factor evaluation indicated that total phosphorus (TP) was the primary exceeding factor in 2011–2012, and its declining concentration drove the water quality upgrade from Class III to Class II in 2013. The CRITIC-entropy weight combination assigned the highest weight (28.96%) to permanganate index, whose decline was the core driver of water quality improvement. The improved fuzzy 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. This study provides scientific evidence for ecological protection and high-quality development of the Yellow River Basin.