Key Takeaways & Executive Findings
- •• • 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.
Abstract
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.
1. Introduction
The Sanshenggong section of the Yellow River, located at the head of the Hetao Irrigation District, is a pivotal control point for water quality in the middle and upper reaches. Its strategic importance lies in its role as the water source for one of China's three largest irrigation districts, supporting agricultural productivity and regional ecological balance. However, rapid industrial and agricultural development has increased pollutant loads, posing significant challenges to water quality. Existing water quality assessment methods, such as single-factor evaluation and traditional fuzzy comprehensive evaluation, often suffer from subjectivity in weight assignment, limiting their accuracy and reliability. This study addresses these bottlenecks by integrating the Mann-Kendall test for trend detection with a CRITIC-entropy weight combination to objectively determine factor weights, thereby enhancing the robustness of water quality assessment.
The proposed methodology overcomes the limitations of conventional approaches by combining non-parametric trend analysis with an objective weighting scheme. The Mann-Kendall test is well-suited for non-normal, incomplete water quality data, while the CRITIC-entropy method reduces subjectivity in weight determination, ensuring that the fuzzy comprehensive evaluation accurately reflects the complex interactions among multiple pollutants. This integrated framework not only identifies critical pollution factors but also captures temporal dynamics, providing a scientific basis for targeted pollution control and sustainable water resource management in the Yellow River Basin.
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YU Yongkun, ZHANG Lieyu, DU Caili, GAO Shengwang, BAI Yangwei, WEI Yimei, TIAN Zhenjun, JU (2026). 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. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202508013
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Frequently Asked Questions
How does the CRITIC-entropy weight combination method improve the objectivity of fuzzy comprehensive evaluation compared to traditional subjective weighting?
Traditional fuzzy evaluation often relies on expert judgment for weight assignment, which can introduce bias. The CRITIC method determines weights based on the contrast intensity and conflict among indicators, while entropy weight reflects the information content of each indicator. By combining these two objective methods, the approach minimizes subjectivity, as evidenced by the permanganate index receiving the highest weight (28.96%), which aligns with its significant impact on water quality.
What are the specific water quality parameters and their trends that led to the classification upgrade from Class III to Class II in 2013?
The upgrade was primarily driven by a continuous decline in total phosphorus (TP) concentrations. In 2011–2012, TP was the main pollutant exceeding the Class III standard. After 2013, TP levels fell below the threshold, allowing the overall water quality to meet Class II criteria. This improvement was sustained, with no subsequent deterioration observed.
How reliable is the Mann-Kendall test for detecting abrupt changes in water quality time series with missing data or outliers?
The Mann-Kendall test is a non-parametric method that does not require data to follow a normal distribution and is robust to outliers and missing values. In this study, it successfully identified 2013 and 2019 as significant change points, consistent with known management interventions and pollution control efforts, thereby confirming its reliability for such analyses.
What are the practical implications of the periodic rebounds in Class II water membership for water resource management?
The rebounds indicate that while overall water quality has improved, there are periods of potential degradation. This suggests that continuous monitoring and adaptive management are necessary to prevent backsliding. The fuzzy evaluation method provides a nuanced view of water quality status, enabling early warning and targeted interventions.
Can the methodology be applied to other river sections or water bodies with different pollution characteristics?
Yes, the methodology is transferable. The combination of Mann-Kendall test and CRITIC-entropy weighted fuzzy evaluation is adaptable to various water quality indicators and environmental contexts. However, the selection of indicators should be tailored to the specific pollution sources and regulatory standards of the target water body.
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