Quantitative Analysis of the Driving Factors of Water Quality Variations in the Minjiang River in Southwestern China

The Minjiang River is an important first-level tributary of the Yangtze River. Understanding the driving factors of water quality variations in the Minjiang River is crucial for future policy planning of watershed ecology protection of the Yangtze River. The water quality of the Minjiang River is impacted by both meteorological factors and anthropogenic factors. By using wavelet analysis, machine learning, and Shapley analysis approaches, the impacts of meteorological factors and anthropogenic factors on the permanganate index (CODMn) and ammonia nitrogen (NH3-N) concentrations at the outlet of the Minjiang River Basin were quantified. The observed CODMn and NH3-N concentration data in the Minjiang River from 2016 to 2020 were decomposed into long-term trend signals and periodic signals. The long-term trends in water qualities showed that anthropogenic factors were the major driving factors, accounting for 98.38% of the impact on CODMn concentrations and 98.18% of the impact on NH3-N concentrations. The periodic fluctuations in water qualities in the Minjiang River Basin were mainly controlled by meteorological factors, with an impact of 68.89% on CODMn concentrations and 63.94% on NH3-N concentrations. Compared to anthropogenic factors, meteorological factors have a greater impact on water quality in the Minjiang River Basin during both the high-temperature and rainy seasons from July to September and during the winter from December to February. The separate quantification of impacts of driving factors on the varying water quality signals contributed to the originality in this work, providing more intuitive insights for the assessment of the influences of policies and the climate change on the water quality.