Evaluation of water quality and its driving forces in the Shaying River Basin with the grey relational analysis based on combination weighting.

The water quality of the Shaying River Basin and even the entire Huai River Basin has been widely concerned. Based on the water quality data acquired in flood and non-flood seasons from 2012 to 2016, the Shaying River Basin was selected as the research object. First, the principal component analysis method was used to identify the main pollution indices. Then, grey relational analysis combined with an analytic hierarchy process and entropy weight method was used to evaluate the water quality of the upper, middle, and lower reaches of the Shaying River Basin, while the single factor evaluation method was used for comparative analysis. Finally, the driving forces of water quality were analyzed and discussed from natural and human aspects. The results show that the main pollutants in the Shaying River Basin are total nitrogen, total phosphorus, and ammonium nitrogen. While the basin is seriously polluted by nitrogen and phosphorus, the spatial and temporal distribution of the pollution varies, although the overall trend toward improving water quality conditions is significant. The midstream region had the poorest water quality, which fluctuated between Classes III and V. The downstream region had generally good water quality, which could be ranked as Class III most of the time. And the upstream region had the best water quality with well-developed ecological conditions; all the water samples were ranked as Class I or II. The water quality improves significantly during the flood season when compared with that in the non-flood season. Seasonal climate variation, non-point source pollution emissions, the release of water from sluices and dams, and water resource management activities are the main reasons for the variations in water quality across the Shaying River Basin.