Understanding the water quality change of the Yilong Lake based on comprehensive assessment methods

•WQI and ETI methods can help understand the water quality change of Yilong Lake.•The water quality of Yilong Lake was considered as generally “medium”•The driving factors of water quality in different periods are revealed.•DO, CODMn, EC, TN, Hg, Chla are the key water quality parameters of Yilong Lake. To understand the water quality changes of the Yilong Lake and reveal the driving factors of water quality changes in different periods, in this study, based on 13 water quality parameters, the water quality of Yilong Lake from 2008 to 2018 was comprehensively evaluated by water quality health comprehensive assessment method (ETI) and water quality index method (WQI), and 6 key parameters were determined by WQI method to develop a more convenient WQImin comprehensive evaluation model. The results showed that: (1) Yilong Lake’s single-factor water quality evaluation result was inferior to category V, CODCr and TN had a greater impact on the water quality evaluation results; while the ETI and WQI evaluation methods show the water quality of Yilong Lake is at a moderate level (53.58 ± 7.46 and 0.53 ± 0.13). Among them, nutrients such as nitrogen and phosphorus have the greatest impact on WQI and ETI values. (2) The comprehensive evaluation method reveals that the water quality changes of Yilong Lake can be divided into two stages. Among them, the water quality of lakes declined from 2008 to 2013, and the main driving factors are the increase in industrial sewage discharge and the increase in pollution load caused by tourism development; and from 2014 to 2018, lake water quality showed an improving trend, and the spatial change shows that the water quality of the west is slightly better than the middle and the east, which is mainly due to the increase of the lake's self-purification ability, the implementation of various treatment measures, the water supplement project and the location of pollutant discharge; (3) Based on the WQI method, six key parameters of DO, CODMn, EC, TN, Hg, and Chla were determined and the Yilong Lake WQImin model was developed. The water quality evaluation results of the WQImin model considering the weight are more accurate, and the water quality change characteristics of Yilong Lake can be understood simply and quickly.