## 3 Results
### Analysis on Driving Factors of TSS Change
#### 3.3.3 The Influence of Human Activities on TSS Concentration  
On the whole, due to the Shijiao Town being located in the middle reaches of the Jiuzhoujiang River and upstream of Hedi Reservoir, it is an important import source of water pollution in Hedi Reservoir from the upstream of the watershed. The water the Shijiao area generates has severe impacts on the water quality of the whole reservoir and the concentration of TSS there is the highest compared with other areas in Hedi Reservoir. According to the consequences of this study, there are two reasons to exemplify this phenomenon. On the one hand, Shijiao Town is located across the boundary of Guangdong and Guangxi Provinces and is affected by the water quality of the upstream Jiuzhoujiang River. On the other hand, field investigations and related research have shown that illegal sand mining activities in the region are frequent and the concentration of TSS is subject to the effect of large suspension. In 2016, the concentration of TSS in Xin'an Town began to increase, and the concentration of TSS reached 31.84 mg/L and then decreased to a normal level in 2018. After a historical view of the Google Earth image and field investigation (shown in Figure 10), it was found that there were large areas of eucalyptus planting and large-scale illegal sand mining activities in the area. Besides, eucalyptus has great economic benefits, and people are also exerting a subtle influence on the surrounding water environment while pursuing the economic benefits of eucalyptus. Particularly, eucalyptus deforestation and fertilization periodically have a serious impact on water quality [59], which is also an important reason for the sudden increase in the TSS concentration in Xin'an Town in 2016. Consequently, to further reveal how anthropogenic activities, especially the illegal sand mining activities, affect the concentration of TSS, this research also compared the differences in TSS in different seasons under such human interruption. The frequent sand mining activities in Shijiao Town make the concentration of TSS distinctly higher than that during the flood season. The reason is that the water level in the dry season is lower and affected by the large need for sand in the market, which is more conducive to the collection of sand for the period. The sand mining activity causes the TSS of the water in the reservoir area to resuspend, which promotes the increase in the concentration of TSS, accompanied by the decrease in water transparency. Just as the research in China's four largest freshwater lakes also draws similar conclusions, the concentration of TSS becomes high in the dry season and low in the flood period after sand dredging activities begin [7,37]. Figure 9: Comparison of time series variation of TSS concentration and daily precipitation in Shijiao Town, where the average TSS concentration is signed by orange triangles and the daily precipitation is signed by blue dots. Despite the fact that human-induced changes severely threaten the local water environment to bring about the problem of soaring TSS, the natural conditions, especially precipitation and seasons, also cause identical effects on TSS and they become the most significant factor to reveal the dynamic changes in TSS. Regularly, there is a significant seasonal variation in the effect of precipitation on the concentration of TSS. The TSS concentration in the upstream (Shijiao Town) is more affected by rainfall in the dry season with an average TSS of 33.66 mg/L than in the flood period with an average of 22.45 mg/L. The reason for this phenomenon can be attributed to the fact that the catchment in Shijiao is the TSS deposition area produced by the upper river discharge and the surrounding load runoff. This unavoidable phenomenon, where the TSS concentration before a strong rainstorm in the dry season is lower than the one in the rainy season, was also found in the study by [PERSON] et al [31]. The changing law of TSS depends on the two aspects of natural conditions and human activities.