## 3 Results
### Analysis on Driving Factors of TSS Change
#### 3.3.2 Effect of Precipitation on the Concentration of TSS  
Normally, precipitation may generate some effects on water quality, where especially heavy rain and the surface runoff can flow into the lake or reservoir and produce a turbid phenomenon in the water body, even posing a threat to the local water environment.  
In the five years, the TSS concentration changes are smaller in the whole reservoir area. As we can see from the results shown in Figure 5b, the concentrations of TSS in Hechun Town and Xin'an Town near the dam were below 20 mg/L (13.16 and 17.64 mg/L, respectively). However, the temporal and spatial distribution of upstream Shijiao Town changed significantly and the average concentration of TSS was as high as 47.67 mg/L. Besides, because Shijiao Town is located at the junction of Guangdong and Guangxi Provinces, it has accepted the pollution from upstream mining and frequent human economic activities, which seriously impacts the water quality of the area. Therefore, taking this typical area as an example, combined with precipitation and referring to the level of precipitation (GB/T 28592-2012, published by Standardization Administration of China), we analyzed the feature of TSS change. In practice, we obtained the accumulated rainfall data of two weeks before the date of satellite imaging and used the average value as the daily precipitation. Taking the acquired multi-source satellite imagery of the five years as a time series, the daily precipitation and the trend of the corresponding concentration of averaged TSS in the region are plotted (Figure 9). Obviously, during the dry season, the daily precipitation is negatively correlated with the change in the TSS concentration within the reservoir area of Shijiao Town. The concentration of TSS in this area decreases with the increase in daily precipitation. It is not difficult to find that, in Figure 9, the rainfall decreased from 17.90 on 27 September 2014 to 3.87 mm 13 October 2014, but the TSS concentration increased from 27.98 to 36.97 mg/L from strong mankind activity for same period. However, in the flood period, the daily precipitation change is consistent with the trend of the TSS concentration, which is positively correlated. From Figure 9, the rainfall increased from 7.17 on 12 July 2015 to 28.31 mm on 14 September 2015, while the TSS concentration increased from 17.17 to 33.25 mg/L. In fact, suspended matter in water shows a slow settling process [58]. Due to the influence of different rainfall intensity, the settling process of TSS will change, and the TSS concentration will produce different variations. When the rainfall intensity is small, especially in the dry season, the concentration of TSS in water from mankind activity of upstream is greater than the resuspension rate caused by rainfall, so the TSS concentration is negatively correlated with rainfall during the dry season. On the contrary, the rainfall intensity in the flood season is greater, and the settling rate of TSS in water is less than the resuspension rate caused by rainfall, so the TSS concentration increases with the rainfall intensity.  
Figure 8: Comparison of annual average TSS concentrations in the reservoir area of Shijiao Township from 2014 to 2018 in different seasons.