## 3 Results
### Spatiotemporal Characteristics of TSS Concentration
#### 3.2.2 The Temporal and Spatial Patterns of TSS Distribution in Hedi Reservoir  
The spatial and temporal distribution of the concentration of TSS in the reservoir area was studied by applying the three-band model to satellite imagery and plotting the temporal and spatial distribution of TSS in Hedi Reservoir. Based on the results of TSS inversion in the long-term sequence of the reservoir area (2014-2018), the average concentration of TSS and spatial pattern distribution in the reservoir area were analyzed and compared (Figure 5).  
Figure 5a reflects the distribution of the average concentration of TSS in the reservoir area during the five years, which is \"High in the upper reaches of the Hedi Reservoir and low in the downstream\", and the upstream concentration of TSS reaches 2.5 times in the downstream. From 2014 to 2018, the concentration of TSS in the reservoir area was between 7.92 and 60.67 mg/L, with an average of 16.06 mg/L. The average concentration of TSS in the reservoir area of Shijiao Town was 31.54 mg/L, and the concentration above 40 mg/L was farthest away at 2.1 km toward the south of Shijiao Town. As shown in Figure 5b, the quality of water in the reservoir area was stable in 2018, and its average concentration of TSS has remained at 14-20 mg/L. The concentration of TSS in the Hechun Town area is consistent with the changing trend of the whole reservoir area. On the contrary, the concentration of TSS in Shijiao Town changed significantly from 2014 to 2018 and reached a maximum of 40.46 mg/L in 2015. Until 2018, the concentrations returned to the normal level of the reservoir area (13.84 mg/L).  
Figure 4: A number of 35 measured spectra of typical water collected by ASD in the 2015 cruise over Hedi Reservoir, where the different color lines represent the different spectra of 35 water samples.  
The water quality in the study area was mainly influenced by the water in the Shijiao area where the TSS were the highest.  
To further reveal the temporal and spatial variation in the water quality in Shijiao Town, the average TSS gradient map in the region was analyzed at intervals of 20, 30, 50 and 80 mg/L (Figure 6). Overall, the average concentration of TSS in Shijiao Town was 31.54 mg/L between 2014 and 2018. In the first two years, the concentration of TSS increased and gradually spread downstream. The area of concentrations ranging from 50 to 80 mg/L was increased by 66.42%, accounting for 31.84% of the reservoir area of the region. On the contrary, in the last two years, the average concentration of TSS in Shijiao Town decreased to 22.41 mg/L, and the areas of 30-50 and 50-80 mg/L decreased by 84.48% and 96.88%, respectively. Besides, the area of the 0-20 mg/L range increased from 26.56% to 79.32%, and each gradient concentration distribution changed significantly. In the five years, the annual average concentration of TSS in the water body of Shijiao Town reached a maximum of 40.46 mg/L in 2015, and it reached the lowest value in 2018 and fell to 13.84 mg/L, which decreased by 65.79% and reached the first-level water quality standard. In fact, since September 2017, the local government has fully implemented a strong work plan for the river chief system, reflecting the good effectiveness of national policy implementation.  
Figure 5: The spatial distribution of TSS concentration based on OLI imagery in Hedi Reservoir, where different colored lines represent different boundaries of towns: (**a**) mapping of the averaged TSS concentration in Hedi Reservoir from 2014 to 2018, and (**b**) the comparison of the annual average TSS concentration in different areas within the reservoir. The color scale is the legend of the average TSS concentrations, in mg/L. The location of the red star in the picture is the government of Shijiao Town.