# An Effective Water Body Extraction Method with New Water Index for Sentinel-2 Imagery
## 4 Results  
### Comparison of the Water Indices Enhancement Performance  
In order to compare the enhancement performance of NDWI and SWI, the mean values of water indices for the different water types (turbid water, floating ice, and salt water) and nonwater types (building, vegetation, and bare land) are shown in Figure 3. The mean value of the SWI for the water types ranges from 0.78 to 0.89, while that of NDWI in the water types ranges from 0.18 to 0.41. For the nonwater types, the mean values of the NDWI and SWI range from \(-\)0.24 to \(-\)0.07 and from \(-\)0.18 to \(-\)0.09, respectively. Compared with the mean value range of water and nonwatery bodies, the difference of the SWI is greater than that of the NDWI, which suggests that the SWI can achieve better enhancement performance. Water bodies enhanced by the NDWI and SWI in the four study areas are shown in Figure 4. Figure 4a is a composite image of the Taihu Lake area. The enhanced water bodies utilizing the NDWI and SWI are shown in Figure 4b,c, respectively. The NDWI and SWI can effectively distinguish most of the water bodies and the background objects are suppressed. Comparison of the results of the NDWI and SWI for Taihu Lake shows that the difference between water bodies and nonwater bodies using the SWI is more significant than that using the NDWI. This is because the reflectance of polluted water in Taihu Lake is lower in Band 11 than in Band 8 and the SWI can effectively enhance polluted water. Figure 3: Mean value of SWI and NDWI in different water and nonwater bodies. Turbid water is easily confused with buildings and bare land. Figure 4d shows the composite image of turbid water in the Yangtze River and the water body enhanced images of NDWI and SWI are shown in Figure 4e,f, respectively. Compared with Figure 4e,f, the performance of the NDWI cannot enhance turbid water from the background in the Yangtze River and coastal waters, but the SWI can effectively separate the water features from the background. Combined with the spectral curve in Figure 2b, it can be observed that the reflectance of turbid water in the green band (Band 3) is consistent with that of the general water body and the reflectance in the NIR band (Band 8) is larger than that in the general water body. Thus, the NDWI value of turbid water is smaller than that of general water bodies. For the SWI, the reflectance of turbid water in the vegetation-sensitive red-edge band (Band 5) is greater than that of general water bodies and the reflectance in the SWIR band (Band 11) is consistent with that of general water bodies. Compared with the general water bodies, the mean value of turbid water is 0.89 by using SWI calculations, which makes it easier to distinguish turbid water from buildings and bare soil. Chaka Salt Lake is a shallow salt lake. Figure 4g shows a composite image of Chaka Salt Lake. The enhanced images of the water body were obtained by using the NDWI and SWI (Figure 4h,i, respectively). The results show that the NDWI cannot effectively enhance most of the water body information, while the water bodies can be highlighted using the SWI. This is because the reflectance difference between the green band (Band 3) and NIR band (Band 8) is smaller than that between the vegetation-sensitive red-edge band (Band 5) and SWIR band (Band 11), which renders the mean value of the SWI larger than that of the NDWI in Chaka Salt Lake. The Sentinel-2A image of Chain Lake is shown in Figure 4j and flowing ice is present in Chain Lake. The enhanced images of the water body were obtained by using NDWI and SWI (Figure 4k,l, respectively). As we can see from the enhanced water body performance, the NDWI misses some of the floating ice, while the SWI can effectively highlight floating ice. Combined with Figure 2, the reflectance of floating ice is similar to that of general water bodies from the visible light band (Band 3 and Band 5) to the NIR band (Band 8) and the reflectance of floating ice is larger than that of general water.