# Water Body Mapping Using Long Time Series Sentinel-1 SAR Data in Poyang Lake
## 4 Discussion  
### Analysis of the Influence Factors  
Although the test accuracy of WaterUNet is satisfactory, there are still some factors that influence its classification accuracy. Firstly, all the Sentinel-1 SAR data used in this work were used in Interferometric Wide Swath (IW) Mode [70], which allows a large swath width (250 km) and can cover Poyang Lake with one or two consecutive scenes. The IW mode images three sub-swaths using Terrain Observation with Progressive Scans SAR (TOPSAR) [70], and Poyang Lake is in the middle and far sub-swaths. Although the TOPSAR technique ensures homogeneous image quality throughout the swath, the backscatters near the edge of the sub-swaths are different and more pronounced in the VH  
Figure 16: The relationship between the water level and water area for Poyang Lake. The red line is for the fitted curve, with “\(y\)” for the water area and “\(x\)” for the water level. Figure 15: The water area series obtained from Sentinel-1 SAR data using WaterUNet for Poyang Lake from 2014 to 2021 and its histogram. The black dots are for the water area, and the blue line is for the trend of water area. The green bars are for the count in each water area interval (64 bins). polarization images (the blue box in Figure 17b), which can be observed in most data and may lead to the misclassification of water bodies (Figure 17). In addition, the scalloping effects in the Sentinel-1 TOPSAR SAR data will also affect the backscatter in radiometric quality in the order of 1-2 dB, especially in VH polarization (the ripples are shown in the yellow and green boxes in Figure 17b) [71; 72]. Another known problem in the use of SAR data for water body mapping is the floats and sediments. When the upstream flood flows into Poyang Lake, the floats or sediments will cause a change in the water surface's backscatter, especially in VH polarization. Figure 18 shows the Sentinel-1 SAR data acquired on 27 May 2020; 2 June 2020; and 8 June 2020, at the water levels of 14.6 m, 16.0 m, and 16.9 m. From 27 May 2020, the water level began to rise dramatically (red dots in Figure 2) due to the flow caused by upstream precipitation, which brought many floats and sediments, resulting in the change of the backscattering from specular reflection to Bragg scattering. Consequently, in the data acquired on 2 June, there were large areas where the backscatter was different from that of water surface due to the floats and sediments, leading to the misclassification of areas as non-water and resulting the omission of water bodies (Figure 18b). In addition, the boats on the lake will also lead to omission due to the strong backscatter caused by the hull's metal material (Figure 17b). [PERSON] et al. [73] mentioned the restraining factor of weather conditions (rain and wind) in C-band ENVISAT ASAR data. Wind-induced waves also affect backscatter for C-band Sentinel-1 SAR data [31] and cause the misclassification of water bodies. Figure 19 shows the SAR data acquired on 15 February 2020, when it was raining or snowing with a level 5 north wind around the lake ([[https://lishi.tianqi.com/](https://lishi.tianqi.com/)]([https://lishi.tianqi.com/](https://lishi.tianqi.com/)), accessed on 12 December 2021), Ducang County, and Poyang County, maybe with stronger wind over the lake surface. The stronger wind caused waves on the lake surface, which strengthens the backscatter from the water surface by altering the backscattering from specular reflection to Bragg scattering [31] and cause misclassification. Figure 17: The influences of subswath and scalloping effects on water extraction, taking the data acquired on 14 July 2020, as examples. The blue box in (**b**) shows the enhanced subswath edge, while the green and yellow box show the scalloping effect. The corresponding area in (**c**) shows water misclassified as non-water, with blue for water and green for non-water. (**a**) Sentinel-1 SAR data in VH/VV/(VH + VV); (**b**) Sentinel-1 SAR data in VH; (**c**) classification result.