# Water Body Mapping Using Long Time Series Sentinel-1 SAR Data in Poyang Lake
## 3 Results
### Annual and Interannual Variation of Water Area  
After pre-processing all the Sentinel-1 SAR data, we used the trained WaterUNet to extract water body data and then obtained the water body distribution maps of Poyang \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Validation** & **Kappa** & **Accuracy** & **Precision** & **F1 Score** \\
**Dataset** & **Coefficient** & **Accuracy** & **Precision** & **F1 Score** \\ \hline Testing samples & 0.96 & 0.98 & 0.98 & 0.98 \\
11 July 2015 & 0.98 & 0.99 & 1.0 & 0.99 \\
21 April 2019 & 0.97 & 0.98 & 0.99 & 0.98 \\ Average & 0.97 & 0.98 & 0.99 & 0.98 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Accuracy assessment for WaterUNet classification results. Figure 10: Accuracy–loss curves for WaterUNet with a batch size of 128. Lake corresponding to all 324 SAR observations. Poyang Lake, as previously stated, has distinct wet and dry seasons. During the wet season, each small lake is connected to the main lake and water covers most of the study area. Due to the severe flood that took place in Poyang Lake basin on 20 July 2020, the water area reached its highest level for the past 6 years (2714.08 km\({}^{2}\)), covering 96.4% of the study area (Figure 10(a)). The water surface shrank throughout the dry season, and the small lakes were cut off from the main lake. On 4 January 2020, the water area reached its lowest value in the past 6 years (634.44 km\({}^{2}\)), covering 22.5% of the study area (Figure 10(b)). After obtaining the water body distribution maps corresponding to all 324 SAR observations, Equation (8) was used to calculate the inundation frequency (IF) for Poyang Lake. The spatial distribution of the IF factor can be used to reflect the spatial-temporal distribution features of water bodies. The longer the water body lasts, the greater the IF will be. The larger the water body extent is, the greater the IF will be. \[\textit{Inundation frequency}=\frac{\textit{Water Observation Times}}{\textit{Total Observation Times}}\times 100\% \tag{8}\]  
Figure 12 shows the IF distribution map corresponding to all the SAR data for our study area. From Figure 12, we can see that most of the study area is covered by seasonal water body area (areas with green color), becoming inundated by water and turning into mudflats and grasslands. The second larger part of the study area is a perennial water  
Figure 11: The maximum and minimum water area from 2014 to 2021. Blue is for water and green is for non-water. (**a**) maximum water area on 20 July 2020; (**b**) minimum water area on 4 January 2020.  
area (areas with blue color, and an IF greater than 0.75) that is covered by water almost all year round. The IF for some of the perennial water area is close to 1.0, meaning that these areas are always covered by water all year round. Some study areas are non-water areas (areas that are bright green with an IF close to 0.0), such as the small islands in Poyang Lake. According to the shape of perennial water bodies, water tanks and water transfer channels that link tanks inside the lake system can be identified. The spatial distributions of IF also highlight the different storage areas according to the duration of the water presence and their geographical location within the hydrological system. Some of the study area is mudflats (areas with orange color and an IF between 0.5 and 0.75). The lower the terrain is, the more likely it is to be flooded, and the IF distribution also reflects the topography of the lake basin [69]. Figure 12 shows that the IF generally varied along the gradient of the elevation, exhibiting maximum values within the deep flow channels and permanently flooded sub-lakes [59]. We conducted an annual statistical evaluation to study the yearly variation in the IF. In the years 2014 and 2021, the SAR data were acquired in the dry season, resulting in a lower IF for the study area. Conversely, in the year 2015, most of the SAR data were acquired in the wet season, resulting in a higher IF for the study area.