## 4 Results
### Submergence Ratio Analysis  
The submergence ratio can document the spatiotemporal dynamic change of the water surface areas of Poyang Lake and is plotted in Figure 6. The area that was submerged 100% of the time was 1307.41 km\({}^{2}\), or 31.85% of the entirety of Poyang Lake; 876.07 km\({}^{2}\), or 29.04% was submerged between 80% and 100%; and 1605.61 km\({}^{2}\), or 39.11%, was submerged for less than 60% of the study period.  
The change characteristics of Poyang Lake had obvious regional differences. To expediently describe the change characteristics in different regions, Poyang Lake was roughly divided into four regions based on submergence ratios (Figure 6, Regions I-IV). As a whole, the submergence ratio was more than 90% for the majority of Regions I and IV, less than 40% for the majority of Region  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Region and Date** & **Category** & **QA (\%)** & **PA (\%)** & **UA (\%)** & **OA (\%)** & **Kappa** \\ \hline \multirow{2}{*}{Poyang Lake, 27 September 2016} & Water & 98.84 & 91.51 & 92.58 & \multirow{2}{*}{96.52} & \multirow{2}{*}{0.8981} \\  & Non-water & 99.67 & 97.93 & 97.61 & & \\ \hline \multirow{2}{*}{Poyang Lake, 9 September 2015} & Water & 99.18 & 94.18 & 93.41 & \multirow{2}{*}{96.16} & \multirow{2}{*}{0.9102} \\  & Non-water & 99.64 & 97.05 & 97.40 & & \\ \hline \multirow{2}{*}{Taihu Lake, 2 February 2016} & Water & 98.51 & 97.16 & 98.73 & \multirow{2}{*}{98.62} & \multirow{2}{*}{0.9690} \\  & Non-water & 99.19 & 99.37 & 98.57 & & \\ \hline \end{tabular}  
QA = quantity accuracy; PA = producer’s accuracy; UA = user’s accuracy; and OA = overall accuracy.  
\end{table}
Table 2: Confusion matrix for the SWI classification accuracy.  
Figure 4: The area of Poyang Lake from 24 May 2015 to 14 November 2016, with intervals of 12 days.  
III, and Region II had a complex submergence ratio. In Region II, there were many isolated lakes whose submergence ratios were more than 90% and had an area of 430.36 km\({}^{2}\) (27% of the total area in Region II), and there were many districts whose submergence ratios were less than 40% with an area of 717.88 km\({}^{2}\) (44% of the total area in Region II). During this study period, the variation rate in the changed area of Region II was the greatest among the four regions. The maximum variation rate was 80% between 12 April and 24 April 2016, with an average variation rate of 13% in Region II.  
Figure 5: The examined SWI classification accuracy based on OLI: (**a**) OLI true color (R band 4, G band 3, B band 2) imagery acquired on 27 September 2016 located in Poyang Lake; (**b**) classification results based on Sentinel-1A imagery acquired on 27 September 2016; (**c**) SWI classification accuracy based on OLI on 27 September 2016; (**d**) OLI true color imagery acquired on 9 September 2015 located in Poyang Lake; (**e**) classification results based on Sentinel-1A imagery acquired on 9 September 2015; (**f**) SWI classification accuracy based on OLI on 9 September 2015; (**g**) OLI true color imagery acquired on 2 February 2016 located in Taihu Lake, China; (**h**) classification results based on Sentinel-1A imagery acquired on 2 February 2016; and (**i**) SWI classification accuracy based on OLI on 2 February 2016.