## 4 Experimental Result and Discussion
### Submerged Rice Area Estimation  
The tropical storm Son-Tinh landed around 19 July 2018, accompanied by heavy rain, causing serious flooding. So, temporal water maps for three days afterwards using S1A images available on 22 July 2018 (Figure 9(a)), 3 August 2018 (Figure 9(b)), and 15 August 2018 (Figure 9(c)) were created. The affected rice map was produced by stacking temporal water and rice maps together. It can be seen that most of the rice-growing areas were inundated during the stormy period, which is shown in Figure 9(a). Depending on the terrain and weather conditions, rice in areas with rapid drainage will be able to recover after storms. However, for areas with long-term inundation, rice will have less resilience and could be considered damaged rice (Figure 9(b),c). The flood occurred during the storm Son-Tinh mainly in agricultural areas. Submerged rice maps and flood maps are quite similar because rice is the main agricultural crop in the region. The GSO reported serious rice damages by flooding at the locality at different times (i.e., 21 July 2018 in Bac Ninh, 29 August 2018 in Thai Binh, 23 August 2018 in Ha Noi, 8 August 2018 in Nam Dinh and 3 August 2018 in Ninh Binh). We relied on the statistical report's dates to make a comparison of affected rice from our estimation. The corresponding S1A flood maps on 22 July 2018 for Bac Ninh and Nam Dinh, on 15 August 2018 for Thai Binh and Hanoi, and on 3 August 2018 for Ninh Binh, were created. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**OA(\%)** & **F1—Water** & **F1—Non-Water** & **Kappa** \\ \hline
90.94 & 0.88 & 0.93 & 0.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Results of water map assessment with inspection data. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Measures** & **Water** & **Non-Water** \\ \hline Precision & 0.80 & 0.99 \\ Recall & 0.98 & 0.87 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Precision and recall of permanent water extraction model. Figure 9: Some typical cases of misclassified water regions. Figure 10: Visualization of flood map with permanent water map (**left column**) and rice map with submerged rice map (**right column**) on 22 July 2018, 3 August 2018 and 15 August 2018, for 10 provinces in the RRD. For Tien Du, Thuan Thanh and Luong Tai districts in Thai Binh province, the differences between the affected rice area and the statistics were 1519 ha, 1114 ha, 1375 ha, respectively. For Tien Hai, Thai Thuy and Kien Xuong districts, the total difference was less than 2676 ha. In Hanoi and Ninh Binh, estimations were higher than the statistics of 4086 hectares and 6611 hectares. In Nam Dinh, the largest difference was observed at 19,866 hectares (Table 10). In general, the affected rice area from S1A data tended to be larger than statistical reports. The GSO surveyed and made reports after the storm. Besides, the fieldwork had many difficulties due to bad weather conditions, those may cause errors in estimation. Otherwise, fast assessment of affected rice areas caused by flooding from remote sensing will yield immediate and convincing results. The affected rice maps of the RRD on 22 July 2018, 3 August 2018, and 15 August 2018, were created to evaluate rice damage levels. The affected rice area decreased over time depending on local drainage conditions. As can be seen in Table 11, about of 260,971 hectares of rice were inundated directly after the storm made its landfall on 22 July. The inundated area was reduced to 142,054 hectares and 57,343 hectares on 3 August 2018 and 15 August 2018 for 10 provinces in the RRD, respectively. Long-term flooding made rice waterlogged and then the rice needed to be re-cultivated in some areas. As of 15 August 2018, almost one month after the storm, there was still about 57,343 hectares (10.6%) of rice remaining inundated in the RRD which was mostly in Hai Phong (7773 ha), Ha Noi (6411 ha), Nam Dinh (17,514 ha), Ninh Binh (12,578 ha), and Thai Binh (5315 ha). In these areas, rice might be completely damaged and replanted.