## 4 Experimental Result and Discussion
### Submerged Rice Area Estimation
In Ha Nam, Hung Yen, Vinh Phuc and Bac Ninh, the inundated rice areas are smaller than 3000 ha on 15 August 2018. It may be due to their higher elevations and better drainage systems than other provinces. However, Vinh Phuc, at a high elevation, still has severe flooding which causes serious damage to some areas (rectangle A in Figures 11 and 12). For Quang Ninh, with the RoN of 55, the estimated number is 10,121 hectares and 11,119 hectares on 20 July 2018, and on 1 August 2018, respectively. There are no available data for 13 August 2018. To assess the extent of flooding impacts not only during the storm, we also produced the affected rice map for the whole summer-autumn season using nine S1A images and stacked them up to create flood frequency maps for the rice area (Figure 11). Flood frequency maps will help to visually assess severely affected rice areas and future flooding risk. Areas with a high frequency of flooding (>5 times) will be considered seriously damaged rice areas while areas with a flood frequency of 4 to 5 times are vulnerable by flooding in the future. Some areas such as Lap Thach, Vinh Phuc (Figure 11, A rectangle), Chuong My, Quoc Oai, Hanoi (Figure 11, B rectangle), Nho Quan, Ninh Binh (Figure 11, C rectangle) and Nam Dinh (Figure 11, D rectangle) have the highest flooding risk. Those regions were reported as serious flooding areas with largely damaged rice areas and human life lost during the Son-Tinh storm. Figure 12 illustrates the high correspondence between the rice-flood frequency map and the actual flooding areas in Lap Thach, Vinh Phuc. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Location** & **Mapped Data (ha)** & **Affected Rice Map Date** & **Statistical Data (ha)** & **Reported Date** & **Diff (ha)** \\ \hline Tien Du & 3019 & & 1500 & & 1519 \\ Thuan Thanh & 4014 & 22 July 2018 & 2900 & 21 July 2018 & 1114 \\ Luong Tai & 3375 & & 2000 & & 1375 \\ Tien Hai & 2379 & & & 29 August 2018 & \textless{}2676 \\ Thai Thuy & 411 & 15 August 2018 & \textgreater{}2000 & & \\ Kien Xuong & 1886 & & & & \\ Ha Noi & 12,069 & 3 August 2018 & 7983 & 23 August 2018 & 4086 \\ Nam Dinh & 38,373 & 3 August 2018 & 31,762 & Before 8 August 2018 & 6611 \\ Ninh Binh & 26,797 & 22 July 2018 & 6931 & 3 August 2018 & 19,866 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Comparison of affected rice area reported by GSO statistics our proposed method. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Affected Rice Area by Flooding Caused by the Son-Tinh Storm Landing in RRD Region**} \\ \cline{3-8}
**Province** & **Rice Area** & \multicolumn{2}{c}{**22 July 2018**} & \multicolumn{2}{c}{**3 August 2018**} & \multicolumn{2}{c}{**15 August 2018**} \\ \cline{3-8}  & & **Ha** & **\%** & **Ha** & **\%** & **Ha** & **\%** \\ \hline Bac Ninh & 34,306 & 23,103 & 67.3 & 10,070 & 29.3 & 1728 & 5 \\ Hai Duong & 56,449 & 3220 & 5.7 & 9031 & 16 & 2709 & 4.8 \\ Hai Phong & 34,355 & 20,087 & 58.4 & 16,832 & 49 & 7773 & 22.6 \\ Ha Nam & 33,451 & 15,912 & 47.5 & 4454 & 13.3 & 515 & 1.5 \\ Ha Noi & 90,675 & 37,961 & 41.8 & 12,069 & 13.3 & 6411 & 7 \\ Hung Yen & 32,433 & 17,385 & 53.6 & 2227 & 6.9 & 404 & 1.2 \\ Nam Dinh & 78,868 & 55,786 & 70.7 & 38,373 & 48.6 & 17,514 & 22.2 \\ Ninh Binh & 44,615 & 26,797 & 60.1 & 23,408 & 52.5 & 12,578 & 28.1 \\ Thai Binh & 80,320 & 55,730 & 69.4 & 23,110 & 28.7 & 5315 & 6.6 \\ Vinh Phuc & 29,574 & 4990 & 16.9 & 2480 & 8.4 & 2396 & 8.1 \\ Quang Ninh & 27,165 & - & - & - & - & - & - \\
**Total** & **542,211** & **260,971** & **48** & **142,054** & **26** & **57,343** & **10.6** \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Imundated rice area estimation for 10 provinces during the Son-Tinh storm occurrence. Figure 11: Flood frequency map for summer-autumn rice in 2018. **(A)** Lap Thach, Vinh Phuc, **(B)** Chuong My, Quoc Oai, Hanoi, **(C)** Nho Quan, Ninh Binh, **(D)** Nam Dinh.