## 4 Results
### Soil Salinity Severity Classification  
The derived soil salinity prediction of the HH and VV modes using CCK and IDW in Figures 11a and 9b, respectively, were classified as soil salinity severity according to FAO standards (see Table 2), as shown in Figure 13. The area of soil salinity severity classification of the two measuring modes by sub-district and district is summarized in Table 18.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Methods** & **ME (mS/m)** & **RMSE (mS/m)** & **R\({}^{2}\)** & **PBIAS (\%)** \\ \hline \hline \multicolumn{5}{c}{**HH measuring mode**} \\ \hline IDW & 663.1821 & 1308.5117 & 0.8422 & \(-\)43.6737 \\ OK & 632.8475 & 1247.7234 & 0.8526 & \(-\)41.6760 \\ CCK & 633.2390 & 1240.7618 & 0.8538 & \(-\)41.7018 \\ RK & 570.6292 & 1318.1468 & 0.8280 & \(-\)37.5786 \\ \hline \hline \multicolumn{5}{c}{**VV measuring mode**} \\ \hline IDW & \(-\)80.6340 & 803.9182 & 0.8354 & 3.2346 \\ OK & \(-\)145.4183 & 807.1316 & 0.83436 & 5.8334 \\OCK & \(-\)144.9735 & 806.8829 & 0.83443 & 5.8155 \\ RK & \(-\)420.6817 & 1218.4730 & 0.6665 & 16.8754 \\ \hline \hline \end{tabular}
\end{table}
Table 16: Comparison of model performance from four interpolated methods of two measuring modes for soil salinity prediction.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & \multicolumn{4}{c}{**Score**} & \multirow{2}{*}{**Total Score**} \\ \cline{2-2} \cline{4-5}  & **ME** & **RMSE** & **R\({}^{2}\)** & **PBIAS** \\ \hline \hline \multicolumn{5}{c}{**HH measuring mode**} \\ \hline IDW & 1 & 2 & 2 & 1 & 6 \\ OK & 3 & 3 & 3 & 3 & 12 \\ CCK & 2 & 4 & 4 & 2 & 12 \\ RK & 4 & 1 & 1 & 4 & 10 \\ \hline \hline \multicolumn{5}{c}{**VV measuring mode**} \\ \hline IDW & 4 & 4 & 4 & 4 & 16 \\ OK & 2 & 2 & 2 & 2 & 8 \\ OK & 3 & 3 & 3 & 3 & 12 \\ RK & 1 & 1 & 1 & 1 & 4 \\ \hline \hline \end{tabular}
\end{table}
Table 17: The total score of four methods for identifying a suitable method for salinity prediction in the two measuring modes.  
Figure 13: Spatial distribution map of soil salinity severity classification with a suitable method of two measuring modes in each sub-district. **(a)** HH and **(b)** VV.  
For the HH mode using OCK, the area of the non-saline class is only found in two sub-districts, Dan Chak and Ban Wang, and it covers an area of about 11.25 sq. km. The top three sub-districts with slightly to moderately saline classes are Non Thai, Banlang, and Dan Chak, with areas varying from 12.05 to 24.21 sq. km. In contrast, three sub-districts, Khang Phlu, Kampang, and Samrong, display the minor area of slightly to moderately saline classes varying from 1.59 to 8.25 sq. km. Meanwhile, the top three sub-districts with strongly to very strongly saline classes are Non Thai, Banlang, and Khang Phlu, with areas varying from 56.50 to 82.97 sq. km. On the contrary, three sub-districts, namely Dan Chak, Sai O, and Thanon Pho, display the minor area of strongly to very strongly saline classes, varying from 12.78 to 33.16 sq. km.  
For the VV mode using OK, the area of the non-saline class is only found in one sub-district, Dan Chak, and it covers an area of about 0.54 sq. km. The top three sub-districts with slightly to moderately saline classes are Makha, Ban Lang, and Dan Chak, with areas varying from 9.25 to 21.55 sq. km. In contrast, three sub-districts, namely, Samrong, Khang Phlu, and Sai O, display the minor area of slightly to moderately saline classes varying from 0.17 to 2.53 sq. km. Meanwhile, the top three sub-districts with strongly to very strongly saline classes are Non Thai, Banlang, and Khang Phlu, with areas varying from 62.36 to 83.80 sq. km. On the contrary, three sub-districts, Dan Chak, Sai O, and Thanon Pho, display the minor area of strongly to very strongly saline classes, varying from 26.14 to 33.16 sq. km.