## 4 Results
### Soil Salinity Prediction and Accuracy Assessment
#### 4.4.4 Soil Salinity Prediction and Accuracy Assessment Using the RK Method  
The spatial distribution maps of soil salinity prediction of two measuring modes using RK with stepwise regression method and ordinary Kriging type are displayed in Figure 12. Essential statistical data of soil salinity prediction using RK of two measuring modes by sub-district are reported in Table 14.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Measuring Mode** & **ME (mS/m)** & **RMSE (mS/m)** & **R\({}^{2}\)** & **PBIAS (\%)** & **Number of Samples** \\ \hline HH & 633.2390 & 1240.7618 & 0.8538 & \(-41.7018\) & 121 \\ VV & \(-144.9735\) & 806.8829 & 0.8344 & 5.8155 & 121 \\ \hline \hline \end{tabular}
\end{table}
Table 13: Statistical data for accuracy assessment of CCK method.  
Figure 12: Spatial distribution of soil salinity prediction using RK of two measuring modes: (**a**) \(\mathrm{HH}\) and (**b**) VV.  
In Figure 12, the patterns of soil salinity distribution from two measuring modes at the sub-district level are slightly different. In Table 14, the top three sub-districts of ECe of the HH mode, Ban Wang, Non Thai, and Khang Phlu, show the highest mean values of soil salinity, with a value from about 2451 to 5058 mS/m, while the top three sub-districts, Sai-O, Thanon Pho, and Makha, display the lowest mean value of soil salinity, with a value from about 699 to 995 mS/m. Moreover, the top three sub-districts of ECe of the VV mode, Banlang, Ban Wang, and Non Thai, show the highest mean values of soil salinity, with a value from about 2767 to 3387 mS/m; the top three sub-districts, namely, Sai O, Dan Chak, and Makha, display the lowest mean value of soil salinity, with a value from about 1169 to 1481 mS/m. Unlike IDW, OK, and CCK, the CV value of the HH mode displays the lowest at Banlang, with a value of 16.59% and exhibits the highest at Sai O, with a value of 69.64%. Meanwhile, the CV value of the VV mode displays the lowest at Kampang, with a value of 16.21% and shows the highest at Sai O, with a value of 118.24%.  
At the district level, the mean ECe value of topsoil (HH) is 2156.02 mS/m, with an SD of 1574.99 mS/m and a CV of 73.05%. Meanwhile, the mean ECe value of subsoil (VV) is 2377.38 mS/m, with an SD of 1405.83 mS/m and a CV of 59.13%. Like IDW, OK, and CCK, the variation of soil salinity at the topsoil level is higher than at the subsoil level.  
Moreover, the results of the accuracy assessment of the RK method with the testing dataset are reported in Table 15. The ME and RMSE values of the HH mode are about 570.63 and 1318.15 mS/m, and those of the VV mode are about \(-\)420.68 and 1218.47 mS/m. The R\({}^{2}\) values of the HH and VV modes are 0.8280 and 0.6665, respectively. The PBIAS of the HH mode is about \(-\)37.58%, and that of the VV mode is 16.88%.  
When comparing ME values of two modes, VV has the lower error in the mean. Similarly, the RMSE value of VV is lower than that of HH. Likewise, the PBIAS value of VV is lower than that of HH. However, the R\({}^{2}\) value of VV is lower than that of HH and provides a moderate correlation between measured and predicted values.