## 3 Results
### PLSR Models for Estimating Soil pH and EC with HSI-Resampled Spectra
#### 3.3.3 PLSR Model Inversion Using HSI Images  
With the PLSR models, the soil pH and EC are derived from the bare soil pixels of HSI images. From Table 5, the maximum estimated pH and EC values were 14.65 and 35.72 dS/m, respectively. These values were reasonable and accepted. The minimum estimated pH and EC values were 1.78 and \(-\)55.09 dS/m, respectively. The low and negative values were not reasonable for models. In fact, most of these pixels are in water or wet soil area. The effectiveness of applying the PLSR models to the HSI image was tested against the measured and estimated values. Six sampling sites (marked in Figure 1) are used to assess the accuracy of the estimated soil pH and EC. All the sampling sites were in grassland and farmland near the Zhalong Wetland and rivers. In one sampling site, the ground-measured values of four samples from the triangular cluster plots were averaged to represent the value in an area of 100 m\({}^{2}\). The relationship between the ground-measured values of pH and EC against corresponding image-estimated values from the PLSR models showed the accuracies of model inversion. For the pH level, the linear trend indicated a good agreement between measured and estimated values with RMSE = 1.09 (Table 5). For the EC, the linear trend indicated a low agreement between the measured and estimated values with RMSE = 17.30 (Table 5). Notably, all the image-estimated EC values were negative as a result of the non-reliable PLSR model.  
Figure 4 maps eight alkaline and saline classes of soils in accordance with the above-mentioned pH-EC levels in different color codes. Water and vegetation were separated from bare soils with NDVI < 0.05 and NDVI > 0.3, respectively. All these non-soil land covers are also color coded in Figure 4. Statistically, the alkaline and saline soils covered 46.24% of the mapping area. Slightly and moderately alkaline soils were 24.28% and 16.48% of the mapping area, which were the dominant alkaline and saline classes (88.15% of all the alkaline and saline classes), respectively. The four classes (strongly alkaline, strongly alkaline and slightly saline, strongly alkaline and moderately saline, and strongly alkaline and strongly saline) accounted for 5% of the mapping area. Notably, the two classes (moderately alkaline and slightly saline and moderately alkaline and moderately saline) were not identified in the abovementioned pH-EC levels. The two classes were negligible at 0.31% of the mapping area. In addition, the water, vegetation, and non-affected soil were 4.83%, 2.93% and 46.00%, respectively. In Figure 4, the alkaline and saline soils are near the rivers' entrance to the mouth of the Zhalong Wetland. Compared with the distribution of alkaline soils, the saline soils were rare and overlapping on the strongly alkaline soils.  
\begin{table}
\begin{tabular}{c c c c} \hline  & \multicolumn{2}{c}{**Map Inversion**} & \multicolumn{2}{c}{**Validation**} \\ \cline{2-4}  & **Maximum** & **Minimum** & **RMSE** \\ \hline pH & 14.65 & 1.78 & 1.09 \\ EC (dS/m) & 35.72 & \(-\)55.09 & 17.30 \\ \hline \end{tabular}
\end{table}
Table 5: Performance statistics of PLSR model inversion from HSI images.