# Soil Texture Estimation Using Radar and Optical Data from Sentinel-1 and Sentinel-2
## 3 Results  
### Satellite Data Sensitivity to Soil Texture  
#### 3.1.1 Sentinel-2 Sensitivity Analysis  
Clay minerals and carbonates have characteristic absorption bands in the SWIR [47], which correspond to bands 11 and 12 of the S-2 satellite. Figure 4 plots the reflectance of the S-2 bands over three classes of soil observed in the study area from ground soil measurements (with soil clay contents lying in one of the following 3 ranges: Class 1 for 15% to 30%; Class 2 for 30% to 45% and Class 3 for 45% to 60%). For each class, we compute the mean value of the measurements recorded over the reference fields. The optical data selected for sensitivity analysis was recorded in the summer (July-August 2017) over bare soils only. The summer period was considered to be the best suited to these observations, as it is well separated from the end of the winter cereal production season, with no vegetation, during the autumn period. For many fields, this period can be extended from July to October. For this reason, only those crops with an NDVI lower than 0.25 were selected. As this takes place more than four weeks  
Figure 4: Soil reflectance observed in the Sentinel-2 (S-2) spectral bands, for three different soil textures (distinguished by different ranges of clay content) on the Kairouan Plain site: Class 1 (15–30%), Class 2 (30–45%) and Class 3 (45–60%). after wheat harvesting, there are no crop residues remaining on the surface of the fields, we are rather in a context of low presence of straw, indeed this effect could appear in early July. Moreover, surface spectral data are also generally affected by the confounding effects of soil moisture and soil roughness. For instance, some colored soils, revealed by optical data, could be explained by high roughness dating from later summer plowing. The summer period, which is the driest in the year, also has the advantage of minimizing reflectance noise that can be produced by changes in soil moisture, following rainfall events. The soil reflectances derived for the spectral bands observed by Sentinel-2 are compared, as shown in Figure 4, in order to determine those, which are the most sensitive to soil texture: the B11 and B12 bands can be seen to have the highest sensitivity to variations in clay content. The reduced reflectance at 2190 nm (B12) may be caused by a loss of crystallinity in clayey minerals. This analysis was repeated on eight different dates, between July and August. Bands B11 and B12 are found to have similar linear regressions, and nearly identical values of R\({}^{2}\), and the reflectance in these two bands can be seen to decrease when the clay content increases. Table 4 lists the coefficients of determination computed between clay content and reflectance in these two bands, for each date of observation, showing that the observations with the highest coefficients of determination (close to 0.4) were made on 4 August 2017 (Figure 5), and the lowest \(P\)-value which shows a statistically significant correlation. The lowest correlation was observed for the month of July with the lowed coefficient of determination (under 0.35) and almost highest \(P\)-value. The observations made on this date were thus selected, for the computation of all soil indices, as described in the following sections. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Dates** & **R\({}^{2}\) (B11)** & **\(P\)-value (B11)** & **R\({}^{2}\) (B12)** & **\(P\)-value (B12)** \\ \hline
05/07/2017 & 0.29 & 0.0013 & 0.30 & 0.0011 \\
10/07/2017 & 0.31 & 0.0009 & 0.32 & 0.0003 \\
20/07/2017 & 0.32 & 0.0008 & 0.39 & 0.00008 \\
25/07/2017 & 0.30 & 0.0011 & 0.32 & 0.0004 \\
30/07/2017 & 0.35 & 0.0003 & 0.39 & 0.00008 \\
04/08/2017 & **0.39** & **0.00002** & **0.4** & **0.00006** \\
19/08/2017 & 0.38 & 0.0001 & 0.38 & 0.0001 \\
24/08/2017 & 0.38 & 0.0001 & 0.37 & 0.0001 \\ \hline \hline \end{tabular}
\end{table}