# Soil Texture Estimation Using Radar and Optical Data from Sentinel-1 and Sentinel-2
## 3 Results
### Satellite Data Sensitivity to Soil Texture
#### 3.1.2 Optical Indices and Sensitivity Analysis  
For the purposes of soil texture analysis, two soil-related indices were also tested, using the data recorded in certain S-2 bands, in an attempt to distinguish between different textural classes. These indices are described as follows:  
Simple ratio Clay Index (SRCI)  
The SRCI is defined as the reflectance ratio of two SWIR bands, namely B11 (SWIR 1) and B12 (SWIR 2), derived from S-2 images [48; 49]. It is expressed as:  
\[SRCI=\frac{SWIR}{SWIR}\frac{1}{2} \tag{1}\]  
The SRCI was computed for the images recorded on 04/08/2017 (the driest day, and the date on which bands B11 and B12 had their highest sensitive to clay content). Although the overall trend is for the SRCI to decrease with increasing clay content, these variables are not strongly correlated, with R\({}^{2}\) equal to just 0.19 (Figure 6). Brightness Index (BI)  
The BI [50] combines information from the Red and Near-Infrared bands, represented by bands B4 and B9, respectively, in the case of S-2 data. This index provides an indication of changes in the average level of reflectance. The BI is used to assess whether an observed surface is bright or dark, and is often used for the mapping of soil characteristics, such as roughness, texture, salinity, and moisture [51; 52; 53; 54]. Dry soils with low clay content tend to be strongly reflective, whereas dark soils have low radiometric values in both bands, due to their high clay content. The BI is expressed as  
\[BI=\sqrt{R^{2}+NIR^{2}} \tag{2}\]  
where \(R\) and \(NIR\) are the red and near infrared bands, respectively  
Although the BI is used to distinguish between bright and dark zones, which in principle correspond to sandy and clayey soils, respectively, Figure 7 shows that in the case of the present study, this index does not provide a clear separation between these classes of soil. The low value of R\({}^{2}\) shown in this figure is partly due to confusion between played and clay soils. Figure 6: Clay content as a function of the simple ratio clay index (SRCI). 3.1.3. Analysis of the Sensitivity of Soil Moisture to Clay and Sand Content  
Soil texture is a function of its constituent clay, silt, and sand fractions, and can be strongly affected by water retention and infiltration. Conversely, soil moisture is strongly influenced by the texture of the soil--sandy soils have a porous texture that acts as a filter, allowing water to percolate. Although this soil type is common towards the North-East of the Kairouan Plain, the loamy clay soils dominating the study area retain rain and irrigation water more readily, thanks to their compact texture. This type of soil thus dries more slowly than sandy soil, and can remain wet for several days. The method applied for this analysis involves the monitoring of dynamic changes in the soil moisture profile (directly related to rainfall events), and relies on the observation of a relationship between the soil's mean level of moisture and its texture (clay and sand contents). For this analysis, 14 soil moisture maps were derived from radar and optical data. As soil moisture levels are generally higher for clay soils (slow drying) than for sandy soils (fast drying), we analyzed data recorded during the autumn, between September and December, since this corresponds to a period characterized by relatively strong precipitation events on the Kairouan Plain. The period following the month of December is less suitable for this analysis, as it corresponds to the beginning of the agricultural season for most crops, and is generally accompanied by irrigation, which could affect the soil's natural drying regime. The Figure 8a,b represent soil moisture mean values derived from the soil moisture maps retrieved the neural networks algorithm based on the inversion of the backscattering models (WCM and IEM, as function of the clay content from the ground measurements.