# Penetration Analysis of SAR Signals in the C and L Bands for Wheat, Maize, and Grasslands
## 3 Data Analysis
### Temporal Behavior of the L- and C-Bands According to the SSM and NDVI for Wheat and grasslands
Figure 4a,b show very clearly that in the early season, approximately until the end of February (NDVI\(<0.7\)), the radar backscattering coefficient for wheat and grassland is dominated by direct scattering from the soil. The C-VV variation is due to the SSM variation: \(\sigma^{0}_{VV}\) increases following rainfall (i.e., increase in soil moisture) and decreases during a time span without rainfall due to soil evaporation (i.e., decrease in soil moisture). Between the end of February and the end of April (NDVI\(>0.7\)), the backscattering decreases with stem elongation due to the attenuation of the direct ground scattering. This decrease is due to vertical stems and leaves that produce high absorption of the incident SAR wave associated with weak direct scattering [43; 45]. The SAR signal reaches a minimum value (\(-17.8\) dB) on 20/04/2017 (NDVI \(\sim\)0.9) due to the strong attenuation produced by the longer stems. As a result, for the period extending from the end of February and the end of April, the ground backscatter attenuated by the canopy is the dominant mechanism [41; 42]. At the beginning of the heading phase and during grain filling (between beginning of May and the end of June), the backscattering values start increasing due to the increase in direct scattering from ears and later from grain [42; 43]. Therefore, for this period, the scattering from the upper canopy elements is the dominant mechanism [41]. From the end of June (NDVI\(<0.7\)), the backscattering becomes sensitive to SSM due to the decrease of canopy attenuation associated with an increase of direct ground scattering. This decrease of canopy attenuation is due to vegetation drying [45]. For the grassland plot, until the beginning of April (NDVI\(<0.7\)), the behavior of the SAR signal depends on soil moisture variations: it increases when the soil moisture increases following rainfall and decreases when the soil moisture decreases during days after a rainfall event. Between the beginning of April and the beginning of June (NDVI\(>0.7\)), the C-VV decreases with time and is insensitive to soil moisture, which indicates that the attenuation of the SAR signal by the canopy dominates the soil contribution (the soil contribution is negligible). From the beginning of June (NDVI\(<0.7\)), the SAR signal starts increasing due to vegetation drying and becomes sensitive to soil moisture. As a result, the C-VV backscattering is dominated by ground scattering when the NDVI of wheat and grasslands is less than 0.7, which corresponds to poorly developed and dry canopy cover. Finally, for an NDVI higher than 0.7 (very well-developed wheat and grassland cover), the C-VV is dominated by the vegetation attenuation and the soil contribution is negligible, which explains the insensitivity of the C-band to SSM for an NDVI\(>0.7\). Figure 5: \(\Delta\sigma^{0}\)(difference in \(\sigma^{0}\) between two successive dates) in the L and C bands versus \(\Delta\)SSM (corresponding difference of SSM) for reference plots of wheat and grassland with an NDVI greater than 0.7. Positive \(\Delta\sigma^{0}\) and \(\Delta\)SSM correspond to increases between two successive dates of \(\sigma^{0}\) and SSM, respectively.