## 3 Results and Discussion
### Relationships between Radar Signals and Vegetation Parameters
Previous results have shown that the radar signal decreases with an increase in the LAI for narrow leaf crops (wheat, alfalfa, and barley), and increases for board leaf crops (sunflower, corn, sorghum, and sugarcane) [91,93]. For example, [PERSON] [87] studied the sensitivity of radar signals in C and X bands to the LAI of wheat crops (soil moisture values between 0.05 and 0.20 cm\({}^{3}\)/cm\({}^{3}\)) in using HH and VV polarizations (20\({}^{\circ}\) for the C band and 40\({}^{\circ}\) for the X band). In this study, the signal decreased approximately 8 dB in the X-band for LAIvalues between 0.1 and 3 m\({}^{2}\)/m\({}^{2}\), and the signal decreased approximately 12 dB in the C-band for LAI between 0.1 and 4 m\({}^{2}\)/m\({}^{2}\). The signals at C and X bands increase approximately 7 dB with LAI for LAI values up to 8 m\({}^{2}\)/m\({}^{2}\). Similar results on the behavior of X-VV radar signal according to LAI of wheat were found by [PERSON] _et al._[59] for soil moisture values between 0.05 cm\({}^{3}\)/cm\({}^{3}\) and 0.20 cm\({}^{3}\)/cm\({}^{3}\). [PERSON] _et al._[88] analyzed wheat crops with LAI values between 1 and 4 m\({}^{2}\)/m\({}^{2}\) in wet soil conditions (soil moistures between 0.20 and 0.40 cm\({}^{3}\)/cm\({}^{3}\)); the radar signals in the X and C bands decreased with the LAI by approximately \(-\)2.6 dB by 1 m\({}^{2}\)/m\({}^{2}\) for X-HH and \(-\)2.4 dB by 1 m\({}^{2}\)/m\({}^{2}\) for C-VV. The C-HH and C-HV signals have lower sensitivity, with about \(-\)1 dB by 1 m\({}^{2}\)/m\({}^{2}\). [PERSON] _et al._[92] demonstrated that the radar signal at Ku-VV (50\({}^{\circ}\)) increases with the LAI of corn and sorghum, up to LAI of approximately 2 m\({}^{2}\)/m\({}^{2}\); beyond this, the radar signal saturates. [PERSON] _et al._[89] observed increasing radar responses in sugarcane plots when the sugarcane LAI increased (with the C-band and incidence between 31\({}^{\circ}\) and 39\({}^{\circ}\)). This increase is greater in HV than in HH due to higher volume scattering in HV compared to HH. In a study by [PERSON] _et al._[90], the radar signal in the C band (24\({}^{\circ}\) and 47\({}^{\circ}\)) increased with the LAI in soybean and corn crops. Figure 10 shows the behavior of the SAR X-band signal according to HVE using different soil moisture classes. The results showed that for vegetation heights lower than 50 cm, the radar signal decreases with HVE. This decrease in the radar signal for vegetation heights lower than 50 cm is higher in HV polarization than in HH polarization (approximately -0.5 dB and -1.1 dB by 10 cm in HH and HV, respectively). The attenuation is stronger for HV than for HH due to the vertical plant stems, and the attenuation increases with stem height [81, 82, 86, 94]. Consequently, the soil contribution to the total backscatter is lower at HV than at HH polarization. [PERSON] _et al._[88] showed that for wheat height is between approximately 3 cm and 65 cm, the radar signal at X-HH decreases by 1.3 dB as the HVE of wheat increases by 10 cm. Beyond 50 cm, the radar signal slightly increases at both HH and HV when head element flowers begin to appear at the top layer of grassland vegetation (Figure 9). When plants are higher than 50 cm, the backscattered signal is mainly due to the leaves, stems and the head element flowers [94]. According to Figures 6 and 7, the soil also contributes slightly to HVE higher than 50 cm. The increase in the radar signal from HVE above 50 cm is greater in HV than in HH due to the greater contribution of vegetation in HV than in HH. Figures 11 and 12 show the behavior of the SAR X-band signal according to BIO and VWC using different soil moisture classes. Result shows that the HH polarization appears to be insensitive to fresh biomass and vegetation water content. The results also showed that HV slightly decreases with BIO and VWC (BIO and VWC are well correlated) to a threshold about 1.5 kg/m\({}^{2}\), then increases slightly.