# Sensitivity Analysis of Multi-Temporal Sentinel-1 SAR Parameters to Crop Height and Canopy Coverage
## 3 Results
### SAR Parameters and Their Correlation with Crop Coverage  
The CC measured in field includes measuring the row and plant cover by steel tape in unit area. For validation of the CC calculated from measured data, photographs were taken using the camera in downward position and perpendicular to the ground with 100 cm distance from the camera lens. The CC extraction process is done by application of Python glob image processing package [46]. The photos are first converted to HSV (Hue, Saturation, and Value) model and used as a detector for the type and shape and to do color constancy processing by grouping or classifying the image. Following the HSV conversion green mask is applied to slice the green areas as white ratio and black to the bare soil. The sample classified green area for wheat is given in Figure 10. The unit area from the photograph is calculated using the ground sampling distance (GSD) formula and setting calibration derived from images. The contributing parameters for determination of ground resolution are the camera's height above the ground, the camera's pixel size, and the lens' focal length.  
The coefficient of determination (R2) between Sentinel-1 polarimetric SAR parameters and the measured CC for maize, sunflower and wheat for early and later growing stages are given in Table 4. According to the principle growth stage [20] when the maize reaches to the end of its stem elongation, leaves completely unfold and have full size. When the BBCH-scale is greater than 39 for maize, CC is measured 75%. The CC for sunflower and wheat is measured as 85% and 60%, respectively when the BBCH-scale is greater than 51 and 49.  
Considering Sentinel-1 dual polarimetric SAR parameters and measured CC, the highest correlation is obtained for the VV polarization (R2 = 0.73) and VV+VH (R2 = 0.73) when the CC is lower than 75% for maize while with increasing CC the correlation decreased (R2 = 0.49) for both crops. This is contrary to the findings of [PERSON] et al. [17] where the sensitivity of RADARSAT-2 polarimetric SAR and its correlation with fractional vegetation cover (FVC) in HV polarization is high for maize, and they suggest that VV polarization is not a useful parameter for monitoring broad-leaf crops. Their findings may have conflict with our results due to the sensors properties. Regarding the sunflower, in its early stage (i.e., CC \(<\) 85%), again the higher coefficient of determinations are obtained for VV polarization (R2 = 0.46) and VV + VH (R2 = 0.47). However, comparing the maize and the sunflower as two different broad-leaf crops in our study, R2 of the sunflower is lower than the maize. The discrepancy in correlations might be due to the difference in leaves geometry of maize and sunflower. In other respects, similar to the findings of [PERSON] et al. [17], no correlation is observed for wheat at both stages in our study. This may be due to the wheat structure and leaves geometry as narrow-leaf crop. Wheat can reach its full development at early stage and penetrating from biomass  
Figure 10: Canopy coverage extraction from in-situ photo taken using python image processing packages. Left panel (**a**) is the original photo and the right panel (**b**) shows white ratio calculated as green areas and black to the bare soil in wheat sample field.  
occurs due to narrow leaves, stem affectation and contributing of underlying soil resulting attenuation of SAR backscatter [47]. Figure 11 depicts the parameters which have highest correlation with CC during the growth stage for maize and sunflower.