*5.2. Crops Phenology Monitoring*

The Sentinel-1 temporal backscattering coefficient profiles of diverse land cover types at VV and VH dual-polarizations from the study area during the growing season of the winter crops (from 1 October 2018 to 1 September 2019) are shown in Figure 9; the temporal profiles of the mean σvv and σvh values of urban, vegetation (including other crops), water, bare soil, winter wheat, and winter barley land cover are displayed. As shown in Figure 9, besides the profiles of the water area, which fluctuate significantly due to the weather conditions, the temporal profiles of the vegetation, urban, and bare soil are much more stable than the profiles of winter crops are, which have a significant fluctuation according to their different growth stages. Especially in the σvh profile, the vegetation, urban, and bare soil profiles are generally close to their mean value, regardless of the season. Nonetheless, the variation of the backscattering coefficients of the two winter crops are clearly evident, for example, a peak is seen in early December, followed by a minimum value in early summer and a maximum value in midsummer. Thus, the results indicate that it is feasible to distinguish winter crops from other types of land cover, particularly vegetation and other crops, and furthermore, we are able to identify and study the main phenological stages from germination to ripening (harvesting) by using Sentinel-1 temporal profiles.

**Figure 9.** Sentinel-1 temporal backscattering coefficient profiles of different land covers (vegetation, water, urban area, bare soil, winter wheat, and winter barley) in the study area at VV and VH polarizations from 1 October 2018 to 1 September 2019.

Based on prior knowledge and field research with local farmers, winter wheat and winter barley are both cereal crop types that are planted from October to November. Generally, winter barley is sowed earlier than winter wheat in the Finistère department. Germination, which is the first growth stage of the crops, takes place three to four weeks after sowing, hence, this is in early December for winter wheat and in mid-November for winter barley. The crops remain in their vegetative stage during winter, and stem elongation begins in spring, and it lasts until the plants reach their maximum height, usually in early summer. Lastly, ripening, the final growth stage, and harvesting occur in summer (early summer for winter barley and mid-summer for winter wheat).

In Figure 10, both the raw signal and smoothed trend line of the temporal backscattering coefficient profiles of VV, VH, and the VH/VV ratio for the 2018–2019 growing season are displayed. Looking at the charts, it is shown that large variations occur before the germination due to the interaction between the bare soil and vegetation that is caused by stem-ground double scattering [6,70], while previous research suggests that the fluctuation in the backscattering profiles is mostly induced by changes in soil water content and roughness [6]. Pursuant to previous research, germination as the first stage of emergence of the plant can be recognized as the first maximum value of the profiles before they begin decreasing [25], therefore the germination stage is observed at around 1 December for winter wheat, and in early November for winter barley. Moreover, for winter wheat, this phase is best observed with VV and the VH/VV polarizations as they represent the first peak of the curves; however, the peak is better illustrated at VV and VH polarizations for winter barley. Afterwards the overwintering stage occurs, and the crops remain in their vegetative stage during winter (generally around 1 January); a gentle decreasing and a slight flattening can be observed in the VV polarization curves during this stage for both the crops. Furthermore, a fluctuation of the VV and VH curves of the two crops at around 1 January 2019 is driven by a short pause of rainfall, as the signals are highly affected by the soil water content. The stem elongation stage starts in spring, where the vertical development of the stems and leaves of the plants cause soil scattering attenuation, represented as a continuous and steadily decreasing line, until they reach the heading stage, where the plants achieve their maximum height. After a long decreasing phase, σ◦ reaches the minimum value of the temporal profiles at the heading stage at around 1 May 2019 for both winter crops, and this stage can be better observed in σvv and σvh/vv for winter wheat phenology, and in σvh and σvh for winter barley. However, the sharp decrease in σvv and σvh at the heading stage, specifically in the profiles of the winter barley might be the result of the relative lack of rainfall that occurred after early April. After heading, the inflorescence emergence, anthesis, grain development, and dough development stages occur. As seen on the graphs the curves start to increase during the flowering and grain development stages. These stages are illustrated by a sharp increase in the winter barley, regardless of the polarization, while by contrast the σvv and σvh/vv of the winter wheat shows a smooth increase. At last, the ripening stage, which is the maturation stage, occurs and the crops are ready to be harvested. This phase is shown as the last peak of the profiles during the growing season, followed by a sharp decrease which is caused by the absence of volume and multiple scattering after the harvesting [25]. As the results show, harvesting, which took place around 1 August 2019, is better demonstrated by σvv and σvh for winter wheat, while the harvesting stage was in late June for winter barley, and it is clearly shown by all polarizations, particularly in VV and VH.

**Figure 10.** Winter wheat and winter barley Sentinel-1 temporal backscattering coefficient profiles at VV, VH, and VH/VV polarizations of the northern part of the Finistère region for the 2018–2019 growing season, with the daily precipitation data and three main phenological stages, which are presented by a vertical line as well.

The best polarization for each phenological stage (germination, heading, and ripening (harvesting)) are detailed as follows in Tables 10 and 11. The phenology monitoring of the winter wheat highly relies on VV polarization, while the VH/VV ratio is also very helpful in identifying the germination and heading stages. Otherwise, the VH polarization was used to detect the ripening stage and the harvesting event.



**Table 11.** The best polarization observed for each phenological stage of winter barley in the study.


Meanwhile, the phenology monitoring of the winter barley depends more on VV and VH polarizations, which are able to easily identify the three phenological statuses. In addition, VH/VV polarization is also effective for detecting the ripening and the harvesting stages.

### **6. Discussion**
