## 5 Discussion  
In this paper, we show that the Sentinel-1 can be used to estimate rice height and biomass in Camargue with a high accuracy. The dynamics in rice height and biomass can be observed by the backscatter behaviors in each of the two polarizations (VV and VH). Good results can be obtained by using classical approaches (MLR, SVR, and RF). The validated metric indicates good performance, in which the correlation coefficient \(R^{2}\) was greater than 0.8. We show that there is a better performance using non-parametric methods (SVR and RF) over the parametric MLR method. Thus, these demonstrated results confirm the great potential of Sentinel-1 data for rice height and biomass retrievals. First of all, even with the traditional approaches, we show that good retrieved performance could be yielded with Sentinel-1 SAR data. This is not straightforward due to the speckle noise nature in radar images. Good performance can be explained by the fact that 6 days time series Sentinel-1 SAR data allows not only a good follow-up of the rice growth but also mostly noise-free dataset, thanks to the speckle adaptive filtering. In addition, for rice biophysical parameter estimations, the Day after Sowing information, which can be retrieved directly from Sentinel-1 data within the accuracy of 3 days (our estimation), allows us to mitigate the potential problem on the radar signal  
Figure 8: Rice biomass estimation using MLR, SVR and RF methods. In this figure, only the configurations that provide the best results are considered (with dual-polarization VV and VH model for MLR, and with VH model for SVR and RF). Figure 9: Rice biomass retrieval and in-situ measurements at two reference plots 1G3 and 3M1. saturation. This situation is different from forest applications, where the forest age is challenged to retrieval. For example, for forest biomass estimations, many works have to use prior knowledge (e.g., Landsat-derived tree cover [48]) or advanced techniques (e.g., SAR tomography [49; 50]) to mitigate the radar signal saturated problem. Finally, we note that outside the European zones, there is a limitation for Sentinel-1 data since the revisit period will be 12 days. However, it should be emphasized that, nowadays, the Sentinel-1 constellation is the only system which can provide free and global coverage radar data. Therefore, it is a good candidate for operational rice monitoring tasks. Using in-situ measurements at Camargue, we show that radar backscattering coefficients are sensitive to rice biophysical parameters and have a strong correlation with rice height and biomass. This observation is consistent with reports from the literature [14; 51] due to the sensitivity of radar signals to rice structures. The backscatter of VH and VV polarizations can be separated into two main parts. In the first part, the VV and VH increase with the rice growth until 50 cm height (See Figure 5c). At this stage, the main backscattering mechanism is contributed by the interaction between the stem and the underlying water surface (e.g., double bounce). In the second part (where rice grows from 50 cm to 100 cm (maturity)), although the VV goes down, the VH is still slightly increased. Indeed, at this stage, the backscattering mechanisms are contributed not only by the double bounce interaction but also from the volume stem directly. Beyond 50 cm, the double bounce phenomenon can be reduced, and the stem and leaves of the rice are not remained vertical. While they lead to the power decrease of the VV backscatter, the VH signal is increased due to its sensitivity to random volume backscattering. Similar interactions were also reported by [PERSON] et al. [52]. The same trend of backscattering coefficients versus rice biomass is observed. When rice biomass reaches around 800 g\(\cdot\)m\({}^{-2}\) (See Figure 5d), this is the period when the rice stem is no longer vertical and the leaves have already deviated from the stem. Interestingly, it is worth pointing out that there is a jump in both VV and VH backscatter at 100 DaS days (See figure 5a).