## 4 Discussion
### Response of Backscattering Coefficients in Phenology Retrieval  
Evolution of the backscattering coefficients during the growing season (Figure 7) reflected the characteristics of rice phenology evolution and growth status change. By taking advantage of responses of backscattering coefficients to different polarizations and in different phenological phases, the phenology retrieval method (Figure 8) achieved a high overall accuracy (86.2%). During the transplanting phase, rice plants were very small and short. The amount of scattering attenuation was small because of the low rice height and sparse rice canopy. The Bragg scattering from the rough surface of the flooded ground predicted that VV backscattering coefficients had moderate values (\(-\)12.5 dB on average) [5; 26]. VH backscattering coefficients were low (\(-\)22 dB on average) because of the sparse rice canopy since VH polarization was sensitive to volume scattering from the rice canopy [25; 26]. HH backscattering coefficients were low (compared with later phases) due to the weak double-bounce scattering between the small rice plants and the underlying water surface, since HH polarization was sensitive to double-bounce scattering between rice plants and the ground surface [45]. The difference between the responses at VV and VH was clearly illustrated by the VV/VH ratio, which can be used to separate the transplanting phase from later phases. During the vegetative phase, VH increased significantly with the development of the plants and hence with increasing canopy density. HH also increased significantly because of the larger stalks and increasing plants height. Double-bounce scattering between the rice plants and the underlying surface increased [26]. VV increased slightly because it was affected by an extinction due to the vertical preferred orientation of the plant elements (e.g., stems and leaves) [6]. During the reproductive phase, VH increased due to the denser plants canopy. Rice heads emerged as new scattering elements in the upper part of the canopy. HH decreased because the dense canopy reduced the microwave penetration to the surface and the double bounce off the rice plants [29]. VV decreased because the stems remained predominantly vertical and the amount of double-bounce and surface scattering was small [26]. Because of the different trends in the HH and VH channels, the HH/VH ratio can be used to separate the vegetative phase from later phases. During the maturity phase, VH increased because the amount and size of rice heads increased. HH decreased because of the large amount of attenuation by the rice canopy and the low volumetric moisture content. Stems became drier and the reduced water content caused less vertical extinction, especially for VV polarization [25]. The orientation of plant elements (i.e., stems, leaves, and heads) became random during this period. The attenuation of the VV polarization by the vertical orientation of the cylinders was reduced [25]. Therefore, VV stopped decreasing and the HH/VV ratio can be used to separate the reproductive phase from the maturity phase due to the different trends in HH and VV channels. Most of the misclassification occurred in the transition from the vegetative to the reproductive phase, as shown in Table 4. In this period, backscattering properties of rice plants were mainly influenced by planting density and rice plants growth status, which were usually affected by rice varieties and plants health. Late vegetative (from stage 40 to 49) rice plants had similar backscatter properties with early reproductive (from stage 50 to 59) rice plants because of the gradual development of plants morphological structure. As shown in Figure 12, values of HH/VH decreased monotonously from the early vegetative phase (from stage 20 to 39) to the late reproductive phase (from stage 60 to 69). The slow growth of rice plants and weak rice heads at the turn of the vegetative and reproductive phase resulted in no significant change of HH/VH data.