## 3 Results
### Temporal Evolution of Polarimetric Observables
#### 3.1.2 Polarimetric Decompositions  
* Freeman-Durden Decomposition  
The relative strength of the different scattering mechanisms present in the scene can be explained directly in terms of the proportional scattering contribution, in percentage (%), reported by Freeman-Durden decomposition. Notably, their proportional contribution varied greatly during the growing season for all crops. Figure 6a-c shows the contribution of surface scattering, double-bounce scattering and volume scattering of various crops during the growing season (The values of three main crops (corn, soybean, and winter wheat) in Figure 6 are shown in the supplementary Table S2). In general, the double-bounce contribution was small (less than 10%) for all crops, but for corn at the last date was 20%. The evolution of the scattering mechanism contributions was similar for all crops, showing a decreasing trend of surface scattering and an increasing trend of volume scattering. For tobacco, surface scattering was the main scattering mechanism from April to the end of June, while volume scattering was the main scattering mechanism from July to the end of September. The watermelon field and forage also had similar situations, for example, the contribution of surface scattering in the watermelon field accounted for 43%, and volume scattering accounted for 52% in the whole growing season. From August to September, the surface scattering of corn increased while volume scattering decreased, and the opposite trend was found in the watermelon field. For summer crops, volume scattering was dominant in soybean (>55%) throughout the growing season, although there was also a high contribution of surface scattering (>50%) from April to May. For corn, the volume scattering was dominant in the whole growing season (about 56%), from April to May but surface scattering was dominant (greater than 55%), in June to the end of September because of the crop leaf blight and part of the crop began to be harvested, so the volume was the main scattering mechanism (greater than 60%). Due to leaving crop residue after harvesting, double-bounce scattering also increased in the last date. Surface scattering was dominant in winter wheat from April to June (>50%), and volume scattering was dominant from June to August (>60%) due to canopy or standing residues. * Cloude-Pottier Decomposition  
Cloude-Pottier decomposition can determine the main scattering mechanism of targets on the land surface from the H-\(\alpha\) characteristic plane. The variation trend of entropy (H), anisotropy (A), and \(\alpha\) angle of various crops are reflected in Figure 6d-f. In general, most crops showed multiple scattering mechanisms with moderate to high values of entropy. The maximum entropy for most of crops appeared in August and September, while winter wheat reached maximum entropy at its peak biomass in late June, and soybean achieves the maximum entropy in July. Entropy generally showed a similar change trend in the whole growing season. It first increased and then tended to be relatively stable at the  
Figure 6: Temporal evolutions of polarimetric decomposition parameters for different crops. (**a**) surface scattering (Ps); (**b**) double-bounce scattering (Pd); (**c**) volume scattering (PV); (**d**) entropy (H); (**e**) alpha angle (\(\alpha\)); (**f**) anisotropy (A); (**g**) degree of orientation randomness (**\(\tau\)**); (**h**) magnitude of the particle scattering anisotropy \(|\delta|\); (i) phase of the particle scattering anisotropy \(\phi_{\delta}\). Note error bars denote standard deviation. late stages. During the whole observation period, the \(\alpha\) angle identifying the average scattering mechanism was distributed in the range between 14\({}^{\circ}\) and 45\({}^{\circ}\) for the three crops, indicating an increasing contribution of volume scattering, which is consistent with the contribution of scattering mechanism obtained by Freeman-Durden decomposition.