## 3 Results  
### Temporal Evolution of Polarimetric Observables  
#### 3.1.1 SAR Backscattering  
The average SAR backscattering coefficients in linear polarization (HH, HV, VV), Pauli channels (HH + VV, HH-VV), total scattering power (Span), and backscattering ratios (HH/VV, HV/VV, HV/HH) for various crops throughout the whole growing season were calculated and are presented in Figure 5 (The values of three main crops (corn, soybean, and winter wheat) are shown in the supplementary Table S1). In general, the seasonal patterns of HV and HH-VV were more pronounced than those of co-polarizations (HH, VV), Span, and HH + VV. Taking winter wheat as an example, the dynamic range of HV was about 14 dB, and that of HH-VV was approximately 10 dB, while it was about 6 dB in HH and VV, 7 dB in HH + VV, and only about 3 dB in Span. The seasonal pattern of crops under HV was the most obvious, so the subsequent analysis of the variation of radar scattering values mainly revolved around HV polarization. The seasonal patterns of winter wheat and summer crops are clearly shown in HV. In the study area, winter wheat is sown in October of the previous year, going through dormancy over winter, and starts regrowth when the snow melts the following spring. During the month of April and early May, the HV value was relatively low due to the small biomass of the winter wheat crop. After the weather warmed up more, it started to grow from May to August, and the HV value continued to rise (\(-40\) dB to \(-25\) Db). In mid-August, after winter wheat had been harvested, the HV value showed a downward trend. Watermelon and tobacco are harvested in October after a rapid increase in HV values during sowing and emergence in May, reaching peak biomass in August, and entering senescence in early August, when HV values start to decrease. The difference between watermelon and tobacco is that the HV values fluctuated very little from mid-July to October (\(-25\) dB to \(-27\) dB) for watermelon. The HV response value of soybean was the lowest in the sowing period (early May), and the HV value increased slowly and continuously to mid-July with crop growth. The HV value maintained a high value (about \(-24\) dB) at the peak of biomass (mid-July and mid-August), and then decreased slowly due to the increased increased supply of the winter wheat crop. The HV value increased slowly due to the increased supply of the winter wheat crop. to aging and harvesting. The difference between corn and all other crops is that its biomass peaks in late June when the HV value reached its peak (about \(-25\) dB), maintained a large HV value (about \(-26\) dB) from late June to mid-July, and then began to decline. The average values of three backscattering ratios (HH/VV, HV/VV, and HV/HH) for various crops were calculated and are shown in Figure 5g-i. It can be seen from the figures that most crops were more sensitive to HV/VV and HV/HH than to HH/VV, which showed only a clear temporal pattern for winter wheat. Most crops, such as corn, forage, soybean, tobacco, and watermelon, had stable HH/VV values (about 0-5 dB) throughout the growing season. The variation range of HH/VV value of soybean was 2 dB, and the variation range of HV/VV and HV/HH of soybean was about 8 dB and 9 dB. The variation range of HH/VV and HV/VV values of winter wheat was 9 dB, while the variation range of HV/HH was 5 dB, indicating that winter wheat was more sensitive to HH/VV and HV/VV. In conclusion, all crops except winter wheat were more sensitive to HV/VV and HV/HH. From Figure 5h,i, in general, the HV/VV value of crops was more pronounced than the HV/HH ratio. For example, the variation range of HV/VV of corn, tobacco and watermelon was 10 dB, 9 dB, and 16 dB respectively, and the variation range of HV/HH was 7 dB, 8 dB and 14 dB. Therefore, the following analysis mainly focuses on the HV/VV ratio. Soybean and corn exhibited very similar patterns in HV/VV.