## 3 Results and Discussion
### Level of Contribution of the Time Series Features
Regarding the most relevant time series feature Z-Score VV for the northern Greece/Turkey study area, it is assumed that the backscatter in VV polarization is dominated by the contribution of the forest canopy and moreover the volume scattering represents the main scattering mechanism. Due to the sensitivity of the VV polarization for the double bounce effect, however, a slight increase in the backscattering values at the flood date was detected, which can indicate TFV (Figure 8). The NDVI values were derived from the LANDSAT 8 data and show that the slight increase of the backscatter values in both polarizations at the date of flood can be attributed to the flood event and not to any change in vegetation. For the study area in China, Z-Score VV + VH represents the most relevant time series feature for the derivation of the TFV (Table 5). The time series feature indicates that in both polarizations an increase of the backscatter values has occurred at the analyzed date of the flood. This can be confirmed in Figure 9. Both polarizations increase at the analyzed flood date, whereas the stronger increase can be observed in VV polarization. The sum of the two polarizations intensifies the increase of the backscatter values at the same date. Although only a few cloud-free optical Sentinel-2 images are available that cover the study area in China for the analyzed time period, it can be observed that there is no increase in the NDVI values at the analyzed flood date (Figure 9). This leads to the deduction that the increase in SAR time series data is not related to phenological changes. Figure 5b shows a high-resolution Sentinel-2 image demonstrating the presence of the corresponding TFV area at the analyzed flood date. Figure 8: Multitemporal behavior of the backscatter intensity for TFV areas for VV (dB), VH (dB), VV/VH (linear scale), and NDVI values in northern Greece/Turkey. The blue bars mark the analyzed date at the flood event. Since the TFV type is high grassland and reed (Figure 5b) [64], which is similar to the structure of some crops (rice, wheat), it is assumed that the vegetation can be penetrated by the C-band data [10; 65]. Therefore, it is expected that the increase in backscatter values will occur at the time of the flood in the VV polarization due to their sensitivity to the double impact effect. However, the increase in VH polarization cannot be confirmed by the depolarizing property mentioned above. Instead, the latter increase is a result of the combination of environmental parameters. The contribution of the soil under the vegetation plays a decisive role in this process. Due to of possible unevenness of the soil (microtopography) and the low water level, it is possible that a part of the soil surface beneath the vegetation was only partially flooded. In the flood-free areas underneath the vegetation, the soil moisture may have increased at the date of the flood compared to non-flooded conditions. Simultaneously, water could have accumulated in the sinks, which could also lead to standing water beneath vegetation in the same region. Due to different sensitivities of VV and VH to the respective conditions, the latter can dominate the respective polarization. The analyses from the literature confirm that an increase in soil moisture in both VV and VH causes an increase in the backscatter values and can be detected by the sensor depending on the biomass amount [66; 67]. The TFV, which is represented by the double-bounce effect, can only be detected by VV polarization, as the depolarizing property of VH polarization does not allow sensitivity to double-bounce [68]. The mixture of these two cases by micro topographical differences explains the increase not only of backscatter values for VV, but also for VH and, thus, the relevance of Z-Score VV + VH for the derivation of TFV in the study area of China.