## 3 Results and Discussion
### Flood Inundation in Poyang Lake Basin During the Extreme Precipitation Event  
The extreme precipitation event investigated in this study affected the middle and lower reaches of the Yangtze River from 22 June to 2 July 2017 [1], with cumulative rainfall exceeding 500 mm in some areas. The event caused widespread regional and basin flooding as well as secondary geological disasters. To show its detailed characterization in CYGNSS data, we analyzed the data covering the northern part of the Poyang Lake basin, which is located in the middle of the worst-hit area. Figure 3 shows the SR (CYGNSS-derived) and the inundated areas (d1, d2) of this region during DOY 137 (before the event) and DOY 182 (the end of the event). The SRs are shown in two forms, i.e., scatters of the specular reflection (b1, b2) and 30 km gridded images (c1, c2). Note that the event caused flood inundation in the middle of the Poyang Lake basin, mostly covering the Ganjiang, Fuhe, Xinjiang, and regions around the lake. The coverage of the inundated area is also consistent with the DEM of this region (Figure 3a). Figure 3e1,e2,f1,f2 show the characteristics of SMOS- and SMAP-derived T\({}_{\text{bc}}\) (also gridded to 30 km) for the two corresponding dates, respectively. Due to the coverage limitations of the SMOS and  
Figure 3: The CYGNSS-derived SR (**b1,b2**) and inundated areas (**c1,c2**) of the northern Poyang Lake Basin during Day of Year (DOY) 137 (before the event) and DOY 182 (the end of the event). (**a**) The Poyang Lake basin; (**e1,e2**) Soil Moisture and Ocean Salinity (SMOS) T\({}_{\text{bc}}\); (**f1,f2**) Soil Moisture Active Passive (SMAP) T\({}_{\text{bc}}\). SR: Surface reflectivity; T\({}_{\text{bc}}\). : Total brightness temperature at circular polarization. SMAP data, both the descending and ascending passes were used for these two dates. Qualitatively, Figure 3 shows that the SR and its resulting inundated area are consistent with the T\({}_{\rm bc}\) maps. Figure 4 further compares pixel-wise values of CYGNSS SR and SMOS/SMAP T\({}_{\rm bc}\) for DOY 137 and DOY 182 respectively. Overall, the Pearson correlation coefficients of SR vs. SMOS T\({}_{\rm bc}\) (R = \(-\)0.66 for DOY 137 and R = \(-\)0.70 for DOY 182) are almost the same as that of SR vs. SMAP T\({}_{\rm bc}\) (R = \(-\)0.66 for DOY 137 and R = \(-\)0.71 for DOY 182). The correlations between SR and T\({}_{\rm bc}\) are shown in Figure 4 as a function of (a, b) soil moisture and (c, d) nadir vegetation opacity. The nadir vegetation opacity is an indicator of the attenuation of the electromagnetic signal through the vegetation layer. It showed a dynamic range of 0-0.6 cm\({}^{3}\)/cm\({}^{3}\) for soil moisture along the complete SR dynamic range, while T\({}_{\rm bc}\) appeared saturated at \(\sim\)270 K with nadir vegetation opacity values of \(\sim\)0.7. Similar to results shown in [19] over inundated wetlands, this indicates that CYGNSS data (which is forward scattering) are less affected by vegetation cover over inundated regions. This phenomenon is also consistent with results shown in [12] for Thailand, since these two regions have similar land surface conditions (i.e., cropland with high soil moisture, high-to-moderate vegetation, and moderate-to-low land cover heterogeneity). The threshold (SR = 17 dB) for extracting the flood inundation area is also shown in each subfigure of Figure 4 (see black dashed line). Note that on DOY 182 (the end of the event), most pixels with low T\({}_{\rm bc}\) and high soil moisture could be correctly identified as inundated pixels. This indicates that flood water was the major contributor to the high SR, while effects from vegetation were relatively low. However, on DOY 137 (before the event), pixels with low T\({}_{\rm bc}\), high soil moisture and high vegetation opacity could not be correctly identified as 'water' pixels, particularly for those SMAP pixels (see red triangles in (a) and their corresponding triangles in (c) in Figure 4). This indicates that vegetation attenuation on SR was nonnegligible at this stage.