## 4 Results  
### Analysis of Regional Floods  
Three radar images on 15 (before the rainstorm), 20 (during the rainstorm), and 22 (after the rainstorm) were extracted using threshold segmentation. The images before and after the rainstorm were superimposed to determine the extent of rainstorm inundation. This paper extracted and counted the flood disaster area time sequence changes in each main urban area in three periods after obtaining the flood disaster conditions in Zhengzhou. The change in water body area in each district (7.15, 7.20, and 7.22) is shown in Figure 7, and the detailed water body area is shown in Figure 8.  
By observing the comparison of water body areas in the three time periods in Figures 7 and 8, the storm reached its maximum rainfall on 20 July, with huge fluctuations in water bodies in all five urban areas.  
The inundated area of water bodies within Guancheng District increased the most, from 7897.10 m\({}^{2}\) before the rainstorm to 41,299.30 m\({}^{2}\), which was the most severely affected. As can be seen from the chronological changes in Figure 7, the village of Wuzhuang in the northeast of Guancheng District was severely affected, and the roads in that area were still not restored to normal in some of the affected areas until 22 July. The Changzhuang Reservoir in Zhongyuan District was also severely flooded, with an affected area of 27,174.9 m\({}^{2}\). Due to the continuous heavy precipitation and a large amount of water flowing upstream of the reservoir, the water level in the reservoir remained high. Affected by this, the water level of the river section in this area also rose to a certain extent. In addition, due to the reservoir spillway encroachment blockage and other circumstances, the water level in the Guojiazui reservoir in Erqi District on the 21 July rose significantly to the maximum full overflow water depth of 0.5 m, the occurrence of the reservoir flooding became its 22 July retreat area of the least urban areas.  
Comparing the changes in water bodies in each district in Figure 8, Huiji District was the least affected by this extreme weather, which is inseparable from the unique natural environment within the region. Huiji District is located in the northern part of Zhengzhou City, and the terrain generally shows highs in the west and lows in the east. The Yellow River runs through the northern part of the district for 27 km, making it the richest ecological water system in Zhengzhou. In addition, the high forest cover in the district provides favorable conditions for stormwater infiltration and drainage. When heavy rainfall occurs in the area, the infiltration of water absorbed by the plant soil layer and the timely drainage of the river do not cause large areas of water to accumulate on the ground.  
Figure 6: dB values of typical water bodies in different SAR images. The figure marks the highest dB value.  
Overall, Zhengzhou's \"7-20\" rainstorm affected each of Zhengzhou's five main urban areas to varying degrees of disaster. As of 22:00 on 20 July, the area of water bodies in Guancheng District grew the fastest, with a total increase of 33,402.2 m\({}^{2}\), while the total area of water bodies in Zhongyuan District increased by 27,174.9 m\({}^{2}\). As of 10:00 on 22 July, the total area of water bodies in all urban areas of Zhengzhou City decreased compared to the total area of water bodies on 20 July. The total area of water bodies in all city's urban areas decreased compared to the 20 th. Erqi District recovered the most slowly, with a recovery area of 12,461.1 m\({}^{2}\).  
Figure 7: Water extraction results before, during, and after the rainstorm.