## 3 Results
### Flood Risk Baseline Map Derived from the Baseline Model
#### 3.1.2 Baseline Flood Risk Scenario  
The correlation coefficient matrices (Figure S6) of the factors in Figure 6 and the variance inflation factor (Table S3) proved that there was no strong collinearity among the factors. The historical flood inundation frequency factor played a crucial and objective role in the assessment, as it had a great weight according to the Entropy method (Table 2). However, the low weights assigned to sensitivity factors such as elevation and TWI reflected a limitation of the Entropy method in deviating from common sense, which was effectively addressed by incorporating experts' knowledge with the FAHP method. FAHP provides more reasonable weight estimations than AHP because the latter overestimates certain factors subjectively. Therefore, we adopted the FAHP-Entropy method to ensure scientific weight estimation. The factors were normalized by using the Min-Max method. The risk value obtained from the weighted summation ranged between 0.06 and 0.57. We classified the flood risk baseline map (Figure 7a) into five levels--very high (0.36-0.57), high (0.28-0.36), moderate (0.22-0.28), low (0.14-0.22) and very low (0.06-0.14)--according to the natural breaks (Jenks) method. Similar to the definition of the flood high-risk zone above, the low-risk zone refers to areas classified as being at very low and low risk in the baseline map. Figure 4: The spatial distribution of RMAX3 in the wet season in 2015–2021. According to the official criteria for grading daily rainfall in China, when the value of RMAX3 exceeds 75 mm, a 3-day heavy rainfall has occurred in the area; when the value exceeds 150 mm, a 3-day rainstorm has occurred; a value of over 300 mm indicates that a 3-day severe rainstorm has occurred; and a value of over 750 mm indicates that a 3-day extreme severe rainstorm has occurred. Figure 5: Inundation scenarios for two major flood events in 2020 and 2017. In 2020, the flood evolution process was as follows: lake rising stage (20 June to 2 July), flooding stage (2 July to 14 July), flood receding stage (14 July to 26 July) and stable stage (26 July to 7 August). In 2017, the four stages corresponded to 12 June to 24 June, 24 June to 6 July, 18 July to 30 July and 6 July to 18 July, respectively. Figure 6: Spatial distribution of the factors in the flood risk assessment baseline model. The bottom of each sub-figure shows whether an increase in the factor increases or decreases risk. The results show that elevation, NDVI, proximity to the river system and land use were negative factors, while the remaining factors were positive. (**f**) Level 1 represents the highest susceptibility to flooding, and level 4 indicates no environmental sensitivity in terms of hydrology. (**i**) The resistance to flooding of land-use types in decreasing order was determined as follows: cropland (1), built-up land (2), grass/shrub (3), forest (4) and bare/wetland/water (5). The baseline map (Figure 7a) shows that 11.7% of the land in the PLEZZ (5981 km\({}^{2}\)) is now a high-risk zone primarily concentrated around Poyang Lake and along the water system. The counties in this zone can be classified into the following three types according to risk composition:  
1. Small areas with high-risk city centers, such as the first six counties in Figure 7b, are represented by the six bubbles at the top of Figure 7c. Their high risk is caused by the high-vulnerability characteristics arising from dense populations and developed economies. 2. Large-area counties at relatively high risk surrounding Poyang Lake (ranking from 7 to 23), from Lushan to Pengze (comprising more than 10% of the high-risk zone).