## 4 Results Analysis
### Analysis of Calculation Results
#### 4.2.1 Key Prevention and Controlled Zoning for Flash Floods  
According to the criteria in Table 2, we analyzed the flood control capacity and population size of the small watershed. The results disclosed that 160 small watersheds were classified as key areas for flash flood control and were primarily distributed in Dehong, Kunming, Wenshan, Yuxi, Zhaotong, and other locations, with a total area of 29,549.35 km\({}^{2}\).  
In Yunnan, the largest key flash flood prevention and control zone is Kunming, with a total of 6473.78 km\({}^{2}\) of the key prevention and controlled zoning for flash floods, accounting for 32.81% of the city's administrative division area. Kunming is the capital of Yunnan Province, the central city of the Urban Agglomeration in Central Yunnan, the most populous city in Yunnan [27] and a large city in China. Combined with Table 4, it is evident that the number of people affected by flash floods in the highest danger zones of Kunming is the largest in Yunnan, which is the main reason for the large area of key prevention and controlled zoning for flash floods in Kunming. Figure 5 shows the population density of each city in Yunnan.  
The areas designated as key flash flood prevention and control zones in Dehong, Yuxi, and Zhaotong also accounted for more than 20% of the administrative areas of these cities. Previous studies confirmed that most rural residents in Yuxi live in areas below steep slopes and near streams [28]. During periods of relatively concentrated and heavy rainfall, there is a significant threat of landslides and flash floods. Based on the data analysis in Table 4, Yuxi has the third largest population in the highest danger zone and the largest population in the high-risk zone in Yunnan. Of the flash flood prevention and control villages in Yuxi, 70% have a current flood prevention capacity below the 20-year return period. Therefore, the key prevention and control zoning for flash floods in Yuxi is relatively large.  
Figure 5: Distribution of population density in Yunnan.  
Dehong is located to the south of the Hengduan Mountains and to the west of the Gaoligong Mountains. The terrain is high and steep in the northeast and low, wide, and gentle in the southwest [29]. The terrain is predominately mountainous, with only scattered river valleys and plains among the mountains. The highest point is Dannang Mountain in the north of Yingjiang County, with an altitude of 3404.6 m. The lowest point is in the Jieyang River Valley in the west of Yingjiang County, with an altitude of 210 m and a relative height difference of 3194.6 m. The entire area within the jurisdiction experiences intense neotectonic movement, providing favorable material conditions for the development of flash floods, debris flows, and landslides [30]. The topographic conditions are extremely conducive to the development and strengthening of precipitation cloud systems, and heavy precipitation is prone to occur. When jointly analyzing Tables 3 and 4, it can be seen that the proportion of villages in Dehong with a flood control capacity below the 20-year return period is as high as 78%, and 42% of the analyzed villages have a current flood control capacity below the 5-year return period, which is a reason why the key prevention and control zones for flash floods in Dehong are relatively large.  
Zhaotong is located in the transition zone from the Sichuan Basin to the Yunnan-Guizhou Plateau, where the terrain, geology, and geomorphology are complex, with steep slopes and fast-flowing rivers [31]. Mountainous areas account for 96% of the total area. Between 2006 and 2016, ten flash flood disasters killed at least five people in Zhaotong, an area prone to frequent flash flood disasters. Combining Tables 3 and 4 indicates that Zhaotong has a relatively large number of villages that are vulnerable to flash floods, and the population in danger areas is also relatively large. Therefore, the area of key prevention and control measures for flash floods is relatively big.