## 3 Technology Roadmap and Calculation Method  
_Technology Roadmap_  
The prevention and control zoning of flash flood threat areas is based on the collection and sorting of basic data that can be divided into three types: main river flood control zoning, flash flood threat areas, and local flood threat areas (Figure 3). In China, the main river flood control areas are primarily located in the middle and lower reaches of large rivers flowing into the sea and coastal areas. The terrain is predominately plain and basin, with a small part located in hills or mountains. The local flood threat areas include the Inner Mongolia Plateau, most areas of the Qinghai-Tibet Plateau, and most river areas in Northwest China, deserts, Gobi, and other areas. These areas have low population density, and even if there are floods in local areas, they will not cause disasters because of the sparse population; the flood threat is generally small. Flash flood threat area refers to mountains, hills, platforms, loess hilly tablelands, small river valley plains, small basins, and piedmont plains, which are likely to be affected by flash flooding, debris flow, and other disasters. For topographic and river distribution reasons, the distribution of major river flood control areas in Yunnan Province is small, and there is no local flood threat area. Therefore, the entire area of Yunnan was regarded as a flash flood threat area in this study. The control level of each watershed in the flash flood threat area was determined by referring to the population and critical rainfall affected by the flood, and the control division of the flash flood threat area was obtained. In March 2021, the Ministry of Water Resources of the People's Republic of China issued the \"Technical requirements for the preparation of flood risk regionalization and prevention and control regionalization,\" which indicates that the prevention and control regionalization of flash flood threat areas should refer to the results of the division of small watershed units in the national flash flood disaster investigation and evaluation project, and should conduct the prevention and control regionalization with a basin area of approximately 200 km\({}^{2}\) as the unit. By making full use of the data of flash flood disaster prevention and control planning and flash flood disaster investigation and evaluation, the author sorted the population affected by flash floods (the population within the maximum  
Figure 3: Technical flow chart of prevention and control zoning of a flash flood threat area. possible inundation range or the population within the inundation range of a flash flood once in 100 years) and the critical rainfall of flash floods under general conditions of soil moisture. Based on the combination of rainstorm frequency and economic and social conditions, the small watershed was divided into key, medium, and general control zones (Table 1). In this study, the population affected by flash flooding and the critical rainfall in the sub-watershed were selected as evaluation indices for the areas at risk of flash flood threat. In the \"Technical requirements for the preparation of flood risk regionalization and prevention regionalization,\" the population affected by flash flood prevention regionalization is divided into three judgment levels. Specifically, 200 km\({}^{2}\) is used as the judgment unit. The first level is populations affected by flash floods being greater than or equal to 1000 people, and the second level is that the population affected by flash floods is 500-1000 people. If fewer than 500 people are affected by flash floods, they are classified as Level 3. In the actual implementation process, the areas of small watersheds are often not the same. Focusing on this situation, the process of calculation in this study adopted the area-weighted method to differentiate different areas of a small watershed level affected by flash floods to eliminate the small watershed area of difference between the results of regionalization.