## 1 Introduction  
China is one of the countries experiencing the most severe natural disasters worldwide. Disasters of various types, covering wide areas with high frequencies and heavy losses, are typical national conditions in China [1, 2]. In recent years, with the acceleration of urbanization in China, frequent regional extreme precipitation events and increasingly prominent flood disasters have caused significant damage to regional social and economic security [3, 4]. To effectively prevent and control flood disasters, it is necessary to study and apply flood disaster risk regionalization.  
Floods are the most serious natural disasters in China in terms of deaths and economic losses. With accelerated urbanization and climate change, urban flooding is becoming increasingly serious [5]. Since 1978, China has seen a rapid increase in its urbanization rate [6]--by 2020, China's urbanization rate exceeded 60% and is expected to exceed 70% in 2030 [7, 8]. To address this problem, many researchers have recently studied urban floods [9, 10]. In China, many cities are located in mountainous and hilly areas [11] that are frequently affected by floods, but prior studies have primarily concentrated on cities in the plains. Existing studies and statistical data show that the proportion of population deaths caused by flash flood disasters in recent years has been as high as 62-69% and that flash -flood disasters are the most serious flood disasters in China [12]. Flash floods are sudden floods that last for a short period of time, usually six hours or less [13; 14; 15]. Flash flood disasters not only cause heavy economic losses and casualties but also pose a serious threat to economic and social development [12]. Therefore, the prevention and control of flash flood disasters has always been the focus of flood prevention and control in China [16]. At present, progress has been made in the study of flood prevention regionalization in mountainous areas, including topographic [17; 18] and weighted linear comprehensive methods [19]. Based on cause analyses of flash flood disasters and existing achievements in rainfall regionalization, topographic geological regionalization, and economic and social regionalization, the classification of flash flood prevention and control regionalization [20] was obtained through layer superposition or comprehensive superposition.  
The above method requires a large amount of data accumulation, such as topographic and hydrological data; however, areas with a high frequency of flash flood disasters usually lack the corresponding data. From 2013 to 2015, the Ministry of Water Resources of the People's Republic of China officially started implementing a nationwide investigation and evaluation of flash flood disasters and preliminarily identified the scope of flash flood disaster prevention areas, personnel distribution, social economy, and historical flash flood disasters in China [21; 22]. The analysis and evaluation technology for flash flood disasters solves the problem of classifying flash flood prevention and control regionalization schemes for small watersheds without data. With the development of urbanization in China, mountainous cities have become more sensitive to the impacts of the underlying surface and ecological environment [23]. To address this problem, this study used the results of a flash flood disaster investigation and evaluation, selected Yunnan (a mountainous province in China) as an example, and considered the flood control capacity and population in dangerous areas as the main indicators to evaluate the flash flood control zoning level of small watersheds in Yunnan Province. The research and calculation of the prevention and control zoning of flash flood-threatened areas is helpful for flood control workers and decision-makers in mountain cities to better understand the spatial distribution of regional flash flood disaster risk, and it can provide a reference for flood control planning and decision-making.