## 3 Technology Roadmap and Calculation Method
To ensure that the population density in the small watershed was divided according to a standard method, the prevention and control of flood risk zoning and regionalization were compiled as technical requirements of the agreement. The specific methods used are as follows:  
\[N_{1}=\frac{A}{200\text{ km}^{2}}\times 1000\]  
\[N_{2}=\frac{A}{200\text{ km}^{2}}\times 500\]  
where \(N_{1}\) and \(N_{2}\) are the reference indicators of the population affected by flash floods in small basins and \(A\) is the area of the small basins. Using area weighting, the populations affected by flash floods in small catchments can be divided into three levels. Level 1: \(N\geq N_{1}\)  
Level 2: \(N_{2}\leq N\leq N_{1}\) \begin{table}
\begin{tabular}{c c c c} \hline \hline  & More than 1000 people & More than 500 people were & Less than 500 people were \\ Socio-economic status & floods or the national and & affected by the flash floods & affected by the flash floods, \\ The designed rainfall & provincial important & or the prefecture-level & and no important \\ frequency corresponding & infrastructure and & important infrastructure and & infrastructure or industrial \\ to the critical rainfall & industrial and mining & and industrial and mining & and mining enterprises \\  & enterprises are seriously & enterprises are seriously & were affected by flash \\  & affected by flash floods. & affected by flash floods. & floods. \\ \hline The critical rainfall \(\leq\) & key prevention and & key prevention and & medium prevention and \\ designed rainfall \(p=20\%\) & controlled zoning for flash & controlled zoning for flash & controlled zoning for flash \\  & floods & floods & floods \\ \hline Designed rainfall \(p=5\%\geq\) & key prevention and & medium prevention and & general prevention and \\ The critical rainfall \(>\) & controlled zoning for flash & controlled zoning for flash & control zoning for flash \\ designed rainfall \(p=20\%\) & floods & floods & floods \\ \hline The critical rainfall \(>\) & medium prevention and & general prevention and & general prevention and \\ designed rainfall \(p=5\%\) & controlled zoning for flash & controlled zoning for flash & controlled zoning for flash \\  & floods & floods & floods \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Standard for the prevention and controlled zoning for flash floods. Level 3: \(N\leq N_{2}\)  
where \(n\) is the number of people affected by flash floods in the watershed. By comparing \(n\) with the calculated values of \(N_{1}\) and \(N_{2}\), the grade of the population affected by flash floods in the small watershed is determined. In terms of the selection of another evaluation index and considering the analysis and calculation of the current flood control capacity of villages along the river in the results of the flash flood disaster analysis and evaluation, the frequency of flood peak discharge corresponding to the disaster water level of disaster prevention objects and the cumulative population and house number under the designed flood water level of each frequency were fully considered. This is a more comprehensive index compared with the frequency corresponding to the critical rainfall. Therefore, we chose the status quo of flood control ability as the judgment index that was calculated. According to the current flood control capacity and the population affected by flash floods, a small watershed is divided into key control zoning, medium control zoning, and general control zoning (Table 2). In the process of sorting the data of the two types of indicators, the following principles were followed. If multiple flood-threatened villages are distributed in a watershed, the current flood prevention ability of the small watershed would be the lowest. In terms of the population indicators affected by the flash flood, the number of people affected by the flash flood in the small watershed was the sum of all villages affected by the flash flood in the small watershed. After screening and merging, the decision data for each small watershed were determined.