# Assessment Analysis of Flood Susceptibility in Tropical Desert Area: A Case Study of Yemen
## 2 Methods and Data
### Factor System of Flood Susceptibility and Model Building  
#### 2.8.1 Flash Flood Conditioning Factors  
A significant phase in FSM is the selection of dominant key variables for assessing flood risks [12]. In general, flooding occurs due to natural and human factors. When mapping sensitivity to floods or other natural disasters, the number of conditioning factors must be specified [84; 85]. The flood conditioning factors were chosen based on the geo-environmental condition of the study area and related studies from areas with similar climatic circumstances [86; 87]. Twelve (12) independent variables were prepared as separate maps in R software, spatially registered (Table 2), and resampled to a determined pixel size resolution proportional to the land use map we extracted using ArcGIS 10.3 from a map issued jointly by ESRI and the Impact Observatory Institute (resolution10 m). **Rainfall:** As far as floods are concerned, rainfall is the most significant factor [40]. Due to no more current ground-station rainfall measurements for the study area, gridded data derived from Climate Hazard Infrared Group Precipitation Station (CHIRPS) that were explored on Google Earth Engine (GEE) were used to derive the average of maximum annual rainfall per year. CHIRPS data were also used to attain the average of the most extended period of consecutive days of rainfall per year from 1996 to 2021 (Figure 5h,i). These data can equally be used in difficult-to-reach locations with scant or time-incomplete observational data [88]. This study selects the rain intensity and rain duration to measure the hazard. **Elevation:** Previous research has established that elevation significantly influences flooding [12,22]. The elevation map of the research area was produced from an ALOS PALSAR sensor-derived digital elevation model (DEM) with a 12.5 m pixel size (Figure 5a). **Topographic wetness index (TWI):** The terrain-driven balance of catchment water supply and local drainage for each cell in a DEM is expressed by the topographic wetness index (TWI), which integrates water supply from the upslope catchment area and downslope water drainage [89]. TWI provides information about the spatial distribution and saturation sources contributing to runoff generation. As a result, the TWI has an indirect role in affecting runoff systems in a given area. TWI values were computed in this work using the DEM model, as shown in the following equation [90] (Figure 5b):  
\[\text{TWI}=\ln(As/\tan(\beta))\]  
where _As_ is the specific contributing area and \(\beta\) is the gradient or slope. **Stream power index (SPI):** The power of the stream, shear stress, and velocity are all essential elements in the development of flood damage and the erosion of river channels. SPI is a statistic that measures the erosive strength of discharge compared to a specific area within a watershed. It is also a measure of the erosive force of the flowing water [91]. SPI can be accounted for using the following equation [92]:  
\[\text{SPI}=As\times\tan(\beta)\]  
The SPI map was derived from DEM by applying the equation above in ArcGIS 10.3 software (Figure 5c). \begin{table}
\begin{tabular}{c c c c c} \hline
**No** & **Data Type** & **Source** & **Period** & **Mapping Output** \\ \hline \multirow{3}{*}{1} & \multirow{3}{*}{ALOSPALSAR} & Alaska satellite facility (ASF) & & Elevation, Slope, Aspect, \\  & & & [[https://search.asf.alaska.edu](https://search.asf.alaska.edu)]([https://search.asf.alaska.edu](https://search.asf.alaska.edu)) & 2021 & Curvature, SPI, Drainage \\  & & (accessed on 1 January 2021) & & Density, and TWI. \\  & & & [[https://sciub.copernicus.eu](https://sciub.copernicus.eu)]([https://sciub.copernicus.eu](https://sciub.copernicus.eu)) & \\ \multirow{3}{*}{2} & \multirow{3}{*}{Sentinel 2 (10 m)} & \multirow{3}{*}{(accessed on 6 May 2021)} & \multirow{3}{*}{2021} & \multirow{3}{*}{NDVI map} \\  & & & & \\ \cline{1-1} \cline{5-5}  & & & & \\ \cline{1-1} \cline{5-5}  & & & & \\ \cline{1-1} \cline{5-5}  & & & & \\ \hline \end{tabular}
\end{table}