# 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

Table 2: Lists the sources of data types used in this work. **Slope:** The slope is affected by surface runoff and infiltration and thus is critical in the flood susceptibility mapping [93]. The slope is the ratio of a feature's steepness or degree of inclination relative to the horizontal plane [94]. The slope map was created simply in ArcGIS 10.3 software using the DEM and divided into five categories, as shown in Figure 5d. **Aspect:** Aspect is another factor that influences flooding water flow directions, evapotranspiration, local climate, soil moisture, evapotranspiration, and infiltration [95]. The aspect factor influences the occurrence of natural occurrences on the earth's surface since it is influenced by climatic elements such as precipitation direction and sunshine intensity [6]. Although this element only has a minor impact on flooding, most researchers have included it as one of the factors to consider when mapping flood susceptibility [6; 96]. Aspect was classified into nine groups, each corresponding to a cardinal direction. Flood pixels are spread very evenly throughout these nine types as shown in Figure 5e. **Curvature:** Curvature is another factor affecting floods event; a surface part can be concave or convex and influences the flow's divergence and convergence across the surface [12]. The curvature was extracted from DEM in ArcGIS (Figure 5f). **Normalized difference vegetation index (NDVI):** The normalized difference vegetation index (NDVI), another critical factor for flood susceptibility mapping, is an essential indicator of vegetation cover and its impact on flooding in a catchment [33]. The NDVI value is calculated using the following equation [97]:  
\[\text{NDVI}=(IR-R)/(IR+R)\]  
Figure 5: Maps of conditioning factors for flash flood: (**a**) elevation, (**b**) topographic wetness index (TWI), (**c**) stream power index (SPI), (**d**) slope, (**e**) aspect, (**f**) curvature, (**g**) drainage density (Dd), (**h**) rain intensity, (**i**) rain duration, (**j**) normalized difference vegetation index (NDVI), (**k**) land use, and (**l**) soil. where the \(R\)-value is the red portion of the electromagnetic spectrum and the \(IR\) value is the infrared portion of the electromagnetic spectrum. For this study, the NDVI map was obtained from Sentinel-2 data by combining bands B8 and B6, as shown in the equation below (Figure 5):  
\[\text{NDVI}=(B8-B4)/(B8+B4)\]  
**Land use/land cover (LU/LC):**LU/LC types play an influential role, directly or indirectly influencing some hydrological methods components, such as runoff generation, infiltration, and evapotranspiration [28]. Land-use, land-cover (LULC) maps can be obtained from high-resolution satellite sensors. We derived the research area's land use map from a global map issued jointly by ESRI and the Impact Observatory Institute. Access to the entire global GeoTIFF zip file is available on ([[https://livingatlas.arcgis.com/landcover](https://livingatlas.arcgis.com/landcover)]([https://livingatlas.arcgis.com/landcover](https://livingatlas.arcgis.com/landcover)), accessed on 26 June 2021). The data were accessed and downloaded on 26 June 2021. We then categorized the study area into seven groups: water, trees, grass, crops, scrub/shrub, built area, and bare ground using ArcGIS 10.3 (Figure 5k). **Soil type:** Soil type is an essential factor in flood susceptibility mapping, as primary water infiltration is dependent on soil characteristics [26]. The national soil map of Yemen was compiled in 2006 by Renewable Natural Resources Research Center (RNRRC) in the Agricultural Research and Extension authority (AREA), Dhamar, Yemen [98]. From conversion tools In ArcGIS 10.3, we converted soil map polygon to raster layer, then extracted study area, and categorized into two groups, namely Etc (dry soil, dry sedimentary, soil dry, and limestone soil), and Rcc (dry limestone, soil dry, shallow calcareous soil, and shallow soil) (Figure 5l).