## 3 Data and Methods
### Flash-Flood Potential Assessment
#### 3.3.2 Flash-Flood Conditioning Factors  
Further, in order to calculate the FFPI in the GIS environment, the following 10 geographical factors that influence the surface runoff were derived: slope angle (Figure 4a), profile curvature (Figure 4b), hydrological soil group [91] (Figure 4c), lithology (Figure 4d), topographic position index (TPI) (Figure 4e), topographic wetness index (TWI) (Figure 4f), convergence index (Figure 5a), aspect (Figure 5b), modified Fournier index for 1989 and 2018 [92] (Figure 5c,d respectively), and land use/land cover for 1989 and 2019 (Figure 7a,b, respectively). The first two morphometric factors, namely the slope and the profile curvature, were derived from the digital elevation model (DEM). The relief slope is the morphometric factor that highlights the gravitation force influencing the surface runoff [86; 87; 88; 93]. Consequently, the runoff intensities as the geo-declivity value increases. The profile curvature is another morphometric factor that differentiates convex areas (having negative values), affected by the accelerated runoff form the concave ones (having positive values), facing a decelerated runoff [94]. The hydrological soil groups and lithology were converted from the vector format into raster datasets with a 30-m spatial resolution. As was mentioned before, the spatial distribution of hydrological soil groups was extracted from the Romanian Soils Map (2002) [95]. This property of the soil has a strong influence on the surface runoff [96] as it directs water infiltration into the soil. Therefore, a fine clay texture, which includes the hydrological group D, is supposed to favor the water runoff due to the reduction of infiltration, while a sandy texture, which includes the hydrological group A, will create an increase in the infiltration ratio and a decrease in runoff potential. The classification of soils into four groups are made based on their hydraulic conductivity, which is influenced by the texture [96]. The soil group A is characterized by a hydraulic conductivity higher than 40 \(\upmu\)m/s, while the soil group B, with a loamy texture, has a hydraulic conductivity between 10 and 40 \(\upmu\)m/s. The soil group C, with a hydraulic conductivity between 1 and 10 \(\upmu\)m/s, includes soils with a loamy-clay texture. The soil group D has a hydraulic conductivity lower than 1 \(\upmu\)m/s [94]. The lithological layer for the Zabala catchment was extracted from the 1:200,000-scale Geological Map of Romania. TPI (Figure 4e) is a key factor that shows the difference between the altitude of a raster cell and the altitude of the neighboring cells [97]. The TPI map was constructed by reclassifying its values into five classes taking into account natural breaks intervals. TWI (Figure 4f) is a morphometric factor that highlights the areas from the ground that are most favorable to water flow accumulation [98]. Like in the case of TPI, the TWI map was created by dividing its values into five intervals. The convergence index (Figure 5a), derived from the DEM, is a widely used indicator within studies that approach the flash-flood potential [99]. The map of the convergence index was obtained after the reclassification of its values into five classes according to the literature [50]. Aspect (Figure 5b) is an important indicator mainly for soil moisture status. It was taken into account in this study because it has an indirect influence on flash-flood phenomena [97]. The amount of rainfall also plays an important role as its increase determines the rise of rapid runoff potential on the slopes. One of the most important characteristics of the rainfall that influences the flash-flood occurrence is the intensity. In order to assess the spatial variability of the rainfall intensity across the Zabala catchment, the modified Fournier index was used. This index is widely used to estimate the rainfall intensity [97].