## 3 Data Collection and Methods
### SCS-CN Model  
Soil Conservation Services and Curve Number (SCS-CN) method is a simple technique for rain-runoff modeling. The SCS-CN runoff equation was initiated in 1954 by the USDA SCS [35,42] and applied broadly in the Soil Conservation Service (SCS) in the hydrology section of the National Engineering Handbook (NEH-4) [43]. The CN is an empirical factor applied to hydrology for runoff prediction [36,44]. CN values are estimated based on the following three factors: land use, soil moisture characteristics, and soil type. Geological units were discriminated, and the soil types were recognized depending on the land use/cover of the study catchments and sub-catchments (Figure 2). The soil units are then categorized according to Hydrological Soil Groups (HSGs), indicating the infiltration ratios of the soil after rainfall storms. This method provides many assumptions. The first consideration states that the ratio of the exact quantity of direct runoff to the maximum \begin{table}
\begin{tabular}{c l l c} \hline \hline \multicolumn{2}{c}{**Morphometric Indexes**} & **Formula** & **References** \\ \hline \multirow{4}{*}{Catchment geometry} & Area index (A) & ArcHydro analysis & [34] \\ \cline{2-3}  & Length index (L\({}_{\text{b}}\)) & ArcHydro analysis & [34] \\ \cline{2-3}  & Width index (W) & ArcHydro analysis & [34] \\ \cline{2-3}  & Perimeter index (P) & ArcHydro analysis & [34] \\ \hline \multirow{4}{*}{Stream indexes} & Stream order (S\({}_{\text{u}}\)) & Hierarchial rank & [21,36] \\ \cline{2-3}  & Stream length (L\({}_{\text{u}}\)) in km & L\({}_{\text{u}}\) = L\({}_{1}\) + L\({}_{2}\) +...... L\({}_{\text{n}}\) & [21,39] \\ \cline{2-3}  & Bifurcation ratio (R\({}_{\text{b}}\)) & R\({}_{\text{b}}\) = N\({}_{\text{u}}\)/N\({}_{\text{u}}\) + 1 & [21,34] \\ \cline{2-3}  & Stream number (N\({}_{\text{u}}\)) & N\({}_{\text{u}}\) = N\({}_{1}\) + N\({}_{2}\) +...... N\({}_{\text{n}}\) & [40] \\ \hline \multirow{4}{*}{Areal indexes} & Drainage frequency (F) & TSN/A & [40] \\ \cline{2-3}  & Drainage density (D\({}_{\text{d}}\)) & TSL/A & [40] \\ \cline{2-3}  & Drainage basin shape (B\({}_{\text{sh}}\)) & L\({}_{\text{b}}\)\({}^{2}\)/A & [16,40] \\ \cline{2-3}  & Drainage elongation ratio (R\({}_{\text{e}}\)) & 1.128 \(\times\) A0.5/L\({}_{\text{b}}\) & [34] \\ \cline{2-3}  & Drainage circularity ratio (R\({}_{\text{c}}\)) & 4\(\pi\)A/P\({}^{2}\) & [41] \\ \hline \multirow{4}{*}{Relief indexes} & Catchment relief (R) & H = Z - z & [41] \\ \cline{2-3}  & Catchment relief ratio (R\({}_{\text{r}}\)) & R/L\({}_{\text{b}}\) & [34] \\ \cline{1-1} \cline{2-3}  & Catchment ruggedness no. (R\({}_{\text{n}}\)) & D\({}_{\text{d}}\)\(\times\) (R/1000) & [34] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Computation methods of morphometric indexes, where, A = total area of a catchment, TSN = total stream numbers, TSL = total stream lengths, Z = highest point of a catchment, z = lowest point of a catchment, and D\({}_{\text{d}}\) = drainage density. runoff is equal to the ratio of the amount of infiltration to the potential maximum retention quantity. The second assumption confirms that the early abstraction quantity is a fraction of the maximum retention. The SCS number suggestion applies imperial methods to calculate the direct runoff quantity from study watersheds [36; 42]. In regards to floods, the rainfall excess from the catchment outlets is described by a single value of CN and ranges from 0 to 100. In the present study, a rainfall-runoff model with a fixed grid size was initiated. An appropriate curve number value was calculated for each catchment area according to the standard SCS curve number tables [42], in addition to Antecedent Moisture Conditions (AMC).