## 2 Materials and Methods
### Methods
#### 2.2.3 Data Analysis  
The data from the interviews was collected, synthesized, and analyzed using Statistical Package for Social Science (SPSS), version 22 which was widely used in the social and behavioral sciences [82]. Among the factors of the USLE model, only C and P are related to the farming systems. The P factor was calculated based on the land use type and slope of the given land area (Table 3). When we focus on a specific type of land use in the same terrain and soil type, the influence of P factor to soil erosion rate between these locations can be omitted. In this case, we consider that only the C factor has an impact on the soil erosion rate when the land is used in the same way on similar slope degrees. Since acacia is the most popular tree on the research site, we chose acacia land use in order to compare the impact of different farming system on the soil erosion rate (Figure 2). In terms of NIK methods, farmers cultivate acacia according to technical instructions including density, quantity, distancing, etc. These farmers often buy acacia seedlings from nurseries and use indirect transplanting with a density of 2500 to 3000 trees per hectare (Figure 2a). In contrast, the IK is used in acacia fields by applying two methods of acacia cultivation without following technical instructions. One is that they sow acacia seeds that are collected and stored from the previous crop, directly on the field. They also use the natural deciduous tree seed of old acacia trees, so that it is accustomed to growing naturally in the same fields of cultivation. In this farming system, local people often use about 700 g of acacia seeds per hectare. The number of acacia seeds varies from 6600 to 11,600 per kilogram [83]. As a result, the number of acacia seeds that are used fluctuates from 4500 to 8000 per hectare (Figure 2b). Farmers reported that not all households used the IK method. This depends on three main factors. The first is the location of the acacia field, that being its accessibility from the main road (over 500 m). The second is the complexity of the sloping of the terrain. The third factor is dependent upon the individual experience of each farming household. Older and more agriculturally experienced farmers traditionally apply the IK as opposed to their younger counterparts. We used the participatory GIS method in the FGDs. By this method, indigenous people's capacity for accurate spatial perception is factored into the digital map systems in order to identify the different farming systems within a given area. We invited 6-8 participants, who were the most knowledgeable on the subject within their communities to participate in this aspect. Through this, we were able to determine the exact areas where acacia planting was done by indigenous farming systems as opposed to more modern farming systems. This was based on maps drawn by the participants themselves. Then the C factor values of these areas were extracted from the C map (Figure 3e) and compared at a confidence level of 95% (\(p<0.05\)). In total, a mean of 13,719 values of C factors relating to NIK farming system and 16,563 values in IK farming system areas were compared by \(t\)-test [84]. Moreover, the EVI was used to describe the difference of surface cover between IK and NIK farming systems. The EVI was calculated based on the Landsat 8 images by Equation (9) suggested by USGS [85]. \[\text{EVI}=2.5\times\frac{(\text{NIR}-\text{RED})}{(\text{NIR}+6\times\text {RED}-7.5\times\text{BLUE}+1)} \tag{9}\] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Land Use Type**} & \multicolumn{5}{c}{**Slope (Degrees)**} \\ \cline{2-6}  & **0-5** & **5-8** & **8–10** & **10–15** & **>15** \\ \hline Natural forest, Grassland, Shrub & 1.00 & 1.00 & 1.00 & 1.00 & 1.00 \\ Acacia, Perennial crops & 0.50 & 0.60 & 0.80 & 0.90 & 1.00 \\ Rice, Cassava, Corn & 0.27 & 0.30 & 0.40 & 0.45 & 0.5 \\ Residential areas, building & 0.003 & 0.003 & 0.003 & 0.003 & 0.003 \\ \hline \hline \end{tabular}
\end{table}