Soil characteristics
The soil bulk density value on mixed plant, paddy field, and forest land use is higher on slopes > 25% with the slowest soil permeability class (Arsyad 2010), on mixed plant land use of 0.42 cm/hour (Table 1), which can increase soil susceptibility to landslides. Lowland rice plants are dominant on slopes of 0–8% with a bulk density value of 1.22 g/cm3 belonging to the compact soil category (nrm 2021), which does not trigger landslide events but can trigger flooding events in the research location.
Table 1
Relationship between soil physical properties and land use
Land Use | Slope | Bulk Density (g/cm3) | | Porosity (%) | Permeability (cm/hour) | |
I | II | III | average | I | II | III | rata-rata | I | II | III | average | |
Corn | < 25% | 1.15 | 1.23 | 1.19 | 1.19 | 53.93 | 52.94 | 54.03 | 53.63 | 0.99 | 0.99 | 1.03 | 1.00 |
| > 25% | 1.05 | 1.22 | 1.20 | 1.16 | 57.99 | 51.63 | 51.16 | 53.59 | 0.99 | 1.39 | 1.31 | 1.23 |
Mixed plant | < 25% | 1.07 | 1.10 | 1.11 | 1.09 | 55.03 | 54.18 | 54.06 | 54.42 | 1.16 | 1.16 | 1.20 | 1.17 |
| > 25% | 1.15 | 1.15 | 1.17 | 1.16 | 52.98 | 56.08 | 56.31 | 55.13 | 0.42 | 0.40 | 0.44 | 0.42 |
Paddy field | < 25% | 1.23 | 1.23 | 1.20 | 1.22 | 49.04 | 49.34 | 49.54 | 49.31 | 0.71 | 0.82 | 0.87 | 0.80 |
| > 25% | 1.24 | 1.22 | 1.21 | 1.22 | 50.01 | 49.15 | 49.35 | 49.50 | 0.76 | 0.74 | 0.75 | 0.75 |
Forest | < 25% | 1.11 | 1.10 | 1.11 | 1.11 | 53.15 | 53.44 | 53.62 | 53.40 | 1.22 | 1.05 | 1.14 | 1.14 |
| > 25% | 1.20 | 1.30 | 1.29 | 1.26 | 52.43 | 47.83 | 47.88 | 49.38 | 0.76 | 1.16 | 1.18 | 1.03 |
Horticultural | < 25% | 1.10 | 1.19 | 1.20 | 1.16 | 58.21 | 58.40 | 58.38 | 58.33 | 1.07 | 1.18 | 1.16 | 1.14 |
| > 25% | 1.13 | 1.13 | 1.12 | 1.13 | 56.81 | 57.06 | 57.16 | 57.01 | 1.16 | 1.18 | 1.19 | 1.18 |
The statistical test analysis showed a significant result showing the role of soil properties and land use in triggering landslide events and their interaction with a p-value = 0.000. The part of slopes < 25% and > 25% did not show a significant effect on landslide events, with a value of p-value > 0.05 (Table 2). But the interaction between the type of land use, slope, and physical properties of the soil on the occurrence of landslides at the study site shows a strong relationship with a significant p-value = 0.043 less than the α 5% level (Table 2). The R squared value reached 90.9%, indicating the accuracy of the relationship between the soil factor with the land use variable and the slope on the occurrence of landslides.
Table 2
The Anova of soil physical, land use, and slope
Tests of Between-Subjects Effects |
Dependent Variable: Result |
Source | Type III Sum of Squares | df | Mean Square | F | Sig. |
Intercept | Hypothesis | 30832.107 | 1 | 30832.107 | 48071.430 | .000 |
Error | 1.283 | 2 | .641a | | |
Land_use | Hypothesis | 83.518 | 4 | 20.879 | 29.384 | .000 |
Error | 5.685 | 8 | .711b | | |
Slope | Hypothesis | 2.473 | 1 | 2.473 | 4.037 | .182 |
Error | 1.225 | 2 | .613c | | |
Land_use * Slope | Hypothesis | 6.297 | 4 | 1.574 | 1.731 | .160 |
Error | 40.011 | 44 | .909d | | |
Soil_Physic | Hypothesis | 54690.968 | 2 | 27345.484 | 26900.519 | .000 |
Error | 4.066 | 4 | 1.017e | | |
Slope * Soil_Physic | Hypothesis | 3.696 | 2 | 1.848 | 2.032 | .143 |
Error | 40.011 | 44 | .909d | | |
Land_use * Soil_Physic | Hypothesis | 158.256 | 8 | 19.782 | 21.754 | .000 |
Error | 40.011 | 44 | .909d | | |
Land_use * Slope * Soil_Physic | Hypothesis | 16.221 | 8 | 2.028 | 2.230 | .043 |
Error | 40.011 | 44 | .909d | | |
Rep | Hypothesis | 1.283 | 2 | .641 | 1.231 | .652 |
Error | .242 | .465 | .521f | | |
Land_use * Rep | Hypothesis | 5.685 | 8 | .711 | .781 | .621 |
Error | 40.011 | 44 | .909d | | |
Slope * Rep | Hypothesis | 1.225 | 2 | .613 | .674 | .515 |
Error | 40.011 | 44 | .909d | | |
Soil_Physic * Rep | Hypothesis | 4.066 | 4 | 1.017 | 1.118 | .360 |
Error | 40.011 | 44 | .909d | | |
Further tests with LSD showed that almost all land uses showed an increase in BD values either on slopes < 25% or > 25%, directly proportional to the decrease in soil permeability in the slow to very slow category (Fig. 3). But inversely proportional to the porosity of the soil, which is still in the good category, it can trigger an increase in soil saturation which triggers landslides on slopes < 25% and on slopes > 25%. Increasing land use by communities and collaborating with global climate change has reduced the carrying capacity of the soil in stabilizing slopes and triggering landslides and flash floods at the study site.
Soil Micromorphological Characteristics Associated With Landslides
The micromorphological characteristics of the soil that can trigger landslides are the formation of plane voids (micro cracks), striated b-fabric, and a high percentage of clay fractions which have reduced soil resistance (Yurong et al. 2006; Ahmad et al. 2018). The formation of plane voids due to soil swell activity can be used as one of the parameters in assessing the potential for landslides (Ahmad et al. 2022a).
The micromorphological appearance of the soil in lowland rice of land use on slopes of 0–8% shows the formation of plane voids due to the shrinking-swelling of soil, which contains clay fraction of 43–51%, grano-striated, and cross-striated b-fabric (Fig. 4). Collaboration of paddy fields and slopes of 0–8% cannot trigger a landslide.
The appearance of the soil micromorphology in mixed land use (coffee-cacao-banana-timber) showed an increase in the clay fraction of 34–42% at the topsoil. Some minerals have undergone mesomorphous weathering to become clay, and clay coatings are found at the edges of mineral crystals (Fig. 5). Increasing the clay content in the subsoil (46–60%) has increased bulk density and decreased soil permeability (Table 1), with weathering of minerals in the subsoil layer showing catamorphic weathering of minerals. Most of the minerals have been crushed and weathered to form clay minerals accompanied by the intensive formation of planar planes, grano-striated, and striated b-fabric (Figs. 6 and 7).
The clay fraction in the subsoil is higher than in topsoil, especially in the land use of mixed gardens, forests, and lowland rice. Land management, mineral type, and increased rainfall resulted in a more intensive weathering process. At the same time, the porosity was still in good condition. Still, it had decreased permeability in the subsoil, making the soil easily saturated and triggering landslides on slopes < 25% and > 25%. The formation of cross-striated and striated grano can trigger an internal shift (micro-shift) in the soil body.
General Parameter For Landslide Susceptibility Mapping
The most common parameters used to assess landslides used by experts in determining landslide susceptibility classes are the lithology factor, rainfall factor, slope factor, land cover, and land use factor, and population. (Bachri et al. 2020; Batar and Watanabe 2021; Zou and Zheng 2022) (Fig. 8). The OLS analysis results show that the lithology parameter is an essential factor causing an increase in soil susceptibility at the study site with a significant Robust_Probability value at p-value < 0.01 (Table 3). This is in accordance with the results of research from Hong et al. (Hong et al. 2017), which shows that lithology parameters trigger the dominant landslide events.
Table 3
The OLS statistic of landslide susceptibility mapping
Variable | Coefficient [a] | StdError | t-Statistic | Probability (b) | Robust_SE | Robust_t | Robust_Pr [b] | VIF [c] |
Intercept | 2.184028 | 2.090371 | 1.044804 | 0.313817 | 1.513412 | 1.443115 | 0.170994 | -------- |
Population | 0.000054 | 0.000093 | 0.579704 | 0.571328 | 0.000077 | 0.702179 | 0.494075 | 1.137159 |
Lithology | -0.112894 | 0.056641 | -1.993132 | 0.06611 | 0.02333 | -4.839021 | 0.000264* | 1.221578 |
LULC | 0.015147 | 0.048223 | 0.314097 | 0.758083 | 0.016231 | 0.933167 | 0.366546 | 1.067081 |
Slope | 0.005504 | 0.01915 | 0.287429 | 0.777997 | 0.012724 | 0.43258 | 0.671914 | 1.174942 |
Rainfall | 0.000594 | 0.000535 | 1.111308 | 0.285156 | 0.000443 | 1.342139 | 0.200916 | 1.12064 |
*significant on p < 0.01 |
The Koenker value (BP) statistic is 0.588275, which is greater than p-value > 0.01, showing that the model relationship is relatively consistent. But the Jarque-Bera Statistic value is 0.010880 with a p-value < 0.01, indicating the prediction model is biased. So the R2 value is only 30.8%, meaning that there are still 69.2% external factors that influence the dependent aspect, the results of mapping the soil susceptibility do not provide accurate results in predicting the level of susceptibility to landslides in the study location. This shows that the most common parameters used to determine the class of landslide susceptibility have not been able to provide an accurate model in the field of landslide events that occur.
Specific Parameters For Landslide Susceptibility Mapping
Soil parameters have been used by several researchers in assessing landslide susceptibility, especially soil texture, macro-micro porosity, and erodibility (Fonseca et al. 2017b; Conforti and Ietto 2021). The addition of soil micromorphology data in the form of plane voids has significantly affected the physical occurrence of landslides at the study site (Ahmad et al. 2022a). The spatially adding soil organic carbon, soil texture, soil erodibility, and soil micromorphology data (Fig. 9) to assess landslide susceptibility is expected to provide more accuracy and validity in producing susceptibility maps. The results of the OLS regression analysis are presented in Table 4.
Table 4
The OLS statistic of landslide susceptibility mapping
Variable | Coefficient [a] | StdError | t-Statistic | Probability (b) | Robust_SE | Robust_t | Robust_Pr [b] | VIF [c] |
Intercept | -1.63055 | 7.828637 | -0.20828 | 0.839206 | 8.282964 | -0.196856 | 0.847899 | -------- |
Population | 0.005859 | 0.05316 | 0.110206 | 0.914438 | 0.049626 | 0.118054 | 0.908374 | 2.451664 |
C-Organic | -0.172078 | 0.16288 | -1.056466 | 0.315615 | 0.1682 | -1.023053 | 0.330397 | 1.110746 |
Soil-Erodibility | 2.124802 | 0.843201 | 2.519924 | 0.030381* | 0.718342 | 2.957925 | 0.014346* | 2.173448 |
LULC | 0.065958 | 0.051014 | 1.292932 | 0.225122 | 0.031907 | 2.067167 | 0.065582 | 1.668384 |
Slope | 0.019528 | 0.018249 | 1.070104 | 0.309727 | 0.012665 | 1.541891 | 0.154139 | 1.703753 |
Rainfall | -0.000624 | 0.000677 | -0.920981 | 0.378738 | 0.000791 | -0.788648 | 0.448597 | 2.662604 |
Texture | 0.065843 | 0.183515 | 0.358786 | 0.72722 | 0.364225 | 0.723279 | 3.394234 | -------- |
Lithology | 0.092355 | 0.137048 | 0.673891 | 0.515639 | 0.15826 | 0.583564 | 0.572426 | 2.714366 |
Plane voids | -0.000041 | 0.000018 | -2.311536 | 0.043379* | 0.00002 | -2.061113 | 0.066251 | 1.955582 |
*significant on p < 0.01 |
The Koenker value (BP) statistic is 0.149728, which is greater than the p-value > 0.01, showing that the model relationship is relatively consistent. The Jarque-Bera Statistic value is 0.925161 with a p-value > 0.01, which shows that the prediction model is not biased. The R2 value of 66.66% indicates that the independent factor has a significant effect on the dependent factor, so the results of soil vulnerability mapping provide more accurate results in predicting the level of landslide susceptibility in the study location. This shows that soil erodibility parameters and micromorphology, especially plane voids, can improve the accuracy of landslide susceptibility classes and provide an accurate model of landslides occurring in the field.