## 2 Materials and Methods
### Filter Paper Method to Determine the Matric Suction
On the other hand, the lower portion of the curves represented the moisture retained by capillary or surface tension forces between soil particles. Furthermore, the filter paper water content break point for Whatman No. 42 was 45.3% and 54% for Schleicher and Schuell No. 589 [31]. Between the two types of filter paper, Whatman No. 42 was utilized in the study, because it is more consistent and it is the most suitable for a wide range of suction, which has a bilinear calibration curve [32; 33]. Based on the paper calibration curve in ASTM D5298 [31], the slope of Equation (1) is \(-0.0779\) when the filter paper water content is greater than 45.3%, while the slope is \(-0.0102\) when the filter paper water content is less than 45.3%. Moreover, the intercept is 5.327 when the filter paper water content is greater than 45.3%, while the intercept is 1.882 when the filter paper water content is less than 45.3%. \[h=mw_{f}+b \tag{1}\]  
where \(h\) is the matric suction in log\({}_{10}\) kPa, \(m\) is the slope of the paper calibration curve in log\({}_{10}\) kPa/%, \(w_{f}\) is the water content of the filter paper in percentage, and \(b\) is the intercept of the filter paper calibration in log\({}_{10}\) kPa. Predicting the Unsaturated Hydraulic Conductivity Using Empirical Models and the Best Fit Water Retention Curve  
Using the obtained matric suction through the filter paper method, the unsaturated hydraulic conductivity may be predicted using empirical models and the best fit WRC. A way to establish the best fit WRC, or the soil water characteristic curve (SWCC), was proposed by [PERSON] et al. [34]. According to [PERSON] and [PERSON] [35], the proposed equation by [PERSON] et al. [34] may be used for a wide range of soils. Moreover, [PERSON] and [PERSON] [35] suggested that the correction factor may be equal to 1 to simplify the equation of [PERSON] et al. [34]. In this study, the volumetric water content was obtained by using Equation (2), which was proposed by [PERSON] et al. [34] with a correction factor equal to 1. Equations (3)-(6) were formulated by [PERSON] and [PERSON] [36] to determine the fitting parameters and variables shown in Figure 6. Moreover, the equations were in accordance with the relationship between the volumetric water content and suction proposed by [PERSON] et al. [34] plotted in a semi-log graph. Figure 6 shows the different zones in the WRC, wherein the transition zone is from the air entry value to the residual point, and the residual zone is beyond the residual point. \[\theta=\frac{\theta_{s}}{\left[ln\left(e+\left(\frac{\psi}{a}\right)^{n} \right)\right]^{m}} \tag{2}\]  
\[n=\frac{1.31^{m+1}}{\theta_{s}\times m}3.72\times s_{1}\times\psi_{i} \tag{3}\]  
\[\theta_{i}=\frac{\theta_{s}}{1.313^{m}} \tag{4}\]  
Figure 2:0  
Figure 2:1  
where \(\theta\) is the volumetric water content, \(\theta_{s}\) is the saturated volumetric water content, \(\psi\) is the soil suction in kPa, \(a\) is the matric suction at the inflection point in kPa, \(n\) and \(m\) are fitting parameters, \(S_{1}\) is the slope at the transition zone, \(\theta_{i}\) is the volumetric water content at the inflection point, \(\psi_{b}\) is the air entry value in kPa, \(\theta_{r}\) is the residual volumetric water content, \(\psi_{r}\) is the suction corresponding to the residual water content in kPa, \(S_{2}\) is the slope at the residual zone, \(\theta^{\prime}\) is the volumetric water content at the point where the curve starts to drop linearly, and \(\psi^{\prime}\) is the matric suction at the point where the curve starts to drop linearly in kPa. Once the best fit WRC was established, the volumetric water content and corresponding matric suction was obtained to determine the unsaturated hydraulic conductivity using an empirical model. The model used to predict the unsaturated hydraulic conductivity was formulated by [PERSON] et al. [34] and is shown in Equation (7).