## 4 Results and Discussion  
The changes in the volumetric water content over time were examined across four separate soil columns during the infiltration experiments. In each test, the increase in the volumetric water content in the top layer began approximately 1.5 h after the start for TDR1, situated 5 cm from the soil surface, and after 5 h for TDR2, positioned 10 cm below the soil surface. Interestingly, despite the middle layers having higher permeability, delays in water movement through these layers were observed for varying durations (Figure 4). In the fine sand test (SC-3L-FS), an increase in the volumetric water content of TDR-3, positioned below the interface, was noted after approximately 54.5 h. The water content in the fine sand displayed a time-dependent and gradual increase, while a similar rise in the water content of the bottom layer was observed after roughly 114 h. A rise in the volumetric water content of approximately 5% was recorded in the middle layer, whereas 7% was measured in the lower layer. Throughout the 500 h test, no percolation was observed, and the column did not reach a steady-state condition. During the medium sand test (SC-3L-MS), the volumetric water content of TDR-3, positioned below the interface, began to slightly increase after approximately 92.5 h. This observation indicates that the medium sand, characterized by higher saturated conductivity than fine sand, retained water at the interface for an extended duration. While the water content increased by only 3% in the middle layer, the bottom layer became wetted slightly faster and retained more water than the middle layer. Additionally, despite the water requiring more time to infiltrate the middle layer with medium sand present, the volumetric water content in the bottom layer increased earlier (approximately at 102 h) compared to the test with fine sand (SC-3L-FS). Percolation did not occur during the test, and the change in volumetric water content did not stabilize. In the SC-3L-CS test, where coarse sand was placed in the middle of the soil column, no initial change in the volumetric water content was observed in TDR-3. However, a marginal increase in the volumetric water content in the third layer was noted after 68 h. Unfortunately, the test was terminated at 350 h due to issues with the peristaltic pump, and no percolation was observed. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Test Name**} & \multicolumn{3}{c}{**Bulk Density (gr/cm\({}^{3}\))**} & \multicolumn{3}{c}{**Initial Water Content (\%)**} & \multicolumn{1}{c}{**Precipitation Rate (mm/h)**} \\ \cline{2-7}  & **Top** & **Middle** & **Bottom** & **Top** & **Middle** & **Bottom** & \\ \hline SC-3L-FG & 2 & 1.5 & 2 & 8 & - & 8 & 3 \\ SC-3L-CS & 2 & 1.5 & 2 & 8 & - & 8 & 3 \\ SC-3L-MS & 2 & 1.5 & 2 & 8 & - & 8 & 3 \\ SC-3L-FS & 2 & 1.5 & 2 & 8 & - & 8 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The initial condition of the soil layers for each test. Figure 4: Volumetric water content variation with time and depth for soil column infiltration tests. (**a**) SC-3L-FS, (**b**) SC-3L-MS, (**c**) SC-3L-CS, and (**d**) SC-3L-FG. In the SC-3L-FG test involving fine gravel in the middle layer, a notable increase in the volumetric water content in the third layer became evident at the 26 th h, indicating a breakthrough. Similar to the experiment with coarse sand, there were no significant changes in TDR-3 readings. Once the volumetric water contents reached equilibrium, percolation commenced at the 135 th h.  
The measurement of volumetric water content within both the coarse sand and fine gravel columns in the coarse-grained layer could not detect the increase in volumetric water content during the infiltration process. This occurrence might be explained by the presence of preferential flow within the coarse-grained layer, as highlighted in previous studies [23; 32; 38].