## 4 Results and Discussion
### Water Balance  
The water balance was precisely assessed during the experiments, considering the measurements taken throughout the tests. Simultaneously, surface flow was measured at specific intervals during the experiment. Additionally, percolation was carefully measured using a tipping bucket, and measurements were recorded by the data logger at regular intervals. These comprehensive measurements enabled a thorough evaluation of the water balance in the soil column tests.  
The water balance graph of the SC-3L-FG test in which percolation occurred, shown in Figure 8, reveals several significant findings. In the other tests, only water storage occurred, and the time-dependent storage changes across all experiments given in Figure 8 demonstrate that out of the total rainfall received, only 10% was retained within the soil layers, a mere 0.2% infiltrated as percolation, and the majority transformed into surface runoff. This outcome indicates that the amount of rainfall applied exceeded the saturated permeability of the upper soil layer, mainly due to its low permeability. Consequently, a substantial portion of the rainfall became surface runoff throughout the experiment.  
The experiments revealed challenges in sensitively measuring assessing soil infiltration due to the manual measurement of very high runoff and the need to account for evaporation over extended experiment durations. To address these issues and evaluate the infiltration rate during the tests, a modified parameter known as \"net infiltration\" was introduced. Net infiltration accounts for the effective amount of water infiltrating the soil, considering  
Figure 8: The water balance of the cover with fine gravel.  
Figure 7: Final volumetric water content of the top layer.  
factors such as evaporation and measurement errors. Consequently, the net infiltration value was calculated as the sum of water storage and percolation. This adjustment aimed to provide a more accurate representation of the soil column test by addressing these factors. The infiltration rates of the tests are shown in Figure 9.  
Based on the data shown in Figure 9, the infiltration rates in all tests, except for the gravel case, are significantly lower than the saturated hydraulic conductivity of the top layer. This observation suggests that the granular layers remain unsaturated after the breakthrough, which may limit the overall infiltration into the system. In addition, the breakthrough may have occurred in only a portion of the cross-section. This leads to the assumption that the progress of the water to the third layer is restricted for some time after the breakthrough. In the experimental study by [PERSON] et al. (2020), visualizations with colored liquid show that the barrier is initially broken at one point along the cross-section, and over time, the barrier effect disappears along the entire interface. Except for the experiment with gravel in the middle layer, no steady-state flow occurred after the breakthrough in the other sections. When comparing the materials used in the middle layer, it is apparent that the lowest infiltration rate was observed in the cases of fine and medium sand. In contrast, the highest infiltration rate was observed in the experiment employing gravel as the middle layer material.