# Planning MAR Schemes Using Physical Models: Comparison of Laboratory and Field Experiments
## 4 Discussion  
The setup of the laboratory experiments caused distinct differences compared to the FIU measurements. Dimensionality, the lower boundary condition, sidewall flow, and climate are factors that could be identified as reasons for the differences shown by the statistical parameters. The dimensionality of the column led to restricted lateral flow and, thus, an overall higher degree of saturation in comparison with the 3D experiments. It further stimulated the entrapment of air during intermittent infiltration, which could not escape laterally. Entrapped air can lead to reduced hydraulic conductivity and a further increased degree of saturation [33; 34; 35]. Studies showed that this effect becomes less prominent for homogenous soils [36], as well as for coarse soils, where air can escape upwards [37]. Air entrapment becomes stronger the finer the soils [37]. Thus, the higher silt content of the second soil could have increased the effect of air blockage and respective higher saturation degree. This would lead to a more distinct difference between the tension values from the laboratory experiments and the FIU. The lower tension measurements in the deeper layers of the laboratory experiments can be attributed to the free drainage boundary condition [20]. Water starts percolating from the drainage layer only after full saturation has been reached, leading to the build-up of a capillary fringe that can be up to 30 cm thick for sandy soils [38]. The siltier second soil could lead to an even higher built-up of the capillary fringe [38]. Since the measurement point in the lower layer is only 16 cm above the drainage, an influence of the capillary fringe is highly probable. The constant tension levels in the laboratory experiments further point to the restricted free outflow at the bottom. In the FIU, dewatering continues after the initial phase of rapid drainage. In the laboratory experiments, when during the drying phase no water flows in from the top layer, the degree of saturation at the bottom of the profile was insufficient to activate percolation into the drainage layer. Thus, some water remained in the lower layer of the experimental setup. This effect is amplified in the column where restricted lateral flow in combination with constrained drainage led to a significantly higher water filled pore space of the soil. The constant arrival times of the wetting front in the column experiment could be triggered by preferential flow paths, such as sidewall flow. This effect is especially prominent in 1D-systems, where side walls are close to the measurement points and can potentially influence water flow measurements [20; 21]. Here, flow time estimation through column studies is clearly limited, while tank studies show similarities to FIU measurements. Still, tank and FIU data differs in the intensity of flow time increase, indicating that general flow mechanisms are comparable, but a quantitative analysis cannot be made. Wall effects are also a cause for the phase changes, during wet-dry cycles, that were not exactly met by the different setups (Figure 7). Preferential flow paths and fingering could also trigger this behavior. Figure 7: Tension measurements of the upper layer during scenario 2 for (**a**) the FIU/tank and (**b**) the FIU/column experiments and the adapted laboratory tension values by regression function, with a 95% confidence interval (Fit). The water content measurements of the tank and FIU coincide with the tension measurements in terms of temporal reaction to infiltration events and to the subsequent drying (Figures 4 and 5). Assuming similar water retention curves for FIU and tank, the tension measurements would indicate, however, that water content in the FIU must always be below that measured in the tank. Results shown in Figure 5 do not confirm this hypothesis. In fact, water content in the tank ranks only above FIU values during the drying phase in 0.7 m depth.