## 1 Introduction  
In nature, the vadose zone above the water table can be several meters high. In this zone, the moisture stored in the soil matrix does not fill the pores completely. This form of water is no longer in positive potential but negative, as it presents a capillary tensile interaction that binds the soil particles [1]. This binding interaction thus enhances the mechanical properties of unsaturated soil, resulting in less deformation [1, 2] and higher shear strength [3, 4] compared to saturated soil, where water molecules encounter a compressive interaction [5]. Unsaturated soil mechanical behavior is, therefore, dependent on environmental conditions, which enhance or alleviate this binding effect, such as evapotranspiration and draining or flooding and intensive rainfall [1, 2, 6]. On the other hand, due to the air invasion into the soil matrix to form such a capillary water meniscus, the amount of effective hydraulic conductive channels is reduced as well and this degradation of geo-gas and groundwater flow conduits in unsaturated soil are highly concerning for the investigation of solute transport in the vadose zone [6]. For quantifying the contribution of capillary effects as water protentional in energy form and moisture content to the hydro-mechanical properties of unsaturated soil, the soil suction profile and soil moisture content profile need to be determined in order to characterize the soil water retention curve (SWRC), which is the core unsaturated soil constitutive relationship between the soil suction and moisture content (gravimetric/volumetric) [2]. The measurement of SWRC can be conducted either in the laboratory or in the field. In the laboratory, the standard axis translation technique (ATT) is often implemented [1]. Briefly, there is a one-dimensional representative elementary volume (REV) scaled soil specimen placed in a small fluid leakage-free chamber; through adding air pressure on top of the specimen, soil moisture can be expelled out of the soil matrix; the suction is the difference between applied air on top and water pressure at the bottom; the water moisture variation can be determined by measuring the mass of specimen or volume of water expelled out. Although ATT can be implemented in the laboratory due to the small set-up, this technique can only measure the SWRC under the static condition, for which there is no further moisture redistribution and capillary non-equilibrium. In the field, SWRC is usually given by separately measuring the suction profile using tensiometers and the moisture profile using in-situ sampling soil for the oven-drying method [7]. Compared to the ATT technique on a laboratory scale, this approach is more labor-expensive. In addition, sampling of soil disturbs the original configuration of the soil strata further, inducing moisture reconfiguration. This phenomenon is inevitable for in situ measurement and leads to potential falsification. Due to these limitations, there is always disagreement between the SWRC measured using ATT and in situ methods. Moreover, neither approach is able to measure the SWRC under the transient flow condition due to the requirement of equilibrium achievement and sampling procedure. Therefore, even for a homogeneous soil specimen, the conventional methods constrain the understanding of the spatial effect on soil suction determination [8; 9], the temporal effect on moisture reconfiguration [10] and the dynamic effect in SWRC [11; 12; 13]. To overcome these limitations, there is a strong need for a laboratory experiment maintaining the homogeneity of a specimen in a full-scale soil profile without uncontrollable environmental conditions. For measuring the SWRC under the transient flow condition, the spatially distributed moisture content needs to be measured with high temporal resolution. Some previous literature has implemented the soil column experiment using point-wise moisture sensors and tensiometers for logging the transient response of soil moisture and suction [14; 15; 12; 16].