## 3 Experimental Set-Up for Investigation of Soil Water Retention Behavior
### Suction Profile Logging System Set-Up  
There were five UMS T5 tensiometers(r) (Figure 5a) inserted into the unsaturated zone at the elevations of 40, 60, 100, 140, 180 cm, where all were the above constant water pressure head of 36 \(\pm\) 1 cm, as shown in Figure 4b. Moreover, there is another tensiometer inserted into the saturated zone under the water table at the elevation of 20 cm, which is not specified in Figure 4b. There is a spatial resolution of 20 cm for suction profile logging at the column bottom in order to detect the dynamic pore water pressure around the capillary fringe area for the future investigation of SWRC dynamic effects under variable water pressure boundary conditions. In total, there are twenty-four sensors on four columns (six sensors on each column). These sensors are connected to one Geo-Datalogger (DT85G, Pacific Data(r)). The temporal resolution of data logging is set to 30 s. As the UMS T5 tensiometers used a Wheatstone bridge circuit (Figure 5b), according to the logger manual [54], the output value of this data logger is:  
\[B_{out}=\frac{V_{out}}{V_{ex}}\cdot 10^{6} \tag{9}\]  
where \(B_{out}\) is the dimensionless voltage, \(V_{out}\) is the measured bridge output voltage, \(V_{ex}\) is the excitation voltage. In addition, the T5 sensor measuring range is from \(-85\) kPa for suction up to +100 kPa for positive water pressure. Thus, calibration between the dimensionless voltage and water pressure was carried out for each sensor within the positive water pressure range. Due to the perfect linear equation fitting into the calibration data points in Figure 5b, the calibration equation can be applicable for the entire measurement range. Twenty-four sensors share the same slope in the linear calibration equation, except for differences in the interception (\(-0.2\)-\(1.5\) kPa) due to the varying physical offset on each sensor. It should be noted that the physical offset depends on the maintenance of the T5 sensors. After storing them for a long period, the calibration has to be reconducted to determine the new offset values due to the aging issue of the membrane in the sensor body. When the water table drops to the hydraulic head of the constant head tank, and equilibrium is approached, the suction value can also be double-calibrated and validated by calculating the suction head above the groundwater table. Meanwhile, the trend of the dynamic response of pore pressure can also be collected under the transient flow condition. The specification of the T5 tensiometer is summarized in Table 1.  
Figure 5: (**a**) UMS T5 tensiometer with a 20-cm shaft (deionized water fully filled) [53]; (**b**) the electrical circuit of T5 tensiometer [54]; (**c**) the linear calibration between dimensionless voltage bout and water pressure (the red crosses are data measured for calibration using linear regression) [53].  
For an investigation of the soil suction profile in transient flow conditions, the response delay of the pressure sensor is of particular concern. The response time of tensiometers was fully investigated by [PERSON] et al. [56]. For every tensiometer, there will be a time delay in the pressure response between the specimen and sensor body because of the permeability of the ceramic cup on the shaft tip. However, because the T5 sensor only measures a low suction range and the thickness of the ceramic cup is smaller than the HAE disk, there is only minor hydraulic resistance between the sensor and specimen. According to the T5 tensiometer manual [55], it only takes 5 seconds to approach an accurate value, and this is already the fastest option for suction measurement among other methods requiring moisture redistribution in a long equilibrium period, such as filter paper, dew point meters and ATT [1; 2; 57]. As the temporal resolution of suction logging is 30 seconds, such a small delay in response can be neglected for this experimental set-up.