## 5 Instrumental Monitoring
### Monitoring of Volumetric Water Content and Matrix Suction  
As shown in Figure 7a, three matric suction sensors and two volumetric water content sensors were installed behind the main scap at depths of 0.58 m, 1.10 m, 1.63 m and 1.10 m, 1.63 m, respectively. The variation in matric suction at 0.58 m depth is given in Figure 8. Matric suction generally varied between 8.5 kPa and 61.6 kPa during the two-year observation period. Although a matric suction of 208.7 kPa was initially measured, hydraulic equilibrium was achieved in a short time, and matric suction reached around 60 kPa. During the two-year monitoring period, the lowest matric suctions were measured from mid-January to the end of July at a 0.58 m depth. The highest matric suctions were measured from mid-October to the end of December. From the end of July to the middle of October, increasing and decreasing trends were observed in matric suctions. Figure 7: Location of boreholes and installed sensors. (**a**) Location map, EC-5 and MPS-6, (**b**) ECRN-100 rain gauge, (**c**) Diver sensors. The responses of volumetric water content and the matric suction at a depth of 1.10 m are given in Figures 9 and 10. Initially, the volumetric water content was 14%, and the matric suction was 154 kPa. The volumetric water content gradually increased to 17%, and the matric suction decreased to 60 kPa on 1 February 2020. The reason for the rapid decrease in matric suction equilibrium establishment of the sensors with the surrounding soil, according to the second law of thermodynamics. Between 1 February 2020 and 14 February 2020, the matric suction decreased from 60 kPa to 10 kPa, and the volumetric water content increased from 17% to 45.8%. Between 14 February 2020 and 25 May 2020, the volumetric water content was around 47%, and the matric suction was around 9 kPa. The volumetric water content decreased step by step after 25 May 2020 and reached 15% on 5 September 2020. On the other hand, there was no significant variation in the matric suctions, contrary to the expectation of an increase. A similar behavior was also observed in cases where the volumetric water content decreased step by step after 7 May 2021. This phenomenon may be due to a change in the shape of the SWCC because of crack formation in the soil. Cracks develop in soil due to drying, and this affects the mechanical and hydraulic properties of soils [54, 55]. Various studies, such as References [56, 57, 58, 59, 60], have reported that the formation of cracks in clayey soils causes significant changes in the soil water characteristic curve. For an illustration, [PERSON] et al. [59] observed that crack development increased the slope of the soil water characteristic curve at the transition zone. This results in higher volumetric water content changes but more limited matric suction changes beyond the air-entry value. According to [PERSON] et al. [57], the soil cracks deform during the drying and wetting stages, and the changes in crack volume and suction have a direct impact on the soil water characteristic curve of cracked soil. It is possible to explain the differences in the matric suction and volumetric water content values in Figures 9 and 10 based on this information in the literature. In the wetting stage, due to the insignificant number of cracks or closed cracks, the water infiltrating into the hillslope increased the water content of the soil by entering into the fine interparticle pores where the matric sutions are effective and decreased the matric suction almost simultaneously. In the drying phase, due to the crack development, a significant volume of water first drained from these cracks, and the water content of the soil was significantly reduced. Meanwhile, in fine pores where matric suction is effective since the water content was still preserved or changed slightly, matric suction change became slightly.