## 3 Results of the Field Measurements of Soil Temperature  
The data measurements from the sensor were recorded every 4 min and were stored in on-board memory. Then, the stored data were transmitted to the remote monitoring center every hour via the iridium satellite network. The soil temperature was measured 160 days from 10 May to 16 October 2015. Figure 3 shows the variations of the daily averages of air temperature at the location \"X\" and soil temperatures in the sites A and B. Overall, there was a trend where temperatures gradually increased until the 60 th day, before decreasing gradually for an additional 90 days. During the soil temperature measurement period, the air temperature was overall above 0 \({}^{\circ}\)C, except several days (132 th-136 th, 144 th-145 th, and 157 th-160 th from the start of temperature measurement); however, soil temperatures within the depth range of 5-30 cm were above 0 \({}^{\circ}\)C.  
From Figure 3, it was observed that the maximum soil temperatures in the sites A and B were different from each other. The reason for this difference between the sites A and B was their differences of soil stratum and heat transfer characteristics. The soil temperature at the uppermost depth from the ground surface showed more drastic variations during the measurement period. The lower-depth soil temperature change was highly governed by the heat change at the soil surface due to variations in radiation and air temperature. Heat from the soil surface transferred into deeper depths. Therefore, soil temperatures of deeper depths were highly influenced by the upper soil temperature. The soil temperature variation tended to decrease with increasing depth. The maximum measured soil temperature decreased with increasing depth. Generally, a little lag in the soil temperature was observed among these points measured at different depths. Figure 4 reorganizes the soil temperature time history to group those taken from the same depths in the sites A and B. As is clearly seen from Figure 4, the soil temperature measurements of each site showed different temperature distributions according to different soil properties, such as soil density,  
Figure 2: Locations of the soil temperature measurement sensors of the test sites. For the site A, the highest temperature at the uppermost depth of 5 cm was 15.4 \({}^{\circ}\)C. At the remaining depths of 10, 15, 20, and 30 cm, the highest temperatures were measured as 13.4 \({}^{\circ}\)C, 10.9 \({}^{\circ}\)C, 8.3 \({}^{\circ}\)C, and 5.2 \({}^{\circ}\)C, respectively. Overall temperatures from the site B were higher than those from the site A. With increasing depth, the measured peak soil temperatures were 23.1 \({}^{\circ}\)C, 18.0 \({}^{\circ}\)C, 15.2 \({}^{\circ}\)C, 11.1 \({}^{\circ}\)C, and 6.2 \({}^{\circ}\)C, respectively. Figure 5 represents the relationships between the soil temperature measured at a depth of 5 cm (\(T_{ud}\)), i.e., the uppermost depth considered in this study, and soil temperatures at depths of \(i\) cm (\(i\) = 10, 15, 20, and 30 cm) (\(T_{i}\)). Due to the scatteredness of points at each site in Figure 5, the relationship between the two measurements (\(T_{ud}\) and \(T_{i}\)) could not be described in a simple mathematical form. Additionally, the gradient of \(T_{i}\) increased with increasing depth, while gradient of \(T_{ud}\) decreased with increasing depth. This result corresponds to the previous results on air temperature and soil temperature [50,51]. From the soil temperatures at a depth of 5 cm shown in Figure 4a, it is evident that an active layer developed within approximately 160 days (from 10 May to 16 October 2015) as the measured soil temperatures exceeded 0 \({}^{\circ}\)C. As the global warming effect is significantly pronounced at high-latitude regions during the period of active soil layer development, the investigation of soil temperature above 0 \({}^{\circ}\)C at the uppermost depth of a certain location during this period is useful in modeling the profile of soil temperature.