6.2 Active layer depths
Ground thaw starts in mid-June and continues to mid-September. According to Grave (1964) maximum active layer thickness (ALT) reaches 75 cm at the goltsy landscape and 30-45 cm at the swampy landscape (Table 1).
For simulating ALT at the goltsy landscape we varied two types of initial conditions: 1) with lenses of ground ice in upper part of soil profile and 2) without ground ice. In the first case (with ground ice) the average simulated ALT (1957-1964) is 32 cm with the range from 20 cm (1958) to 42 cm (1960) at RFC #1 (Fig. 4). The same values are 33 cm for the period 1966-2012 with maximum 70 cm in 1968 and minimum 2 cm in 1979. In the second case (without ground ice) simulated ALT varies in the range of 50-80 cm for the period 1957-2012.
The average simulated depth of the active layer (1966-2012) at the RFC #2 is 180 cm, at the RFC #3 is 122 cm, at the RFC #4 is 67 cm. In general simulated ALT exceed observed values in those landscapes for which the model parameters were adopted from the study at the Kolyma water-balance station (Makarieva et al., 2018a).
For example, mean ALT values in tundra and rocky talus landscapes are about 1.5 m there, reaching 2.5 m at steep southern slopes due to higher air temperatures and longer freeze-free period (about 130 days compared to 55 days at the Suntar-Khayata Station).
It is important to mention that there are no statistically significant increasing trends of simulated ALT values in long-term period.