3.2. MAGT and ALT during the reference period
Using the collected borehole data, we fitted the meteorological factors
and geographical environmental factors to obtain the MAGT distribution
of the permafrost regions on the QTP (Figure 4). We extracted the MAGT
of the QTP below 0 °C as an average range of permafrost (Chen et al.,
2015), which indicating suitable conditions for permafrost, with a total
area of 1.04 × 106 km2 (excluding
glaciers and lakes). Considering the heterogeneity and uncertainty of
ground temperature on the QTP, the minimum permafrost extent is 0.8 ×
106 km2 (the area within MAGT ≤
−0.5°C), and the maximum extent is 1.28 × 106km2 (the area within MAGT ≤ +0.5°C). Compared with the
pan-Arctic permafrost, the permafrost temperature on the QTP is
relatively high (Obu et al., 2019). Nearly half of the permafrost
temperature area on the QTP exceed -1.0ºC and the average temperature is
-1.35 ± 0.42 ºC. The permafrost temperature is not only affected by
latitude, but also by altitude. As illustrated in Figure 4, the
lower-temperature permafrost on the QTP generally occurs in
high-altitude mountains, and the ground temperature gradually rises with
decreasing altitude, with the lowest value distributes in the Kunlun
Mountain and its surrounding regions. In general, the MAGT on the QTP
was found to be higher in the southern region (GZIR) than that in the
northern region (AEJIR), and higher in the eastern region (WQIR) than
that in the western region (XKLIR).
Based on permafrost extent, the spatial distribution of the ALT for the
entire QTP was obtained (Figure 5). The statistical results indicated
that the average ALT is 2.3 ± 0.60 m on the QTP, and the ALT value of
~ 90% of the permafrost region ranged from 1.6 to 3.0
m. Geographically, the ALT in the eastern part of the QTP is relatively
thinner (generally no more than 2 m) with slight variations. The ALT
along the Qinghai-Tibet Highway and in the central and western plateau
is highly heterogeneous. The overall ALT pattern is thin in the
mountains, thick on the plains, thin in the hinterlands, and thick along
the periphery of the permafrost. The maximum value appears along the
southern boundary of the permafrost and the surrounding sporadic
permafrost (generally ≥ 3.2 m). In general, MAGT and ALT exhibit a
consistent trend in spatial distribution, with a correlation coefficient
of 0.44. The smaller value of MAGT corresponds to thinner ALTs.