1. INTRODUCTION
Permafrost
results from interactions between the land and atmosphere, so its
spatial distribution is mainly determined
by climatic conditions (Ferrians
and Hobson 1973; Harris, 1981; Cheng and Dramis
1992;
Riseborough et al. 2008).
However,
the influence of local environmental conditions such as slope, aspect,
vegetation, snow cover, and soil conditions may outweigh the climatic
background, resulting in heterogeneous permafrost conditions (ground
temperature, active layer and permafrost thickness) at the local scale
(Brown
1973; Williams and Smith 1989; Camill and Clark 1998; Camill, 2000;
Cheng 2004; Heggem et al. 2006; Lin et al. 2019; Luo et al. 2019). Local
conditions may influence the ground thermal regime of permafrost by
controlling, for example, incoming solar radiation, heat convection and
conduction processes, and ground ice conditions
(Cheng
2003).
The
Qinghai-Tibet Plateau (QTP) is one of the highest plateaus in the world,
and represents the largest area of high-elevation permafrost area on
Earth
(Zhou
et al. 2000). The occurrence of high-elevation permafrost is mainly
controlled by climate,
topography,
and surface conditions
(Cheng
1983; Harris 1986; Gorbunov 1988; Cheng and Dramis 1992). At the global
scale,
latitude and atmospheric circulation generally control the distribution
of permafrost, while local factors such as topography and surface
conditions strongly regulate site scale ground thermal conditions (Zhang
et al. 2000).
The
QTP includes
mountainous
topography, and the differences in slope aspect are significant. Due to
the high elevation, low latitude, and clear air, significant solar
radiation reaches the ground surface on QTP (Xu and Chen 2006). As a
result, the absorption or reflection of solar radiation differs greatly
on slopes depending on aspect (Lin et al. 2015a; Wang
et
al. 2016), significantly affecting permafrost conditions (e.g., Gorbunov
1978; King 1986).
The influences of slope direction
on permafrost have been reported in several areas. For example, in
southeast Yukon, Canada, vegetation growth and active layer thickness
differed at four sites with similar altitude and geological conditions
but different slope directions (Price 1971). At Tianshan Mountain,
China, the average annual ground temperature difference between sunny
and shady slopes at the same elevation can reach 4.6 ℃ (Cheng 2003). The
difference in active layer thickness on north and south facing slopes of
Kunlun Mountain, China, was ~1.0 m, and permafrost
temperatures differed by 0.5 ℃ (Lin
et
al. 2015a; Luo et al. 2019). The permafrost thickness on the southwest
slope of Fenghuo Mountain was about 70 m, and about 120-145 m on the
northeast
slope.
A
recent report from discontinuous permafrost regions in northern Mongolia
indicated mean ground surface temperature (MGST) differences of 3-4°C
between north- and south-facing slopes over short horizontal distances
(Munkhjargal et al. 2020). In addition, several studies have focused on
the effect of embankment slope aspect on subgrade engineering by
examining asymmetrical subsidence along infrastructure in permafrost
regions (Hu et al. 2002; Lai et al. 2004; Chou et al. 2008; Niu et al.
2011; 2015; Zhang et al. 2017).
Although
these studies reported on differences in ground temperature and
subsidence on opposing slopes, there has not been an in-depth
explanation of the mechanisms responsible for the differences with
supporting field data. This study
more fully examines differences in
air and ground temperature, moisture content, radiation, and soil
texture and SOM content at two sloping sites with opposing aspects in an
area with warm and ice-rich permafrost on QTP. The aim of the study is
to elucidate the climate-permafrost relationship in this mountainous
area, and attempt to understand the impacts of permafrost degradation
due to slope effect.