Climate warming and engineering activities have various impacts on the thermal regime of permafrost in alpine ecosystems of the Qinghai–Tibet Plateau. Using recent observations of permafrost thermal regimes along the Qinghai–Tibet highway and railway, the changes of such regimes beneath embankments constructed in alpine meadows and steppes are studied. The results show that alpine meadows on the Qinghai–Tibet Plateau can have a controlling role among engineering construction effects on permafrost beneath embankments. As before railway construction, the artificial permafrost table (APT) beneath embankments is not only affected by climate change and engineering activities but is also controlled by alpine ecosystems. However, the change rate of APT is not dependent on ecosystem type, which is predominantly affected by climate change and engineering activities. Instead, the rate is mainly related to cooling effects of railway ballast and heat absorption effects of asphalt pavement. No large difference between alpine and steppe can be identified regarding the variation of soil temperature beneath embankments, but this difference is readily identified in the variation of mean annual soil temperature with depth. The vegetation layer in alpine meadows has an insulation role among engineering activity effects on permafrost beneath embankments, but this insulation gradually disappears because the layer decays and compresses over time. On the whole, this layer is advantageous for alleviating permafrost temperature rise in the short term, but its effect gradually weakens in the long term.
Climate warming and engineering activities significantly impact permafrost thermal regimes on the Qinghai–Tibet Plateau (Cheng and Wu, 2007; Jin et al., 2008a; Wu and Zhang, 2008; Zhang et al., 2008; Yang et al., 2010). However, the response of permafrost to climate warming differs greatly from that of engineering construction (Wu et al., 2007; Gao et al., 2015). This difference is mainly caused by permafrost thermal stability (Wu et al., 2007).
Because permafrost is the result of energy and mass exchange between the ground surface and atmosphere, its response to climate warming and engineering activities is modulated by ground surface conditions, e.g., vegetation, soil, and geological conditions (Brown et al., 2000; Hinkel and Nelson, 2003; Frauenfeld et al., 2004). Permafrost has a close relationship with alpine ecosystems, and changes of permafrost can significantly affect those ecosystems (Callaghan and Jonasson, 1995; Jorgenson et al., 2001; Hinzman et al., 2005; Wang et al., 2006, 2012; Shur and Jonasson, 2007; Gregory et al., 2012). Climate warming has varying thermal impacts on permafrost in different alpine ecosystems (Wu et al., 2015); for example, change in permafrost temperature and active-layer thickness (ALT) of alpine meadows is greater than that of alpine steppe (Wu et al., 2015). Therefore it is a concern whether engineering activities have thermal impacts on permafrost that vary with ecosystems. Further, removing or retaining vegetation in highway or railway construction may cause differences in permafrost change and engineering stability. However, there has been little research in this area.
Engineering activities on the Qinghai–Tibet Plateau, e.g., the Qinghai–Tibet highway (QTH) and railway (QTR), have resulted in a substantial increase of permafrost temperatures, rise of the permafrost table, and thawing of ground ice near the permafrost table beneath embankments (Wu et al., 2002, 2010b; Sheng et al., 2002; Ma et al., 2009; Mu et al., 2012). However, the cited research works treated only the thermal disturbance impacts of the highway or railway on permafrost beneath embankments. There has been little attention to interaction among engineering activities, vegetation or soils near the ground surface, and permafrost beneath embankments, an exception being McHattie and Esch (1983), who studied the permafrost benefits of a peat underlay in roadway construction.
The main objective of the present study was to investigate the thermal impacts of engineering activities on permafrost beneath embankments in various ecosystems, using data and information from a continuous record of permafrost temperature monitoring along the QTH and QTR corridor. We first focus on the analysis of annual means and variability of the artificial permafrost table (APT) beneath embankments in alpine meadows and steppes over the periods 1996–2014 and 2005–2014. We then investigate trends of soil temperature and various impacts of ecosystems in driving changes of soil temperature. Finally, we assess the advantages and disadvantages of removing vegetation during engineering construction.
The soil temperature data used were obtained from nine monitoring sites along the QTH and QTR (Fig. 1). Because vegetation was removed when the QTH was constructed but remained present when the QTR was constructed, data were obtained from six centerline boreholes beneath the highway embankment (three in alpine steppe and three in alpine meadow) and six beneath the railway embankment (four centerline boreholes in alpine meadow and two shoulder boreholes in alpine steppe). For comparison, soil temperature from a borehole beneath a natural surface was obtained from the same location as the centerline borehole beneath the embankment for all sites.
Geographic locations of 12 monitoring sites.
Soil temperature was measured at 12 sites from the Chumaer high plain in the north to the Tanggula Mountains in the south, with six sites along the QTH and six sites along the QTR (Fig. 1). Five boreholes were drilled in the Chumaer high plain (two along the railway and three along the highway); four in the Beiluhe Basin (railway); and three in the Fenghuo, Kaixinling, and Tanggula Mountains (highway). Geographic information and ecosystems of these sites are listed in Tables 1 and 2.
Mean annual air temperature ranges from about
Alpine grassland along the QTH and QTR mainly includes alpine meadow and
alpine steppe (Wang et al., 2006). Five monitoring sites in the Chumaer high
plain are in alpine steppe, with 5–10 % vegetation cover at CMH1, CMH2,
and CHM3, and 20–30 % at CMR1 and CMR2 (Fig. 1). Four sites in the
Beiluhe Basin are in alpine meadow, with vegetation cover 60–70 at BLR1,
30–40 at BLR2, 60–70 at BLR3, and 80–90 % at BLR4. Another three
monitoring sites – in the Fenghuo, Kaixinling, and Tanggula Mountains – are
also in alpine meadow, with vegetation cover
Information on 12 monitoring sites along QTH and QTR.
All monitoring sites were established in 2002 except for FHH1 in 1996 and
CMR1 and CMR2 in 2005. Soil temperature measurements from all sites are
continuous through the present. Soil temperature was measured at depths
0.5–18 m beneath the surface of the embankment centerline at all sites
except for that beneath the sunny-side shoulder at CMR1 and CMR2. All
measurements were made by a string of thermistors at depth increments of
0.5 m. These thermistors were made by the State Key Laboratory of Frozen
Soil Engineering. Laboratory temperature accuracy of these sensors is
We analyzed the long-term trends and variability of APT, ALT, and permafrost
temperature beneath embankments in alpine meadow and steppe from 2002 through
2014. APT and ALT were estimated as the maximum thaw depth in late autumn, using linear
interpolation of soil temperature profiles between two neighboring points
near and below the 0
Climate and environmental parameters at 12 monitoring sites along QTH and QTR.
MAAT: mean annual air temperature; ALT: active-layer thickness; MAGT: mean
annual ground temperature at depth of zero annual amplitude, usually at
10–15 m depth below ground surface on the plateau; FST: frozen soil types,
where H stands for frozen soils with ice, B for saturated frozen soils, F
for saturated frozen soils with excess ground ice, and D for icy soils; VC:
vegetation cover.
Mean APT beneath embankments in alpine steppe between 2002 and 2014 ranged from 6.5 m at CMR1 and CMH2 to 7.83 m at CMH3, with an average of 7.03 m (Table 3). In comparison, mean APT beneath embankments in alpine meadow between 2002 (1995 at FHH1) and 2014 ranged from 3.39 at BLR3 to 8.43 m at KXH1, with an average of 4.86 m, resulting in a difference of mean APT between alpine meadow and steppe of 2.2 m. This difference is similar to results below natural surfaces in alpine meadow and alpine steppe (Wu et al., 2010c; Zhao et al., 2010; Li et al., 2012). For alpine meadow, mean APT beneath QTH embankments (6.47 m) was larger than that beneath QTR embankments (3.65 m). However, for alpine steppe, APT beneath QTH embankment (average 7.22 m) was only slightly larger than that beneath QTR embankment (average 6.74 m).
ALT beneath natural surfaces increased continuously between 2000 and 2014 along
the QTH and QTR (Fig. 2) due to climate warming (Wu and Zhang,
2008; Wu et al., 2012; Zhao et al., 2010). The
annual ALT rate of increase ranged from 1.79 at CMR2 to 5.45 cm a
Active-layer thickness (ALT) beneath natural surface in alpine
meadow
Rate of change for artificial permafrost table (APT).
Figure 3 shows that there is a substantial difference between the QTH and QTR
in annual APT change rate, and that this difference is independent of alpine
meadow or alpine steppe. For the QTH, APT beneath embankments predominantly
increased except at FHH1 (where it decreased at an annual rate of
Artificial permafrost table (APT) beneath embankment in alpine
meadow
Because vegetation was removed during QTH construction but present during QTR construction, it is important to analyze the effect of the vegetation layer on the soil thermal regime in alpine meadow and steppe for both constructions. Therefore, we examined the long-term variability of soil temperature at 0.5 m beneath the embankment base, near-permafrost table temperature, and permafrost temperature at a depth of 10 m (see Fig. 4).
Schema of soil temperature measurements at (i) 0.5 m depth beneath the embankment, (ii) near the artificial permafrost table, and (iii) at 10 m depth.
Figure 5 shows changes of mean annual soil temperature at 0.5 m depth
beneath the embankment base along the QTH and QTR in alpine meadow and
steppe. Mean annual soil temperature at that depth had a decreasing trend
along the QTR but shows an increasing trend along the QTH except for a
decrease at FHH1. At QTR sites, this rate of change varied from
Mean annual soil temperature at 0.5 m depth beneath embankment base
in alpine meadow
Figure 6 shows changes of near-permafrost table temperature beneath
embankments along the QTH and QTR in alpine meadow and steppe. Over the
observation period, this temperature showed a decreasing trend along the QTR
but an increasing trend along the QTH except for a decrease at FHH1. At QTR
sites, the rate of near-permafrost table temperature decrease varied from
Near-permafrost table temperature (cf. Fig. 4) beneath embankment
in alpine meadow
Variation rate of mean annual soil temperature at 0.5 m depth beneath embankment base (EBT0.5), near-permafrost table temperature (NPT), and permafrost temperature at depth 10 m (PT10) beneath embankment.
Figure 7 shows changes of permafrost temperature at 10 m depth beneath the
embankment surface of the QTH and QTR, in alpine meadow and steppe. Contrary
to the shallow temperatures shown in Figs. 5 and 6, the 10 m temperature
shows an increasing trend at almost all sites. At the QTR sites, the rate of
permafrost temperature increase at 10 m depth varied from 0.07 at CMR2 to
0.44
Permafrost temperature at 10 m depth beneath embankment surface in
alpine meadow
Change of mean annual soil temperature with depth beneath the
embankments at BLR1
The results shown in Figs. 5–7 showed that changes of soil temperature at
0.5 m depth beneath the embankment base and near-permafrost table
temperatures could not clearly be related to alpine meadow or alpine steppe,
but rather to air temperature increase on the one hand and the cooling of
QTR ballast pavement and heat absorption of QTH asphalt pavement on the other
hand. Changes of permafrost temperature at 10 m depth were independent of
engineering type (QTH or QTR), and their rate of increase approached 0.28
for QTR and 0.22
Vegetation in alpine ecosystems is important for freezing–thawing processes
and the permafrost thermal regime (Zhang et al., 2005; Shur and Jorgenson,
2007; Hinzman et al., 2005; Wang et al., 2012). The response of permafrost
in different alpine ecosystems to climate change varies greatly (Wu et al.,
2015). Because the vegetation layer beneath embankments can be important for
the change of APT and the permafrost thermal regime, the effect of
engineering activities on permafrost likely varies with the alpine
ecosystem. However, we cannot infer that changes of APT, daily mean soil
temperature, and permafrost temperature are closely related to the influence
of the vegetation layer in alpine meadow and steppe based on our study.
Therefore, we analyzed variations of soil temperature with depth beneath
embankments in alpine meadow and steppe. Figure 8 shows variations of mean
annual soil temperature with depth beneath the embankments at BLR1 (a) and
BLR2 (b) for the QTR, where a vegetation layer in alpine meadow was present,
and at TGH1 (e), FHH1 (f), and KXH1 (g) for the QTH, where this layer was
removed. From this figure it can be seen that for BLR (with vegetation
layer) the mean annual soil temperature gradually approached an almost
isothermal value with depth (about
Generally, an insulation layer within an embankment can mitigate heat disturbance from short-term engineering activities (Esch, 1986; Cheng et al., 2004; Wen et al., 2005). However, such a layer is a disadvantage regarding long-term effects of climate warming over the period of engineering operation (Liu et al., 2002; Sheng et al., 2006), especially for warm permafrost because an insulation layer cannot prevent warm permafrost from thawing in the long term. Although we cannot know what happens to an insulating vegetation layer after it is buried under a railroad grade, we can infer that this layer is compressed over time, altering its thermal properties in an alpine meadow. As a consequence, its insulation effect may gradually weaken (McHattie and Esch, 1983). From Fig. 8 it can be seen that the temperature gradient from the vegetation layer to a given depth beneath the embankment gradually decreases with time, and the trend of permafrost warming gradually weakens between 3 and 4 years after railway construction. This indicates that the heat insulation effect of vegetation changes.
Generally, the vegetation layer in alpine meadow of the Qinghai–Tibet
Plateau, including the humus and root-layer soils, is thin, with maximum
thickness < 60 cm (Li et al., 2007). Vegetation roots mainly reach
depths of
The effect of lateral heat transfer on permafrost beneath embankments can
have two sources: horizontal conductive heat exchange between embankment and
outside air, and convective processes within the embankment slope. The
horizontal heat exchange is generally small due to low horizontal heat
conduction. However, heat transfer by lateral convection strongly influences
permafrost beneath embankments. Water flow can especially accelerate
permafrost thaw (Grandpré et al., 2012). The heat effect of embankment
slopes on permafrost beneath the embankment is mainly due to the differential
input of solar radiation on the sunny and shaded slopes of the embankment
(Chou et al., 2008a). The resulting difference in solar radiation produces
differences in soil temperature and the permafrost table under the shoulder
(Chou et al., 2008b; Wu et al., 2011). Monitoring data of soil temperature
along the QTR show that the differences in temperature and APT between sunny
and shaded slopes of the embankment at WD3 in alpine
meadow (Wu et al., 2012) are generally small (< 1
Varying geometries of the roadbed/railroad have a thermal effect on permafrost beneath the embankment. For example, a large embankment height will increase that difference, because of larger radiation input on the sunny slope (Hu, 2006). The embankment width affects the annual heat transfer rate at the bottom of the embankment (Yu et al., 2007). The annual rate increased by 60 % with doubling of the width of asphalt pavement (Yu et al., 2007). This increased rate was mainly at the bottom of the embankment, resulting in a concentration of heat, which then enters the permafrost through the vegetation layer.
On the Qinghai–Tibet Plateau, snow mainly accumulates in the high mountains, with only little snow in the plateau interior (Li and Mi, 1983; Sun et al., 2014). Snow cover along QTH and QTR is therefore generally thin, with less than 6 cm on average and a short snow cover duration (Li and Mi, 1983; French, 2007; Tian et al., 2014). The insulation of snow cover is weak when it is < 20 cm in thickness (Zhang, 2005; Jin et al., 2008b). Although there is no steady snow cover in winter on the plateau, snow accumulation at the foot of embankment slopes is possible, with thickness < 20 cm. Thus, snow accumulation at the foot of the slope may have no large effect on permafrost beneath the embankment. However, thaw of accumulated snow increases soil moisture at the foot of the slope.
In general, ground temperatures in permafrost regions of the Qinghai–Tibet Plateau are mainly controlled by regional climate conditions, as indicated by strong regional zonation of elevation, latitude, and continentality (Cheng and Wang, 1982). The temperatures are also greatly affected by local factors such as vegetation, snow cover, sand cover, and surface conditions. These influences can increase or decrease ground temperature under certain circumstances (Jin et al., 2008b). The regional and local factors can cause significant offsets between mean annual air temperature (MAAT) and mean annual ground temperature (MAGT) (Zhou et al., 2000; Wang, et al., 2002). However, engineering surfaces such as asphalt pavement cause anomalously high surface temperatures through radiative heating. This causes a difference between MAAT and mean annual ground surface temperature, generating accelerated permafrost degradation under the embankment (Wu et al., 2011; Zhang et al., 2016).
Based on soil temperature observations at nine monitoring sites over the
periods 2002–2014 and 2004–2014 along the QTH and
QTR, we studied the variation of APT and soil temperature beneath
embankments. The results show that alpine ecosystems on the Qinghai–Tibet
Plateau can alter the effects of engineering construction on permafrost
beneath embankments. Average APT beneath embankments was between 4.68 m at
alpine meadow sites and 7.03 m at alpine steppe sites. However, the
variation rate of APT was closely related not to the alpine ecosystem but
rather to the engineering type (railroad/highway). APT beneath QTH
embankments showed an increasing trend, with an average of
13.2 cm a
Soil temperature at a depth of 0.5 m and near-permafrost table temperature
beneath embankments in QTH alpine ecosystems showed an increasing trend over
the observation period, with averages of 0.84 and 0.26
Changes in mean annual soil temperature with depth beneath embankment surfaces in alpine meadow with a vegetation layer differed from that without a vegetation layer. This suggests that the vegetation layer of alpine meadow has an insulation role within the effects of engineering activities on permafrost beneath embankment, but this insulation gradually disappeared as the vegetation layer decayed and compressed over time. Overall, that layer is an advantage for alleviating permafrost temperature rise in the short term, but its impact gradually weakens in the long term.
We provide access to data in
This study was supported by the National Natural Science Foundation of China (grant no. 41330634), National Key Scientific Research Project (grant no. 2011CB026106), and STS Project of the Chinese Academy of Sciences (HHS-TSS-STS-1502). Edited by: C. Hauck Reviewed by: two anonymous referees