2. MATERIALS AND METHODS
2.1 Study
area
The QTP lies in the west of China, and has a long cold season
(October-May) and short warm season (Lin et al. 2015b). The Beiluhe
Basin is in central QTP where elevation ranges from 4500-4700 m a.s.l
(Figure 1). Alpine meadow and alpine grassland are the main vegetation
types, accounting for over 40% of the area (Yin et al. 2017). The
vegetation communities are simple, and dominant plant species includeKobresia pygmaea, Carex moorcroftii, Stipa purpurea, and
Littledalea racemose , etc. (Lin et al. 2019). Most plants are
<15 cm tall and the growth period is short. Strong wind
erosion on the plateau results in a fragile ecological environment (Li
et al. 1996).
Data from Beiluhe Weather Station indicate that the annual mean air
temperature was between -4.1 and -2.6 °C from 2005 to 2016, with an
average value of -3.4 °C. Annual mean precipitation ranged between 229
and 467 mm (Figure 2), while the annual mean potential evaporation was
~1588 to 1626 mm in the same period (Lin et al. 2019).
About 10% of annual precipitation falls as snow.
The Beiluhe basin is undulating covered with fine to gravelly surface
sands. Surficial materials within 30 cm of the surface are predominantly
aeolian sand or alluvial deposits (Yin et al. 2017). Thermokarst lakes
are widely spread in the basin and have been eroding permafrost (Lin et
al. 2010; 2011). Permafrost in the basin is continuous, relatively warm
(near 0 ℃), and has high volumetric ground ice content with a mean value
of ~16% (Lin et al. 2020; Fan et al. 2021).
Active-layer thickness ranges from 1.8 to 3.0 m and the annual mean
ground temperature is -1.8 to -0.5 °C. Sediment textures range from clay
to sandy gravel, which overlies weathered mudstones and sandstones (Lin
et al. 2010).
2.2 Sites
descriptions
The study examined two sloping sites with opposing aspect (Figure 1).
One site was at a south-facing sunny slope (34.8367°N, 92.9206°E) and
the other is a north-facing shady slope (34.8486°N, 92.9268°E). The
slope angle at the sunny slope is about 7.5°, and about 8.1° at the
shady slope (Figure 3). The elevation is 4634 m at the sunny site and
4638 m at the shady site. The dominant plant species on the sunny slope
are Stipa purpurea and Kobrecia parva , and the mean
vegetation coverage is approximately 16.7%. On the shady slope the
dominant plant species are Androsace tapete maxim, Carex
moorcroftii , and the mean coverage is ~7.9%. The
sediment profile to 5.0 m depth is presented in Figure 3, and includes
information on gravimetric moisture content (GMC), excess ice content
(EIC), and permafrost table depth (PT). The ground surface at both sites
is typically covered by gravel or sandy silt. Below this layer lie
coarse-grained deposits that are rich in flake gravel. The stratigraphy
is similar up to 5 m depth at both sites, however the ice content
differs (Lin et al. 2020). Soil texture information with depth is
presented in Figure 4. The near-surface soil at the sunny slope was
~41% silt, 14% clay, and greater than 50% silt and
clay combined. Most of the near-surface samples from the shady slope
were sand dominated (63%), with only ~40% silt and
clay combined. The soil at the sunny site was generally
>60% silt at 3-4 m depth, and included high excess-ice
content in this frost-susceptible layer. The soil organic matter (SOM)
content in Beiluhe Basin soils is very low (Liu et al. 2014), and the
measured SOM content at the shady slope was higher than that at the
sunny slope (Figure 5).
2.3Temperatures
A HOBO Pro v2 (U23-004) external temperature data logger was used to
measure air temperature (Ta) and ground surface
temperature (Ts) at each site. The built-in sensor was
installed in a solar radiation shield at 2.0 m height and an external
temperature sensor was used to measure soil temperature at
~1-2 cm depth. The reported measurement accuracy of the
sensors is ±0.21 °C from -40 to 100 °C. Data collection began in
September 2016 and measurements were recorded every 30 minutes.
A drilling programme to install temperature sensors was conducted at the
two sites in July-August 2016. A total of 18 boreholes were instrumented
to 5 m depth to determine the variation in ground thermal conditions and
associations between permafrost temperatures, soil moisture, and slope
aspect. Each site included nine boreholes drilled 5 m apart in a 10 x 10
m rectangular grid. The multiple measurements at each site are meant to
improve the evaluated accuracy of ground temperature characterization.
At each borehole, the drill core was extracted with a 10 cm diameter dry
drill. Three HOBO soil temperature sensors (TMC20-HD; Onset Computer
Corporation, Bourne, MA, USA) were installed at 5 cm, 250 cm, and 500 cm
depth. The three measured depths represent the near surface temperature
(Tns), the temperature near the permafrost surface
(Tps), and the permafrost temperature
(Tg), respectively. The three sensors were fixed to the
outer wall of a polyethylene aluminium composite tube placed in each
borehole. The holes were filled with dry sand and packed with a long rod
in order to improve contact between the sensors and surrounding ground.
Ground temperatures were recorded by a HOBO UX120-006M 4-Channel Analog
Logger. The reported measurement accuracy of the sensors is ±0.21°C from
-20 to 70 °C. Data collection began in September 2016 and measurements
were recorded every 4 h.
2.4 Soil moisture content
At each site a 1.5 m deep soil profile was excavated. HOBO soil moisture
sensors (S-SMD-M005) were inserted directly into the soil profile at
depths of 25, 50, 100, and 150 cm. The volumetric moisture content
(m3/m3) was collected using a HOBO
H21-002 Micro Station. The reported measurement accuracy of the sensors
is ±0.031 m3/m3 (±3.1%) from 0-50
°C for mineral soil up to 8ds/m and ±0.020
m3/m3 (±2%) with soil specific
calibration. Data collection began in September 2016 and measurements
were recorded every 4 h.
2.5 Solar
radiation
A CNR4 Net Radiometer (Kipp&Zonen, Delft-The Netherlands) was installed
at 1.5 m height at each site to measure the energy balance between
incoming short-wave and long-wave (Far Infrared, FIR) radiation versus
surface-reflected short-wave and outgoing long-wave radiation. The CNR4
net radiometer consists of a pyranometer pair, one facing upward, the
other facing downward, and a pyrgeometer pair in a similar configuration.
All 4 sensors were calibrated individually for optimal accuracy.
The spectral range (50% points) of short wave measurements is 300 to
2800 nm and 4500 to 42000 nm in the long wave spectral range (50%
points). The sensitivity of the sensors is 5 to 20 µV/W/m² and the
temperature dependence of sensitivity (-10 to +40 ºC) is less than 4 %.
The instruments can operate in temperatures of -40 to +80 °C and 0-100%
RH. The instruments were factory calibrated. Two data loggers (CR1000,
Campbell Scientific, Edmonton, AB, Canada) were separately employed to
sample at 30 minute intervals and data were stored as 1 h averages for
both sites. Data collection began in September 2016. Following
collection, obviously erroneous measurements were removed and gaps were
filled by interpolation.
2.6 Laboratory test of soil texture
and
SOM
Samples were collected for transport to the State Key Laboratory of
Frozen Soil Engineering (Lanzhou), Chinese Academy of Sciences (CAS) to
examine the soil texture and organic content at both sites. The dried
soil samples were crushed and put through a 2 mm sieve. The
particle-size distribution of soil that passed through the sieve
(<2 mm) was determined using a Malvern Mastersizer 2000
Particle Size Analyzer (Malvern Panalytical Ltd, Malvern, UK). The
resulting particle-size distributions were divided into three texture
classes: (1) sand (2 mm ≥ sand > 75 μm), (2) silt (75μm ≥
silt > 5 μm), and (3) clay (5 μm ≥ clay).
The SOM of the pulverized homogenized samples were quantified by dry
combustion using a Vario EL elemental analyzer (Elementra, Hanau,
Germany). To measure the soil organic carbon content (SOC), 0.5 g
air-dried soil samples were pretreated with HCl (10 mL, 1 mol L-1) for
24 h to remove carbonate.
2.7 Data
processing
Annual mean air temperature, annual mean ground temperature, the surface
offset, and freeze-thaw indices were calculated as outlined in Lin et
al. (2019). Net short wave radiation (Rs, W·m-2), net
long wave radiation (Rl, W·m-2), and
net radiation (Rn, W·m-2) can be
computed using four components (eq.1-3):
\(Rs=DR-UR\) (1)
\(R_{l}=DLR-ULR\) (2)
\(R_{n}=DR-UR+DLR-ULR\) (3)
Where DR and UR are downward and upward shortwave radiation,
respectively. DLR and ULR are downward and upward long-wave radiation,
respectively. The four parameters were measured at both sites using the
CNR4 Net Radiometer.