2. MATERIALS AND
METHODS
2.1 Study area
This study was conducted at 102°54′12″ E and 23°37′13″ N and an altitude
of 1300–2500 m (Figure 1), in Jianshui County, Honghe Hani and Yi
Autonomous Prefecture, Yunnan Province, China. This region is a karst
graben basin with strong stratigraphic erosion since the Tertiary Period
(Cao, 2021). The red weathering crust of carbonate rocks, known as
carbonate laterite, is formed in tropical and subtropical environments
through a long weathering process that is unique to the karst graben
basin. The long-term average annual temperature of this area is 19.8 °C,
with most of the 805 mm mean annual precipitation occurring between May
and October. The summer/autumn wet season lasts from April through
September and provides approximately 70%–90% of the total annual
rainfall.
The E. robusta forests in the experimental area are approximately
1.5 km2, which cover the Nanpan River water system in
the upper reaches of the Pearl River and the middle and upper reaches of
the cross-border Red River Basin. Three study sites (2.8 m × 2.8 m each)
were selected for field tests and marked as Sites 1, 2, and 3. The bulk
density of the soil was approximately 1.2 g/cm3 and
the pH was 6.5. Table 1 details the physical and chemical properties of
the tested soil.
2.2 Experiment design
2.2.1 GRP surveys and ground
truthing
This study was divided into field survey and infiltration experiment
stages. The soil cracks and their properties were characterised in
accordance with the GPR lines and excavation of the soil pedons using an
embedded Linux GPR system (CAS-S500, IECAS, China)
with a shielded antenna of 500
MHz. Horizontal and vertical GPR lines were deployed in accordance with
crossed grid principles and with horizontal lines parallel to contour
lines. The GPR grid (2.8 m × 2.8 m) consisted of 15 horizontal (H1–H15)
and vertical (V1–V15) parallel survey lines with 20 cm between them
(Figure S1). The horizontal lines were oriented downslope with a local
5° slope. 90 radargrams (3 sites × 30 survey lines) were collected over
the study area, with data acquired from the distance travelled by a
survey wheel equipped with a position sensor.
The sampling frequency of the
system was 25.6 Hz, and the wheel moved at a uniform speed (scanning
rate of 390.625 traces/s) in conformity with the survey line. The time
window was set at 40 ns and, for each 3 m long survey transect,
258–276 traces and 38 time windows were recorded. Based on the signal
fluctuations appearing in the radargrams, six pedons (three sites × two
random pedons) were stochastically excavated for verification (Figure
S1). The length and depth of each pit were 120 cm and 60–70 cm,
respectively. Various soil crack shapes were screened through the
radargrams and pedons of the soil profile (Figure 2b).
2.2.2 Infiltration experiment
design
Preferential flow paths were
simulated to account for the natural cracks and pore structures of soils
detected by GPR. Based on the identification of crack configuration in
the field which was defined as “I”, “V”, and “Λ” morphologies, the
infiltration experiments were conducted to explore the effect of
isolated cracks on water infiltration. In addition, we found that soil
cracks were filled with sand grains or rock fragments of different grain
sizes when excavating the soil profile. It has been shown that
inclusions (Yang et al., 2016; Liu and She, 2020), width (Ou Yang,
2020), and configuration (Liu and She, 2020) influence the infiltration
process. For simplicity, soil cracks are always assumed to resemble
smooth parallel plates (Tsakiroglou et al., 2012; Rouchier et al.,
2012). Therefore, three properties of different inclusions (sand grains
and rock fragments), crack widths (1, 1.5, and 2 cm), and configurations
(I-shape, V-shape, and Λ-shape) for infiltration were simulated (Figure
2d and Table 2). A soil column without cracks (CK) was used as the
control. Based on these properties, all experiments were divided into
six groups (Figure 2d) in which four soil columns (including CK) were
conducted simultaneously, with each experiment repeated three times.
Disturbed soil samples were collected in September 2021 from the upper
70 cm of pedons in layers. The tested soil was passed through a 2 mm
sieve, air-dried, thoroughly mixed, and hierarchically backfilled into
Plexiglas columns (20 cm × 5cm × 60 cm, Figure 2c) to ensure that the
particle size composition of the backfilled soil was as consistent as
possible with the field site. For
homogeneous soil columns,
the backfilled soil needed to be
compacted, and the surface was roughened every 5 cm
before the next layer was added.
The top layer was covered with a metal mesh after backfilling soils of
50 cm to prevent water flow impact on top soils. Different crack
inclusions were replaced by sand grains and rock fragments. The diameter
of all sand grains was approximately 2 mm, whereas the rock fragments
ranged 2–5 mm. All these debris (i.e. sand grains and rock fragments)
was wrapped with 500-mesh nylon netting to prevent soil particles from
passing through and blocking the paths (Figure 2c). From the field
excavation of representative sites and GPR large-scale detection, cracks
are mainly scattered approximately 10 cm from the surface. Therefore,
the lab-based experiments of cylindrical “channel” was placed in
column when backfilled to 30 cm from the upper edge of the column to
simulate various cracks.
Brilliant Blue FCF allows clear
visualisation of the infiltration process of non-uniform flow in the
soil (Lipsius et al., 2006); as such, a Brilliant Blue FCF dye volume of
5 L (4 g/L) was added to the Marriott bottles to visualise the
preferential flow paths. The volume of the dye solution used
corresponded to the amount of water collected in a 20 mm rainfall event,
considering more than 65% of regional rainfall events were for 20 mm
during 2009–2018. Therefore, a
constant pressure head of 20 mm was supplied using Marriott bottles
(Figure 2c). For each experimental group, four images (three variances
and a control) were taken with a digital camera (Canon EOS 60D, Canon,
Japan) directly in front of the soil column at 3, 5, 10, 20, 30, 50, 70,
and 90 minutes after the Brilliant Blue FCF dye solution was applied.
The infiltration continued until the wetting front penetrated the bottom
of the column. The dye infiltration depth and wetting front trajectory
of each soil column were recorded simultaneously using a ruler with
millimetre precision attached to the edge of the soil column. The
infiltration rate was determined by measuring change in water level in
Marriott bottles over time.
2.3 Data analysis
2.3.1 GPR data processing
GPR data were processed following a conventional routine (Dal et al.,
2019; Guo et al., 2014) via Radar View V18.08 (Institute of Electronics,
Chinese Academy of Science, China) to enhance the signal-to-noise ratio,
which follows the sequence: 1) background filter wiped off antenna
reverberation; 2) time-zero correction calibrated the radar antenna
start time; 3) energy gained compensated energy attenuation with
propagation depth; 4) Hilbert filter removed high- and low-frequency;
and 5) DC-drift suppressed low-frequency noise and remedied average
amplitude. Finally, the high signal areas float compared to their
surroundings in the radargram, and were converted to average envelope
curves (Figure 2a). The envelopes can estimate the configuration of soil
cracks, and the abnormal signals, defined as cracks on the radargram,
were used to calculate the soil crack densities at the study site.
Total
amplitude area (TotAra) is the area enclosed by the envelope curve and
the coordinate axis. Accumulative amplitude (Acuma) is
the accumulation of the amplitude at every 1 cm soil depth on the
envelope curve (The crack signal illustration method is shown in Figure
S2).
2.3.2 Image processing and data analysis of preferential
flow
The captured images were cropped,
and the threshold was adjusted using Photoshop 2020 (Adobe Systems Inc.,
San Jose, CA) for further analysis. The stained area was replaced with
black (0) and the non-stained area was replaced with white (255) via the
colour replacement function. Separation was then conducted using Image
Pro Plus 6.0 (Media Cybernetics Inc., Rockville, MD) to obtain only 0
and 255 dual-value matrices after bitmap analyses were done.
Dye coverage (DC ) is defined as the ratio of the
stained area to the total soil vertical profile, and the area was
measured using a matrix converted from pixel grids and was calculated as
follows:
, (1)
where D is the number of matrix values of 0 andND is the number of matrix values of 255.
Many historians have argued that the matrix flow depth
(UF ) is defined as the depth at which the stained
area ratio is greater than 80% (Schaik, 2009;
Bargues Tobella et al., 2014). In
contrast to preferential flow, water infiltration in the matrix flow
region is relatively uniform and continuous. Matrix flow often leads to
a delay in the preferential flow.
The preferential flow ratio (PF ) is defined as
the ratio of the stained area of the preferential flow area to the
entire vertical profile; a high PF value leads to
a high degree of preferential flow.
This can be calculated as
follows:
, (2)
where W is the width of the Plexiglas column (W = 20 cm)
and DT is the total stained area of the soil
profile (cm2).
The length index of preferential
flow (Li ) describes the heterogeneity of the dye
penetration, which is higher in the preferential flow region than in the
matrix flow region. A high Li value leads to a
high degree of preferential flow. This can be calculated as follows:
, (3)
where n is the number of vertical soil layers in the soil profile
(one pixel down along the vertical profile named one layer, the size of
which is determined by the number of pixels contained in the maximum
infiltration depth), and Dc(i+1) andDci are the stained area ratios corresponding to
soil profile layer i +1 and i , respectively.