3. RESULTS
3.1 Soil profile
characteristics
The general characteristics of soil pedons are illustrated based on
pedon 1 (Figure 2b). At 0–10 cm soil depth, the soil was dark red with
numerous wormholes found due to the high organic matter content of the
surface layer. Cracks began to appear below 10 cm, with a few soil
profiles showing various crack configurations. For instance, pedon 1
contained a V-shaped crack at 18–21 cm and an I-shaped crack at
approximately 19–33 cm, while most of the profiles contained more
I-shaped cracks. Pedon 1 consisted of four genetic horizons, delineated
as A (0–8 cm), B1 (8–21 cm), B2 (21–54 cm), and BC (54–60 cm)
(Figure 2b), which were classified as loamy skeletal, mixed,
super-active, or mesic, respectively (Soil Survey Staff, 1999).
This experiment applied GPR technology to physically probe soil cracks
and identify natural crack configurations before the infiltration
experiment was unfolded. High-signal areas floating on the radargrams
were identified, and the envelopes were calculated as the average crack
coverage amplitude (Table 3). Soil cracks existed or did not result in
different envelopes. The envelope of non-crack profiles had no
substantial fluctuation with reflection wave conduction, while the
profile with cracks showed more signal fluctuations at 0.3–0.6 m. The
extracted crack signal parameters TotAra,
Maxa, Mina, and Acumasupport the phenomenon (Table 3). Furthermore, the amplitude fluctuation
of the I-shaped crack was less marked than that of the V- and Λ-shaped
cracks (Figure S3). Thus, the different soil crack configurations, as
determined by GPR and excavation, could provide practical support for
conducting infiltration experiments. Notably, geophysical methods cannot
directly explain all the properties of soil cracks. Whether the crack
width, inclusion, and signal fluctuations corresponded to each other
based on the raw data remains unclear.
3.2 Comparison between Brilliant Blue FCF solution and water
transport
Figure 3 shows the infiltration trajectories with time, showing that the
Brilliant Blue FCF solution penetrates faster than water. The difference
between water and dye penetration was observed at the beginning of
infiltration, as early as 3 min. In the VR1.5 treatment (Figure 3c), the
matrix flow was notably dominant throughout the infiltration process,
and no preferential flow occurred. This phenomenon indicates that water
was probably not transported down V-shaped cracks (Figure 3b). The
matrix flow dominated before 30 min, while preferential flow dominated
later in the IR1.5 and ΛR1.5 treatments (Figure 3a–b).
The dye-stained and wetting areas show a linear fit with the primary
function, in which the R 2 values were all above
0.8 (Figure 4). Regarding the I-shaped crack filled with rock fragments,
slight variabilities were found between Brilliant Blue FCF solution and
water when the dye-stained area was less than 21%, at which point the
former infiltrated marginally faster than the latter; however, these
differences gradually became more potent after the turning point,
indicating that there was a lag in
the effects of the Brilliant Blue FCF solution compared to water (Figure
3–4). Comparison with the I-shaped crack showed that the retardation on
the solution infiltration in the Λ-shaped crack appeared later, around a
dye-stained area ratio of 24%–27%. This result may explain, at least
in part, that the retardation on the Brilliant Blue FCF with time was
influenced by the crack configuration, and the flow pathways of the
Brilliant Blue FCF were not fully representative of water infiltration
paths.
3.3 Effect of crack properties on infiltration and
preferential
flow
The effects of the crack properties (inclusion, crack width, and
configuration) on the infiltration rate,
maximum dye-penetration depth,
cumulative infiltration, and wetting advancing rate are shown in Figure
5–7. Generally, a noticeable trend in the infiltration rate was
observed, which could be divided into a sharp decline and a steady
decline for all treatments. The turning point occurred at approximately
40 min, after which, the infiltration rate reached a relatively smooth
decline for all treatments (Figure 5a, 6a, and 7a). The maximum dye
penetration depth and cumulative infiltration increased at a slower rate
with time after 40 min (Figure 5b–c, 6b–c, and 7b–c).
3.3.1 Crack inclusion
Regarding the influence of
inclusions, the infiltration rate was faster by 18.8% on average when
filled with rock fragments than when filled with sand grains (Figure
5a), and the existence of cracks increased the dye-penetration depth,
cumulative infiltration, and wetting front depth by at least 5.2%,
63.2%, and 4.4% (Figure 5b–d). These results were apparent in the
IR2, IS2, ΛR1.5, and ΛS1.5 treatments, indicating that cracks filled
with rock fragments enhanced the infiltration capacity of the soil. The
ΛR1.5 treatment had a notably higher dye penetration depth, cumulative
infiltration, and wetting front depth at the end of infiltration (Figure
5b–d). For example, the 70-min wetting front depth of the ΛR1.5
treatment was 46.5 mm, whereas that of the ΛS1.5 treatment decreased by
17.0 % (Figure 5d). Similarly, the maximum dye-penetration depth and
cumulative infiltration decreased by 14.9% and 4.4%, respectively
(Figure 5b–c).
3.3.2 Crack width
The volumes corresponding to the three crack widths were 8.0, 17.5, and
31.5 cm3, respectively. Regarding crack widths,
significant differences (p <0.05) were identified in the
infiltration rates among the three treatments with different crack
widths (Figure 8b). After the infiltration reached relative stability,
the infiltration rate was faster for the 1.5-cm width crack (Figure
6a). The maximum dye-penetration
depth, cumulative infiltration, and wetting advancing rate were the
highest when the crack width was 1.5 cm (Figure 6b–d), and there was a
significant difference (p <0.05) in the maximum
dye-penetration depth with the 1.5-cm width crack (Figure 8b). Our
experimental results indicate the existence of a certain optimal crack
width between 1 and 2 cm, which allowed the infiltration rate to be
maximised. After the crack volume
reached a value range of 8.0–31.5 cm3, moisture was
more inclined to be stored in the crack than to infiltrate downward.
3.3.3 Crack configuration
Regarding the configuration, the chart shows that the ΛR1.5 treatment
had a substantially higher penetration rate than other configurations,
ranging from 54% to 120% higher than other two configurations (Figure
7a). The maximum dye-penetration depth and cumulative infiltration of
the Λ-shaped configuration increased steadily at the beginning of
infiltration, crossing the line for the I-shaped configuration at
approximately 50 min (Figure 7b and 7c). When infiltration proceeded to
a certain process, the Λ-shaped configuration increased the rate of
liquid passage, but the effect on the wetting advancing rate was not
obvious. The wetting front depth nearly overlapped between the three
crack configurations, with variations ranging from 0.03% to 38.39%
(Figure 7d). Significant differences (p <0.05) were
observed between the IR1.5 and ΛR1.5 treatments, but there were no
significant differences (p <0.05) between these and the
VR1.5 treatment (Figure 8c).
3. 4 Contribution of crack properties to preferential flow
The I- and Λ-shaped treatments, filled with rock fragments, generated
preferential flow, whereas the V-shape was not sensitive to preferential
flow (Table 2). Thus, the analysis of dye coverage and preferential flow
indices was displayed only for IR1, IR1.5, IR2, and ΛR1, ΛR1.5, and ΛR2
treatments.
Figure 9 illustrates the turning range of the preferential flow ratio
with increasing crack width for the same configuration and inclusion.
The experiment proved that the preferential flow extensively slowed down
the decline in dye coverage and remained steady in the IR1.5 and ΛR2
treatments (Figure 9b and 9f). For example, the percentage of
preferential flow dye coverage in the IR1.5 treatment ranged from 60.7%
at 15 cm to 14.4% at 20 cm, while in the ΛR1.5 treatment, it ranged
from 75.9% at 14 cm to 7.5% at 17 cm. The variations indicated more
preferential pathways in the ΛR1.5. However, a rebound point at a soil
depth of 15 cm was observed in the IR2 treatment, in which the
morphology of the stained end appeared radially orbicular (Figure 9c).
This demonstrates the configuration has a greater effect on preferential
flow and a wider linear crack filled with rock fragments likely leads to
a lateral flow at the end of the crack.
Overall, the preferential flow ratio increased with increasing crack
width, similar to the length index of the preferential flow. Generally,
the preferential flow ratio and matrix flow depth showed opposite
trends; however, this was not always the case (Table 4). Treatment with
IR1.5, however, did not match the above regularity. The nonuniformity
and spatial variability of the linear crack was more distinct, which led
to an obvious preferential flow phenomenon. The response of the Λ-shaped
crack to the preferential flow with time lagged. For instance, the depth
of matrix flow for I-shape was 0.9%–33.3% lower than that for
Λ-shape; however, the length index of preferential flow was higher by
3.9%–11.4% (Table 4).