4.
DISCUSSION
4.1 Effect of crack properties on preferential
flow
During the infiltration event, the infiltration rate of Brilliant Blue
FCF was over 1.5 times slower and the infiltration depth was 1.2–3.8
times lower than those of water, which may be attributed to the fact
that the viscosity of water was different from that of Brilliant Blue
FCF. A similar phenomenon was observed by Liu and She (2020a), who
showed that a large organic molecule size and nonlinear adsorption
behaviour within the soil matrix led to slow migration rates. This may
be due to the retardation caused by the reaction between Brilliant Blue
FCF and Ca2+ in carbonate laterite areas (Flury and
Flühler 1995; Nobles et al., 2010). Therefore, using Brilliant Blue FCF
as a staining tracer, the infiltration depth in the soil profile may not
be the true water infiltration depth. The results of this experiment can
be used as a basis for calibration in future field tests.
Soil cracks showed undulated reflections and individual features,
whereas CK showed a more continuous interface, as derived from the
envelope curves (Figure S3). Moreover, the morphological characteristics
in the vertical direction of the I-shaped cracks showed a smaller
envelope reflection amplitude than those of the other shapes. Cracks are
always filled with sand grains or rock fragments, which had a
significant role in providing high-amplitude reflections (Han et al.,
2016; Dal et al., 2019) . Although the co-offset results only
distinguished the envelope curves of different crack configurations, the
soil cracks always had different widths and inclusions as well
(Tsakiroglou et al., 2012; Yang et al., 2016), where the radar
reflections provided by could not be distinguished.
The volume proportion of “manmade cracks” occupied approximately
0.16%, 0.35%, and 0.63%, corresponding to the 1-, 1.5-, and 2-cm
cracks, respectively, as shown in the infiltration experiments. Although
the volume of cracks was less than 1%, the existence of cracks
considerably accelerated the water infiltration rate, while the
infiltration rate, maximum dye-penetration depth, cumulative
infiltration, and wetting advancing rate of CK were lower than those of
other treatments containing cracks. Cracks provide channels or pores for
fluid flow, even though only the I- and Λ-shaped cracks produce
preferential flow. Our findings did not concur with those of Liu and She
(2020a), who suggested that crack volume is not a good predictive
parameter for infiltration characteristics. However, Yang (2020)
concluded that the crack volume is an important indicator of the initial
infiltration rate, and that they are positively correlated.
Preferential flow was observed only in the I- and Λ-shaped treatments,
along the crack pore paths. These results are consistent with those of
Liu and She (2020a) and Yang et al. (2016), who demonstrated that cracks
influence preferential flow. Liu and She (2020a) showed that I-shaped
cracks retained more water than V-shaped or non-cracks, in which
preferential flow was the dominant flow type. Yang et al. (2016)
revealed that the orientation of subsurface flow varied in accordance
with the connectivity of the cracks or underlying structure, which
resulted in lateral flows and deep percolation occupying approximately
32%–36% of the total rainfall.
4.2 Necessity of combining geophysical surveys and column
experiments
GPR was successfully applied in karst topography to reveal structural
information about the subsurface and achieve the transformation of
invisible geological features into trended signals (Estrada-Medina et
al., 2010; Fernandes et al., 2015), thus providing more detailed
information about soil profiles (Busch et al., 2013). This geophysical
technique retrieves information with the advantages of low-cost,
efficiency, and less labour-intensive. We explored the distribution and
characteristics of soil cracks on a large scale in the field, and
verified the configuration by GPR. On the one hand, geophysical surveys
provide a reference for the crack properties before the simulation
experiments. The column experiments were conducted based on the results
of GPR and excavations and visualise the flow infiltration process (Liu
et al., 2020a). On the other hand, it provides a new insight for
interpreting isolated cracks from radar signals. The combination of GPR
with field observations and column experiments has shown great potential
for investigate the effect of isolated soil cracks on preferential flow.