1. INTRODUCTION
Karst areas in southwest China, a
humid monsoon region with distinct dry and rainy seasons, are
characterised by typical regional soil and vegetation that distinguish
them from other regions at the same latitude (Peng et al., 2008).
Carbonate-derived laterite is widely distributed in karst areas with
high clay contents, low permeabilities, and high water-holding
capacities (Waller and Wallender, 1993; Liu, et al., 2016; 2020). These
soil features determine the calciphile and drought endurance of local
vegetation (Peng et al., 2008). Moreover, the complex and unique
topographical and geological conditions of karst areas lead to water and
nutrient loss (Wang et al., 2019). Given these circumstances, the
importance and specificity of these regions have alarmed people about
their ecological conditions.
Cracks are physical phenomena that exist in clayey and expansive soils
(Sima et al., 2014; Acharya et al.,
2015). In karsts, the alternating
wet and dry conditions and the special dilation-shrinkage
characteristics of carbonate-derived laterite allow for soil cracks to
form with ease (Liao et al., 2000; Zhang et al., 2021). Additionally,
shallow subsurface cracks may be influenced by plant roots (Shem et al.,
2009; Zhu et al., 2020), burrowing fauna (Li et al., 2019), and
artificial factors (Mossadeghi-Björklund et al., 2016; Tang et al.,
2016). For example, Eucalyptus robusta can absorb approximately 9
t hm−2 year−1 of carbon dioxide
while releasing oxygen (Ghannoum et al., 2010; Teixeira et al., 2020);
further, rapid water absorption through its root systems result in soil
crevices and cracks (Shem et al., 2009).
Cracks can enhance water
infiltration and induce preferential flows (Zhang et al., 2014), making
water one of the most important factors limiting karst forest
development (Zhang et al., 2018; Liu et al., 2020). Preferential flow,
generated by the rapid penetration of moisture through soil pore
channels, is a nonuniform and unstable water flow (Bouma and De Laat,
1981; Sheng et al., 2014). Crack properties including configuration,
orientation, inclusion, angle, depth, and volume can affect water
distribution and preferential flow (Zhao et al., 2014). For example, the
configuration of a linear crack has a high saturated hydraulic
conductivity compared with other shapes and thus develops preferential
flow (Liu and She, 2020), whereas regular rectangular crack networks
made of steel sheets generate preferential flow only under
high-intensity precipitation or irrigation conditions (Zhu et al.,
2018). Inclusions within soil cracks are normally coarse particles and
rock fragments in karsts (Chen et al., 2011). Although cracks and
fissures in nature exist with complex combinations of properties (Zhang
et al., 2014; Zhu et al., 2020), the small-scale control of the
preferential flow of isolated cracks is critical for water loss in
soil-epikarst zones (Liu and She, 2020).
Preferential flow visibility is usually achieved by Brilliant Blue FCF
(C.I. Food Blue 2), which is a tracer that marks the area where stained
water flows (Bouma and De Laat, 1981; Hagedorn and Bundt, 2002) and
traces the flow paths to reveal soil profile characteristics (Hagedorn
and Bundt, 2002; Sheng et al., 2009). Recently, ground-penetrating radar
(GPR) has been successfully applied to karsts to reveal structural
information about the subsurface and transform invisible geological
features into trended signals (Estrada-Medina et al., 2010; Fernandes et
al., 2015), thereby providing more detailed information about soil
profiles. GPR can accurately locate soil cracks and identify crack
widths as low as 1–2 mm (Levatti et al., 2017). The combination of‘
time-lapse GPR surveys and infiltration experiments has revealed
channelling soil water characteristics (Di Prima et al., 2022). However,
the combination of field-based GPR data and lab-based infiltration
experiments on crack properties has not been tested to generalise the
flow infiltration process.
To clarify whether soil cracks enhanced preferential flow and effected
infiltration processes, a case study approach was chosen. GPR was
applied to physically identify the natural soil cracks and place
“manmade cracks” in transparent Plexiglas columns to observe
infiltration over time. This study aimed to i) identify the soil crack
properties through GPR envelopes with excavated pedons; ii) evaluate the
effects of crack inclusion, width, and configuration on infiltration
processes and preferential flow; and iii) discuss the preferential flow
characteristics caused by different cracks to further explore the
infiltration mechanism in karsts.