1. Introduction
Soil detachment is defined as the separation and dislodging of soil
particles from the soil mass surface by the force of raindrops and
overland
flow (Lautridou, 1990; Wang et al., 2014; Zhang et al., 2002). Scouring
by overland flow is the dominant process resulting in detachment and
transport of soil particles. The mechanism of soil detachment by open
channel flow has become a research focus (Cao et al., 2009; Wang et al.,
2016; Xiao et al., 2017). In the case of clear water, maximum soil
detachment rate referred to as soil detachment capacity (SDC) is a key
parameter used to describe the soil erosion process (Nearing et al.,
1991; Sun et al., 2018). SDC is often used to estimate the rate of soil
erosion resulting from overland flow and has been widely used in a range
of representative erosion models, such as the Water Erosion Prediction
Project (WEPP) and other process-based models (Misra and Rose, 1996;
Morgan et al., 1998; Nearing et al., 1989). Meanwhile, SDC is to a
certain extent affected by the hydraulic parameters of overland flow and
the relationship between SDC and hydraulic parameters can be established
to quantitatively simulate or predict soil detachment (Nearing et al.,
1991; Zhang et al., 2002, 2003). For this reason, past studies have
conducted laboratory control experiments to reveal the internal
relationship between SDC and hydraulic parameters (Zhang et al., 2002,
2015). The results of these past studies have indicated that soil
detachment is strongly influenced, and in some cases controlled, by
hydraulic parameters, such as shear stress (Nearing et al., 1991), unit
energy of the water carrying section (Hairsine and Rose, 1992a, 1992b),
stream power (Wang et al., 2018; Xiao et al., 2017) and unit stream
power (Morgan et al., 1998). However, since these studies did not employ
standardized experimental conditions, there remains no consensus on the
optimal hydraulic parameters to predict SDC and the dynamic mechanism of
the soil detachment process remains unclear. For example, Morgan et al.
(1998) found that the unit stream power is the best parameter to predict
SDC, whereas Nearing et al. (1999) demonstrated the strong relationships
of shear stress and stream power with SDC. Therefore, obtaining
consensus requires further numerous experiments under different dynamic
conditions to reveal the dynamic mechanism of soil detachment driven by
overland flow.
Past research has demonstrated that soil type, bulk density, soil
moisture, freeze-thaw and the root system all have strong relationships
with SDC (Gyssels et al., 2006; Van Klaveren and McCool, 2010; Ye et
al., 2017; Zheng et al., 2000). The seasonal soil freeze-thaw process is
a common phenomenon in mid-latitude regions, such as the northern part
of the Loess Plateau in China. The freeze-thaw process changes soil
physical and mechanical properties and reduces the stability of soil
aggregates, thus affecting soil SDC (Kværnø and Øygarden, 2006; Sun et
al., 2018). And the SDC would be first decreasing and then increasing
with an increasing number of freeze-thaw cycles (Sun et al., 2018). The
soil freeze-thaw process can result in increased soil and water loss
during the spring thawing period of temperate regions to >
50% of annual loss (Chow et al., 2000; Ellison and D., 1945; Froese et
al., 2001). An analysis of soil detachment under the freeze-thaw process
can provide an important theoretical basis for developing soil erosion
control and prevention mechanisms for the seasonal freeze-thaw region.
However, most previous studies on the effect of freeze-thaw on SDC were
conducted on bare soil, whereas many affected areas have vegetation, and
the extent of area with vegetation is continuous increasing with the
implementation of a series of ecological restoration projects (Wang et
al., 2014). Past studies have gradually highlighted the importance of
the effect of the plant root system on soil properties and soil erosion
in this region. The root system plays a key role in increasing soil
stability, thereby reducing soil erosion (Baets et al., 2007; Jiao et
al., 2012). The root system not only promotes the formation of large
soil aggregates by consolidating fine soil particles, but also increases
soil organic matter and improve soil structure through root exudates and
plant residues (Amezketa, 1999; Burylo et al., 2012; Whalley et al.,
2005; Ye et al., 2017). Soil with vegetation can have an SDC that is
23.2–55.3 lower than that of bare soil due to the effect of the root
system (Wang et al., 2014). Gyssels et al. (2006) found that the SDC can
be reduced by > 50% through the presence of herbaceous
plant root systems compared with that of soil without roots. However,
current research on the
root
system effect on the soil detachment process is mainly focused on
non-freeze-thaw conditions, and therefore the relationship between the
root system and SDC under freeze-thaw condition remains unclear.
Quantitatively analyzing the effect of the root system and freeze-thaw
on the soil detachment process and the development of a high-precision
SDC prediction model is of great significance for the improvement of
soil erosion prevention and control strategies in the seasonal
freeze-thaw region and for the accurate assessment of local soil erosion
(Laflen et al., 1991; Lal, 1989).
Therefore, the northern part of the Loess Plateau, China, as a typical
seasonal freeze-thaw region, was selected by the current study as the
study area. The current study investigated the sand and loessal soils
that are widely distributed in this area. A flume experiment was
conducted to explore the effect of the root system and the freeze-thaw
cycle on SDC. In addition, an SDC prediction model was developed based
on hydraulic parameters. The objectives of this study were to: (1)
investigate and compare the effects of the root system, freeze-thaw and
freeze-thaw combined with the root system on the soil detachment
process, (2) develop a model to simulate SDC under the effects of the
root system, freeze-thaw and freeze-thaw combined with the root system.