Introduction
Rill erosion is an important process of slope hydraulic erosion, being
the main component of slope hydraulic erosion prediction models (Foster
et al., 1995; Shen et al., 2015; Assouline et al., 2017; Zhang et al.,
2019). Rill erosion not only accelerates slope erosion and the transport
of erosion products (Guo et al., 2020; Gordon et al., 2012, Vereecken et
al., 2016), it also controls the development of watershed geomorphology
and the evolution of morphological characteristics (Mancilla et al.,
2005; Oz et al., 2017; Wu et al., 2018). In rill erosion processes, rill
morphology is the result of interactions between hydrodynamic factors
and soil properties (Schneider et al., 2013; Assouline et al., 2017; Wu
et al., 2020). Although the presence of rills results in more water
flowing into rills from inter-rills, thereby increasing rill flow
erosivity and the sediment transport capacity of runoff (Robichaud et
al., 2010; Wu et al., 2018), an increase in runoff erosivity and
sediment transport capacity aggravates rill morphology evolution
(Schneider et al., 2013; Zhang et al., 2019). Therefore, the evolution
of rill morphology, hydrodynamic characteristics and soil erosion form a
complex mutual feedback process. Quantitative description of rill
morphology and the relationship between rill morphological evolution,
runoff and sediment yield are important to fully understand rill
erosion, promoting the development of soil erosion simulation.
Currently, investigations have been undertaken on rill cross-section
shape (rill width, depth, width depth ratio, etc.) (Mancilla et al.,
2016; Shen et al., 2016; Oz et al., 2017) and rill network
characteristics (average rill azimuth, rill density, rill cutting degree
and average rill bending complexity, rill number and discontinuous rill
number, rill fractal dimension) (Gómez et al., 2003). The development
process of rills has been quantified from three perspectives (Govers et
al., 2007; Shen et al., 2015; Zhang et al., 2017; Wu et al., 2018), as
well as using the degree of connectivity (the number of combined nodes
and bifurcation ratio) (Schneider et al., 2013; Zhang et al., 2017).
Although these methods provide indices for characterizing rill networks,
the indicators ignore rill evolution during rainfall runoff and soil
erosion processes. In order to improve erosion model accuracy, these
rill characteristics have recently been incorporated into slope erosion
models, resulting in an improvement in model results. For example, Wu et
al. (2020) constructed a model to embed rill evolution module into
hillside rainfall runoff erosion processes. However, their experimental
materials were mainly concentrated in one or a few types of soil, and
the effects of soil properties on rill erosion morphology and sediment
yield were not systematically considered. Due to changes in soil
properties and the complexity of soil erosion processes, each function
can only be used for the soil erosion model under similar soil or
experimental conditions.
The effects of soil properties on slope erosion and rill morphology are
often ignored. Soil type and soil texture have significant effects on
soil separability, particle size distribution of eroded sediment and
morphological characteristics of erosion (Knapen et al., 2007). The
difference of soil physical properties is the internal influencing
factor of erosion processes and rill erosion (Dong et al., 2014; He et
al., 2014; Wu et al., 2017). Soil properties determine the soil shear
strength and stripping rate, directly affecting the occurrence of slope
erosion. Bonilla et al. (2012) found that the correlation between silt
content and soil separability was the largest, followed by sand content,
while the correlation between clay content and soil erodibility was the
weakest. Processes of soil separation and sediment transport are also
closely related to the size of primary and aggregate soil particles
(Rienzi et al., 2013). At the same time, soil properties affect the
formation and stability of soil crusts, change surface soil
permeability, and regulate runoff and runoff velocity, having an
important impact on slope erosion and rill morphology (Vaezi et al.,
2017; Mahalder et al., 2018; Sun et al., 2020). Soil properties are also
closely related to soil structure stability. Under different rainfall
intensities, different soil structure stabilities will have an important
impact on gully wall stability (Xu et al., 2015; Guo et al., 2019).
As a key agricultural area in northwest China, the Loess Plateau has
suffered from serious soil erosion for decades, and rill landforms
formed by water erosion have serious impacted agricultural production
(Wu et al., 2018; Sun et al., 2020). At the same time, soil in the Loess
Plateau has a consistent particle size composition (0.05-0.005 mm silt,
accounting for 58% - 75%) and zonality of particle size distribution.
Zonality shows that grain size of loess gradually becomes fine from the
northwest to the southeast, coinciding with sandy soil, light soil,
medium soil and heavy soil zones, respectively. Loess grain size
differences in the north-south direction have a profound impact on
landforms and soil erosion. Although many experiments have been
undertaken to study the process and mechanism of water erosion in the
Loess Plateau (Fang et al., 2015; Shen et al., 2016; He et al., 2017),
it is still unclear how changes to soil properties in different regions
affect rill morphology during surface erosion, and the accuracy of the
regional erosion prediction model with multiple soil types needs to be
further improved. Therefore, a series of simulated rainfall experiments
were carried out in different soil zones to examine the following
objectives: (1) to examine the effects of soil characteristics on rill
morphology (2); to discuss differences in different types of soil slope
erosion responses to rainfall intensity and slope angle; and (3) to
establish a prediction model of slope erosion based on soil
characteristic factors and rill shape factors. Findings from this
investigation will improve our understanding of erosion mechanisms of
multiple soil zones.