The
relative contributions of slope gradient and vegetation cover on erosion
characteristics of riparian slopes along the lower Yellow River, China
Zihao Caoa b, Qinghe Zhao a *,
Shengyan Dinga, Yifan Zhanga
a Key Laboratory of Geospatial Technology for the Middle and Lower
Yellow River Regions, College of
Environment and Planning, Henan University, Kaifeng 475004, China
b State Key Laboratory of Earth Surface Processes and Resources Ecology,
Faculty of Geographical Science,
Beijing Normal University, Beijing
100875, China
*Corresponding author: Qinghe Zhao
College of Environment and Planning,
Henan University,
Jinming Avenue, Longting District,
Kaifeng 475004, P. R. CHINA.
Tel.: +86-15903783593
E-mail: zhaoqinghe@henu.edu.cn
ABSTRACT
Slope gradient and vegetation cover
play key roles in soil erosion process. Exploring the effects of slope
gradient and vegetation cover on runoff and sediment yielding
characteristics is therefore of great importance for minimizing soil
erosion. In this work, based on
field scouring experiments on the
riparian
slopes of the lower Yellow River, China, variation in
total
runoff, accumulative runoff sediment concentration, erosion sediment
yield, and sediment particle size composition under four slope gradients
(5°, 10°, 15°, 20°) and three
vegetation
cover levels (0%, 15%, 30%) were analyzed. Runoff and sediment yield
were greatly influenced by slope gradient at steep slope gradients (15°
and 20°), while they were mainly affected by vegetation cover at medium
slope gradients (5° and
10°).
The main enriched particle size of the eroded sediment showed a trend of
first increasing and then decreasing with the increasing slope gradient.
There was an interaction between slope gradient and vegetation cover,
the effect of vegetation cover on erosion sediment yield weakened
gradually with increasing slope gradient (at medium slope gradients of
5° and 10°), while the effect of slope gradient increased at steep slope
gradients (15° and 20°).
Key words :
riparian
zone; runoff and sediment yielding characteristics; slope gradient;
vegetation cover
1. Introduction
The
Yellow River basin in China is one of the most severely eroded regions
in the world (Wang et al., 2016). In particular, slope erosion, which is
related to climate, soil properties, slope gradient, vegetation cover,
and their interactions (Römkens et al., 2001; Sachs and Sarah, 2017), is
very common in the Yellow River basin. Among these factors, climate and
soil are relatively stable and change little over short periods.
Variation in slope erosion is mainly driven by changes in slope gradient
and vegetation cover, both of which can retard or accelerate the process
of soil erosion (Joshi and Tambe, 2010; Alatorre et al., 2011; Sharma et
al., 2011; Donjadee and Chinnarasri, 2012; Mohamadi and Kavian, 2015).
Understanding their relative contribution to soil erosion is an
important question for many stakeholders of the Yellow River.
Researchers have long known the importance of vegetation in soil erosion
(Srivastava et al., 2010; Sandercock and Hooke, 2011), and the
observation of a strong correlation between vegetation cover and runoff
and soil loss is common (Nunes et al., 2011; Xin et al., 2011; Gurnell,
2014;
Duan
et al., 2016). Subsequently, several studies under different
environmental conditions have demonstrated the positive effect of
vegetation cover in reducing soil erosion and improving the ecological
environment (Srivastava et al., 2010; Mohammad and Adam, 2010; Nunes et
al., 2011). From the canopy to the roots, vegetation cover of different
spatial structure can weaken the intensity of soil erosion caused by
rainfall and runoff scouring by reducing slope sediment transport and
soil and water loss (Molina et al., 2010; Srivastava et al., 2010; Duan
et al., 2016). Moreover, vegetation cover can increase soil infiltration
and water conservation (Chen et al.,2007), reduce runoff and flow
velocity (Vásquez-Méndez et al., 2010), and improve soil corrosion
resistance and anti-scourability (Wang et al., 2016). Generally, runoff
generation and sediment yield decrease with increasing vegetation cover
(Breshears et al., 1998; Zhang et al., 2015b; Wang et al., 2016), and
different vegetation covers can produce different effects on
soil erosion process
(Srivastava et al., 2010; Sharma et
al., 2011; Wang et al., 2016). For example, a study on the purple soil
in Southwest of China showed that high vegetation cover reduces runoff
and sediment yield (Liu et al., 2015a). A study of slope rainfall and
runoff erosion under different herbaceous vegetation cover found that
soil erosion intensity decreased significantly at vegetation cover in
the range of 20% ~ 60%, while soil erosion became more
serious when the vegetation cover was less than 20% (Zhu et al., 2010).
Slope gradient is probably the most important topographic factor
controlling nutrient, contaminant, and particle transfer within a
watershed (Valmis et al., 2005;
Sirjani and Mahmoodabadi, 2014; Zhao et al., 2015a). Slope is the
dominant factor associated with the hydrological response in soil
erosion processes, and has been studied via numerical simulation,
experiments, and analytical solutions (Bergkamp, 1998; Assouline and
Ben-Hur, 2006; Joshi and Tambe, 2010; Sirjani and Mahmoodabadi, 2014).
Further, a better understanding of relationship between slope gradient
and slope erosion is important in predicting soil erosion and the
conservation of soil and water resources (Assouline and Ben-Hur, 2006;
Zhao
et al., 2015a; Wu et al., 2018). However, due to differences in
experimental methods (Zingg, 1940; Fox and Bryan, 2000; Joshi and Tambe
,2010; Sirjani and Mahmoodabadi, 2014) and the regional environment
(Valmis et al., 2005; Assouline and Ben-Hur, 2006; Liu et al., 2015b;
Zhao et al., 2015a), the observed results are often different. On the
one hand, erosion sediment yield rate has been shown to be greater on
steeper slopes (Assouline and Ben-Hur, 2006; Wu et al., 2018). On the
other hand, there is a critical slope gradient for soil erosion
processes. For example, the first empirical relationships between soil
erosion and slope gradient established by Zingg (1940) indicated that
the amount of soil erosion increased with increasing slope, while in the
early version of the USLE and its
revised version (RUSLE), soil erosion was predicted as a power and liner
function of slope gradient respectively (Fox et al., 1997; Liu et al.,
2017). Moreover, Zhang et al. (2015) found that the relationship between
erosion sediment particle size and slope was linearly negatively only
under a certain slope gradient range. Nonetheless, a number of field
observation data and indoor artificial rainfall experiment data showed
that the relationship between soil erosion and slope gradient can be
established only within a certain slope gradient range. When the
slope gradient exceeds the threshold
value, the relationship between them could even reverse, indicating that
there may be a critical slope gradient in slope erosion processes
(Bergkamp,
1998; Zhao et al., 2015a; Liu et al., 2017).
The riparian zone is an important
ecotone,
exchanging energy and information between streams and hillslopes, and
has unique ecosystem structure and ecological service functions (Mander
et al., 2005; Stella et al., 2013; Méndez-Toribio et al., 2014; Tang et
al., 2014). The main factors affecting the structure and function of
riparian ecosystems are geomorphology and vegetation (Srivastava et al.,
2010; Stella et al., 2013; Méndez-Toribio et al., 2014; Cadol and Wine,
2017). Previous studies have shown that the riparian zone serves an
important function in reducing soil erosion via intercepting soil
particles in surface runoff, and preventing scouring and collapse
(Srivastava et al., 2010; Méndez-Toribio et al., 2014). However, most
studies have focused on the effect of only a single control factor
(e.g., slope gradient, or vegetation cover) on soil erosion (Srivastava
et al., 2010; Stella et al., 2013; Cadol and Wine, 2017). Yet, studies
considering the interaction between slope gradient and vegetation cover
are relatively few. Moreover, existing research results are mainly from
simulated rainfall experiments indoors or outdoors on slopes with
artificially planted vegetation (Srivastava et al., 2010; Sirjani and
Mahmoodabadi, 2014). Documenting erosion and sediment yield under
natural vegetation conditions is rarely considered, especially in the
riparian zone. These issues are
critical for the Yellow River, which is world-famous on account of heavy
silt-carrying, and in combination with long-term impact of human
activities, its riparian zone is suffering serious
threats.
As the last barrier to prevent water and sand from entering rivers, the
riparian zone plays an important role in soil and water conservation
(Srivastava et al., 2010; Méndez-Toribio et al., 2014).
According
to the discharge per unit width produced by heavy rain at the plot scale
in the study area, we ran a series of field runoff scouring experiment
to analyze soil erosion and sediment yield variation under different
slope gradients and vegetation covers.
The
rainfall experiment which can be found in many specialized literatures
(Aoki and Sereno, 2006; Arnaez et al., 2007; Verbist et al., 2009), and
is an important way to analyze the different processes involved in
erosion
(Sangüesa
et al., 2010). Because the riparian zone is long and
narrow
and the erosion effect of a rainfall event is usually less than that of
runoff from upslope, we conducted a field runoff scouring experiment.
Soil
erosion is generally shown to be highly scale-dependent and vary
significantly across different spatial scales, such as plots,
watersheds, and regions (Chen et
al., 2012). In order to get a better understanding of soil erosion
issues on riparian slope of the lower Yellow River, this study was
conducted at the plot scale. The advantages and progresses of soil
erosion research at the plot level are significant. During the past
several decades, some experimental techniques and (semi-) quantitative
methods have documented soil erosion processes at the plot scale (Chen
et al., 2012). Meanwhile, field measurements have provided large
quantities of basic data and important parameters for model calibration
and validation, such as flow length (FL), directional leakiness index
(DLI), the universal soil loss equation (USLE), and the revised
universal soil loss equation (RUSLE)
(Chen
et al., 2012). Processes at these small scales could also reflect key
changes in direction or trends at regional or global scales
(Muñoz-Robles et al., 2013).
The main purpose of this study was to document the relative contribution
of slope gradient and vegetation cover to erosion characteristics of
riparian slope along the lower Yellow River, using field runoff scouring
experiments. Specifcally, we
examined: (1) runoff processes under
different slope gradients and vegetation covers; (2) sediment yield
characteristics under different slope gradients and vegetation covers;
and (3) the relative contribution of
slope gradient and vegetation cover to slope erosion and sediment yield
in the riparian zone of the lower Yellow River.