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.