Xiao Peiqing^{1, 2} and Yao Wenyi^{1, 2}

1.State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling Shaanxi 712100;

2. Institute of Hydraulic Research Yellow River Conservation Commission, Zhengzhou Henan.450003)

Abstract: The Loess Plateau has the highest erosion rates in the world. Study on soil erosion process in the Loess Plateau has received more attention in recent years. Soil erosion on gullied rolling loess plateau has clear sheet zone, rill zone and shallow gully zone from watershed to gully edge. Vertical erosion distribution zone is an important feature on the loess plateau. Infiltration, sediment concentration, erosion pattern and characteristic of sediment and water transportation have a clear distribution too. So, spatial variability of slope erosion pattern, sheet, rill, shallow gully and permanent gully erosion process dominated at different erosion zone as well as slope gully coupling system were discussed in this paper. And then, further research issues have been presented in the future.

Key words: Soil erosion process; Vertical erosion distribution; The Loess Plateau

1 Introduction

Soil erosion on gullied rolling loess plateau has clear sheet zone, rill zone and shallow gully zone from watershed to gully edge. Vertical erosion distribution zone is a striking feature on the Loess Plateau. Soil infiltration rate, sediment concentration, erosion pattern and characteristics of sediment and water transportation have a clear vertical distribution too. Jiang Deqi first pointed out the source of sediment in 1960s, which indicated that sediment sources mainly came from gully slopes and the reason of producing sediment was discussed according to the runoff distribution in the catchments. But the proportion of increased sediment yield along with runoff through the gully slopes was not into account by him. Zeng Boqing(1980) investigated that slope gully erosion relationship at Yang Daogou basin in the gully–hilly loessial middle reaches of the yellow river and found the sediment sources from gully slopes could decrease by about 58.77 and 77.8% of the total runoff and sediment of the entire slope (slope plus gully slope) if surface from upslope was intercepted completely. Jiao Juying et al. (1994) discussed erosion actions of surface runoff on gully slope and found about 50% of the sediment yield from gully slopes was produced by surface runoff. Meanwhile, upslope runoff and sediment have significant impacts on downslope erosion process. But with the limits of research methods, there are not much data to quantify upslope runoff and sediment effects on downslope erosion process under different conditions. In order to study Erosion and Sediment Yield in Different Zones of Loess Plateau and quantify soil erosion-transportation-deposition process, recent progress of soil erosion process and slope gully system were presented in this paper.

2 Spatial distributions of vertical erosion zones

Vertical distribution of soil erosion on the Loess Plateau is closely related to the pragmatic layouts of soil and water measures. Fig 1 shows vertical erosion distribution of sheet erosion, rill erosion, and ephemeral and permanent gully erosion on the Loess Plateau. The earliest research was proposed by Zhu Xianmo (1956). He studied vertical erosion distribution characteristics from the occurrences and evolutions of soil erosion. Luo Laixing (1956) pointed out the runoff along the loess slope is alternative change, which has three alternative stages of increase-decrease-increase. Based on the previous study Cheng Jicheng (1963) pointed out runoff along the slope is a complex process affected by many factors such as slope shape, slope length, anti-erosion forces, rainfall intensities and runoff agent. But he pointed out the general trend along the slope is the erosion amount various from small to large then become small. Chen Yongzong et al. (1976) also studied slope distribution and analyzed the effects of rainfall, slope angel and slope length on soil erosion and depicted the relationship of different topography and runoff erosion. In 1980s, Liu Baoyuan divided the soil erosion zone in a detail on the gullied rolling Loess Plateau based on more observed datum of field trip. Tang keli (1983) had a systematic analysis of the regional characteristics of soil erosion in the Loess Plateau and had given a clear demonstration in the Loess Plateau region. Wu Faqi (1993) studied the development of gully and slope considering the construction and the main factors using satellite images. All the research outcomes accelerated the understanding of Loess Plateau environment, soil erosion pattern and its erosive distribution, which has provided important basis to study process–based soil erosion model.

Fig.1  Vertical erosion distribution on Loess Plateau in Shaanxi Province, China

2.1 Sheet erosion

Splash erosion is the initial erosion process on slope, which usually occur near the watershed. Raindrop action on bare soil disrupts aggregates, dislodges soil particles and compacts the erodible soil surface. If rainfall exceeds infiltration, a surface film of water forms, will be built up into flows 2-3 mm deep. Continuing rainfall causes turbulence within the flow that may increase the water's erosive effect up to 200 times. The texture of loess soil is loose and has low anti-erosion ability; the intensity of anti-erosion is 0.5-1.0 kg/cm². The raindrop of 2-4 mm can smash the soil particle after continuous 1-2 minutes. It is evident the effect of splash is near the watershed on the Loess Plateau. Splash erosion and patch erosion sediment yield were viewed as before rill forming. The observed sediment yield in Zhizhou field runoff plots is usually between 3-40×10³ kg/km², the maximum can reach to 667 kg/m³. So, on the Loess Plateau high sediment content can cause only by splash erosion, which is rare in other loess region.

2.2 Rill erosion process

Rill erosion is an important process on farmland slope in the Loess Plateau. Rill erosion results from surface water following into deeper, faster-flowing channels on depressions or low points. Rill erosion is common on agricultural land devoid of vegetation, which is almost straight and parallel. Following intense rainfall cultivated topsoils overlying denser cohesive subsoil often exhibit rill erosion. According to the observed data of Luo Laixing, rill didn’t occur on slope angle between 4-12° under 6-10 m slope length of the watershed. If the watershed area was grassland the slope length of rill happened was increased to 12.2 m. The observed data from Zhizhou runoff plots of slope length of 60 m and slope angel 22^° from 1963-1967 showed the chance of rill forming is 45%-60% and the erosion amount of rill accounted for 68-91%. Many scientists did more work about critical condition of rill occurrence. Affecting factors of rill erosion forming and developing such as rainfall energy, soil erosion durability, slope length, slope degree and land management were conducted by Zheng Fenli (1988) using simulated rainfall and field investigation. And she studied the non-linear relationship of critical slope length and slope angel under certain rainfall on the Loess Plateau. The runoff scouring experiment was done by Zhang keli (1998) to study the critical characteristics of developing processes from sheet erosion to rill erosion. His results showed Froude number was recognized as a parameter reflecting rill erosion occurrence and the relationship between the critical runoff of rill erosion occurrence and slope gradient was.

2.3 Ephemeral gully erosion

Ephemeral gully erosion is a special erosion type on the Loess Plateau formed by erosion processes and plough activities, are wider and deeper than rills, but they can be tilled across and filled in partially or completely. Ephemeral gully erosion is a transition zone between sheet erosion and gully erosion on Loess Plateau. Along the rill development and water flow towards the slope ephemeral gully erosion occurred. Farmer’s cultivation accelerated the ephemeral gully erosion development. In the hilly-gully region of the Loess Plateau, the ephemeral erosion amount takes up over 60% of the total soil loss at steep hillslope.

The depth of ephemeral gully is between 0.5-1 m. The spatial distribution of ephemeral gully is tegular and parallel. Zhang Keli (1991) evaluated the characteristic of ephemeral gully erosion from aerial photo interpretation and field survey. His results explained the critical slope of ephemeral gully is 18^°, and 40 m in critical slope length, 650 m² in runoff converged area. From Jiang Yongqing’s results of Zhou Tungou watershed using aerial photograph in 1999, the ephemeral gully covers 31.3% of total area of hillslope in the watershed. Its average frequency (density) is 7.2/100m on the cross-section of hillslope, the frequency range 4-9/100m accounted for 78%. In fact, tile-roofed shape ephemeral gully is a more stable geographical feature, which can yield higher silt. Runoff converged area is the priority condition to form ephemeral gully erosion. The data from Chen Yongzong showed the runoff converged area is much bigger than 700 m². The ephemeral gully erosion will be transformed to gully quickly on the feeding runoff area of more than 2300 m².

2.4 Gully erosion process

Gully erosion means the loss of large volumes of soil. An evident feature of the hilly of the Loess Plateau is the presence of many, large, permanent gullies. Deep wide gullies, sometimes reaching 30m deep, severely limit the use of the land, while off-site deposition of soil causes water quality decline in streams or rivers and sedimentation of dams and reservoirs. Derbyshire (1989) found that Chinese losses are prone to slab development that is due to tensional stresses in the loess. They also confirmed the finding of Lohnes & Handy (1968) that stability analysis can be uses to show that loess slopes would have near vertical slopes at the top (70-85°) and slopes of 51-59° lower down. In the loess region, gullies are relatively permanent which experiences ephemeral flows during rainstorm. Gullies are almost always associated with accelerated erosion and therefore with landscape instability. Numerous studies record the formation of gullies by pipe or tunnel collapse. Tunnel develops particularly where the clays are of low permeability and loose. Loess is one of the most erodible soils that great part of Loess Plateau in hill slope is covered by it. So, gully erosion represents an important sediment source and an effective links for transferring runoff and sediment from uplands to valley in the loess region. Topography has important impacts on the gully development. The development of gully is affected by gravitational erosion especially for the expanding of gully slope.

3 Research on the slope-gully system

3.1 Slope-gully erosion relationship

The quantitative erosion relationship of slope and gully land has got some progresses on the Loess Plateau in recent years. Jiang Deqi discussed the ratio of upslope area (all land upslope of the gully boundary edge) and gully area (all land downslope of the gully boundary edge) on the down slope and got the sediment amount was 52.9%-69.8% of gully area under gully edge accounted for the total area. Zeng Boqing got the gully area fed by upslope runoff and sediment was 4.5 times of the sediment area of no runoff feeding from the observed area in Yang Daogou area in Shaanxi. Xu xueliang pointed out slope area was 35.3% of the total runoff and 30.8% of the total sediment. Chen Yongzong analyzed the sediment of having upslope runoff is 5 times the sediment of no sediment feeding but the different results is 3.5 times provided by jiaojuying. Zheng Fenli analyzed the upslope area has many vegetations the runoff has no influences on the down slope gully area. But the sediment is 1.7-4.8 times if the vegetation is reclaimed which had given impacts of human activity on soil erosion. The detailed outcomes and research methods based on the main three analysis methods by other scientist were analyzed in the following table.

Table 1   Erosion distribution of upslope and gully area in the Loess Plateau

3.2 Erosion sediment distribution on the Loess Plateau hillslope

According to the early study results put out by Jiang Deqi, the total sediment yield of sheet erosion, rill and ephemeral gully erosion accounted for 28.7% of the total sediment. Tang keli did detailed research in xingzhihe watershed and found the area of ephemeral gully erosion accounted 72%-90% of the total upslope area, rill area only accounted for 10%. The total sediment area of rill and ephemeral gully accounted for 25%-30% and the sediment is mainly from the topsoil. So upslope land harness should be received more attentions in the watershed. Gong Shiyang got the conclusion upland area was 43.3%-61.8% in the gully watershed; and the total sediment of farmland was 53.9%. Based on the observed data of many years, Zhang Keli did some researches on experiment plots, rill erosion was 30% and ephemeral gully erosion was almost 70%. Recent years Zheng Fenli did some filed experiments on field plots and explained the erosion sediment distribution was restricted by rainfall intensity and rainfall energy, and the evolution of different erosion pattern occurred mainly between the transition zone of rill erosion and ephemeral zone. Furthermore, she got a conclusion rill erosion sediment was 26.1%-36.8%; ephemeral gully erosion was 52.1%-64.1% in Zhi Wuling reclamation land. All the results analyzed the vertical erosion distribution, which are very significant on studying slope-gully system research.

4 Further research issues

       Many researches about slope soil erosion and transportation processes have been done from the above mentioned, but many scientific problems of soil erosion process still not clear owing to the complexity of soil erosion process.  Some issues should be done in the future are as follows: (1) Further research of physical soil erosion process is needed to combine the advanced technology of GIS and REE (rare earth element) with small and large scale runoff plots; (2) The temporal and spatial change as well as soil detachment-transport-deposition process should be studied in a more detailed quantitative information; (3) More studies are needed on critical hydraulic conditions leading gully initiation and development, and the hydraulics parameters analysis and its relationship with sediment transportation; (4) multiple box system are to be designed to study effects of runoff and sediment form upslope on downslope erosion process under steep slope gradient and high sediment concentration.

Acknowledgments

This study was funded by National Nature Science Foundation of China (50239080) and Foundation of State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau (10501-122).

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Source:  www.yellowriver.gov.cn   Editor:HuangFeng