GeoJournal 24.2 195 -200 © 1991 (June) by Kluwer Academic Publishers

195

Soil Erosion Processes in the Loess Plateau of Northwestern China

Zhang Zonghu, Prof. Academician, Ministry of Geology and Mineral Resources, Institute of Hydrogeology and Engineering Geology, 050803 Zhengding, Hebei, China

ABSTRACT: The soil erosion processes in the Loess Plateau may be divided into three types: namely, waterflow erosion; gravitational erosion; wind erosion. The waterflow erosion is most widely distributed and is the main erosion action in the Loess Plateau. The main factors dominating the occurrence and development of the soil erosion in the Loess Plateau are: l. rainfall; 2. topography; 3. vegetation; 4. soil character. The energy of erosion action depends upon the rainfall and topography, but erodiblity depends upon the vegetation and soil properties. The degree of soil erosion in the Loess Plateau changes with variations of interaction of erosion and anti-erosion measures.

Introduction

Soil erosion is an important environmental problem in Loess Plateau. The soil erosion processes may be divided into three types: 1. water erosion; 2. gravitational erosion; 3. wind erosion.

Water Erosion in the Loess Plateau

The potential energy of the waterflow is influenced by the topographical gradient, thus the topographic changes belong to the main factors to directly influence waterflow and erosion intensity. The main geomorphological types in the Loess Plateau can basically be summarized as follows: Yuan in combination with gully and valley; Liang in combination with gully and valley; Mao in combination with gully and valley. In the geomorphological type of Yuan in combination with gully and valley, the topogra- phy is characterized by the smooth Yuan plane in combi- nation with the steeper gully and valley slope incised with different depths. Concerning the other two types, the to- pography is characterized by the slope plane with differ- ent gradient and with incision of steeper gullies and valleys. Waterflow occurring on the different topographies causes various complicated erosion processes and the occurrence of various landforms of erosion, transport and deposition.

According to the waterflow pattern on these different topographies and its characteristics of erosion action, the waterflow erosion can be basically classified as two erosion patterns, namely: 1. Gully erosion; 2. Sheet erosion.

Gully Erosion

A fundamental characteristic of gully erosion is that the eroding waterflow runs along the more or less fixed gully channels (on gully floor). The surface runoff incises the surface soil layer and causes side erosion at the same time. The waterflow changes the shapes of gully channel and gully slope when it deeply incises into gully floor and erodes gully slope. In the meantime, the eroded materials from the gully slope will accumulate in the gully channel. The gullies take in various shapes due to the interaction between the gully slope and gully floor. The present gully shape is only the consequence of a certain development period. No eventual gully shape is reached until the erosive action ceases.

The gully erosion process can be grasped by studying gully morphology, this is because the gully morphology reflects the effect of incision. The gully characteristics can be determined by the following main symbols: depth, gully length (main gully), gully system length, gully system area, gully system volume rate, and lithology and gully

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floor. The change of gully length, gully system volume and longitudinal section is mainly decided by the depth of the "Talweg" and by the catchment area. The depth is the key element. If it changes, the other parameters will also be changed. The longitudinal section of gully floor is seri- ously affected by the lithology of the gully floor. The lithology of gully floor in the Loess Plateau shows three types: 1. The gully floor is composed of loess of the Middle and Late Pleistocene and can be eroded rapidly. 2. The gully floor is composed of loess of Early Pleistocene or red soil of Pliocene with harder lithology, and it is eroded slower than that of the former one. 3. The gully floor is composed of bedrock (sandstone and sandy shale) with hard lithology, and it is eroded very slowly. The different lithology often causes interchanges between gentle and steep slopes, sometimes a sudden change occurs. Such changes directly influence the erosion action in the gully slope nearby.

Incision deepens and widens the gully floor, promotes the soil erosion of the gully slope and gully end, the gully floor extends, and transports the eroded silt. Although the erosion which occurs at the gully ends is connected with sheet erosion on the slope, both types are quite different owing to the essential distinction between the basic regu- larity of incising erosion and that of sheet erosion.

Sheet Erosion

Sheet erosion is characterized by erosion on the soil surface by a bedless waterflow. It causes the denudation of the surface. During rainfall, sheet flow will begin on the slope when the rainfall intensity becomes higher than in- filtration. The waterflow will change into a turbulent rill flow, while encountering unsmooth surface in the process of movement. The flow direction of rill flow is changeable and unfixed. In addition to that raindrops cause splash erosion. Splash erosion is loosening the soil surface by destructing the soil aggregates but it cannot transport the eroded soil. When the sheet waterflow turns into rill flow, the eroded soil grains can be transported to the bottom of the slope. It should be noted that formation of sheet water flow is dependent upon not only the rainfall amount, but also the soil layer, especially the permeability of surface soil layer. The surface runoff begins very late on soil layers with high permeability.

The rills are continuously changing the flow direction individually, but seen on the whole, they have a common flow direction and are connected with each other to form various flow lines with netted, parallel and stellate shapes.

As a result a shallow gully will be formed on the slope. These minor gullies exist only for a very short time owing to the fact that they are easily destroyed by natural or human activities. These shallow gullies, however, are often the source of incising erosion; there sheet erosion may change into incising erosion, while erodes and en- larges shallow gullies much quicker.

The processes of sheet erosion are quite different along the different segments of the slope. For example, on the uppermost part of the slope, the slope gradient is very low and erosion phenomenon is not evident. When the slope gradient amounts to a certain degree and velocity of water movement can cause erosion, the upper part of the slope actually becomes a pure erosion area. But on the middle and lower parts of the slope both erosion and accumulation can occur. The amount of erosion, transportation and ac- cumulation is influenced by soil structure, especially by the contents of aggregates, and the soil permeability. They are also controlled by the topography of the slope. The development and covering degree of vegetation also play an important role. The erosion and accumulation on different parts of the slope can be directly observed at many places in the Loess Plateau. They are the main process of geomorphological development in the Loess Plateau.

Changes of the slope gradient and the slope length may cause great changes of sheet waterflow with a result that various complicated erosion patterns appear. On slopes with an uniform lithology the slope gradient and slope length will play an important role on surface runoff and erosion, if also the vegetation cover is similar. Therefore, studies on the slope shape in the Loess Plateau and its classification is a prerequisite to grasp the regularities of the sheet erosion. The classification of various slope shapes in the Loess Plateau should be conducted with much con- sideration to the relationship between slope shape and flow direction of surface runoff. The various and changeable slope shapes in the Loess Plateau can be classified as two types namely, unitary and combined slope shape, accord- ing to the aforesaid principles.

Gravitational Erosion in the Loess Plateau

The occurrence of destruction and displacement of soils and loess sediments due to gravitational processes is called gravitational erosion. It differs essentially from the water erosion. In the Loess Plateau, gravitational erosion occurs usually in the form of landslides collapse ans slipping. Certain topographic conditions and some other inducing factors such as movement of groundwater and earthquakes are absolutely necessary in addition to the gravitational action of the soil mass. Thus the gravitational erosion often occurs in certain places and shows the regularities of regional distribution.

Landslides, one kind of gravitational erosion in the Loess Plateau, cause the largest damage. It mostly occurs on the steeper, deeply incised slopes of the gullies and valleys, which are situated in the boundary area of Yuan or at the border of gentle and steep slopes of Mao and Liang.

The landslide causes the movement of soil masses on a large scale. It changes the topography of the upper part

of the gully and valley slope and blocks up the gully and valley floors. In the Loess Plateau, landslides mostly oc- cur in deposits of the upper part of Early Pleistocene and

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Characteristics of Morphological Elements

.2

(5

Subsurface Funnel = , In round or approximate round funne shape, 1 m to' several m in diameter, 2-5 m piping erosion / in depth, distributed separately =

[ Paternoster caves = I Round or irregular shape small caves, 1 m to several m in diameter, several m in

] depth with no sediments at bottom distributed in clusters individually connected = [ with blind gullies = : =

[ Blind gully [ With round or irregular shape entrance, t m to Several m in diameter, concealed in

I incis=ing erosidn

t t

:

Wind erosion Usually palaeo-sand dunes or loess after eroded by the wind resid'ual dunes:

qnd e ros ion Several m to more than 10 m in height, distributed individually or in clusters, residual soil mass after wind )lumn erosion

Tab 1 Class i f i ca t ion of soi l e ros ion types in the Loess P la teau

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of Pliocene red clay with high density and low permeabil- ity. These deposits form the sliding bed for the overlying loess. In gullies and valleys with the above mentioned geological conditions, landslides are very common. Heavy rainfalls or earthquakes may then initiate landslides. Most of the gravitational processes occur in loess of the Middle and Late Pleistocene or of the recent loess formation. Basic conditions may be weathered layers on the upper part of slope or vertical fissures formed due to gravitational shearing. The slipping erosion occurs on slopes with loess of Early Pleistocene. The soil mass removed by slipping erosion is accumulated at the foot of the slope. The slipping of soil masses is caused by the seasonal changes of soil moisture.

It must be pointed out that the consequence brought about by gravitational erosion is basically destruction of the land and change of topography. However, gravitational erosion does not transport the eroded soil mass over a long distance. It only prepares a great amount of material for a transport by waterflow to the rivers.

Wind Erosion in the Loess Plateau

Wind erosion is based on a blowing wind action with certain velocity. It destroys the topsoil and transports and accumulates the eroded soil. Wind erosion mainly occurs in the N part of the Loess Plateau and in the S and E parts of Yikezhao League, Inner Mongolia autonomous region. The loess has a component of fine sand and sand, which is not well cemented. It will be eroded first and then transported and accumulated at lower places such as river valleys and river courses. At last they will be successively transported to the Yellow River by temporary floods or rivers.

The wind erosion occurs in the Loess Plateau under the following conditions:

1. At the beginning, the loamy topsoil protects the silty and sandy loess strata. As soon as the topsoil is eroded and destroyed by waterflow, the underlying subsoil with loose structure will be exposed. In such case, the wind erosion develops rapidly and destroys the soil. The eroded soil will be accumulated in situ and form new dunes or will be transported to the gullies and valleys. This phenomenon can be commonly observed around Jingbian, Anbian and Dingbian in the N part of Shaanxi province.

2. If the topsoil is not well cemented or has a loose struc- ture, it will be directly removed by wind erosion. The sand, silt and clay will be blown away, while the coarser material such as gypsum crystals and calcium carbonate nodules remain in situ. After the soil layer has been denudated, the original roots of growing xylophytes will be exposed above the ground surface.

This process is one of the important factors of the increasing desertization in the northern part of the Loess Plateau.

Another consequence of the wind erosion in the Loess Plateau is that an enormous quantity of sand and silt is sedimented temporarily in the gullies and valleys, after- wards transported by water to the Yellow River. Therefore, the amount of erosion by wind, though its importance is evidently less than that of the water erosion, should not be calculated only on the basis of the occurrence of sand accumulation and dunes. Both wind and water erosion are parts of a huge transport system which ends by sedimen- tation in the lowlands along the Yellow River.

Classification of the Soil Erosion Types in the Loess Plateau

The process of soil erosion contributes to changes of geomorphology to a large extent. The landforms which are caused by erosion, show certain geometric shapes and occupy a certain space on the surface of the land. These "erosion marks", which are distributed in different places, have different characteristics and occur in different stages of development (Tab 1).

The Vertical Zonation of Erosion Type Distribution of the Loess Plateau

The various types and subtypes of soil erosion men- tioned above are distributed from the top to the bottom of the loess landscape in a zonal distribution. This is the basic regularity of soil erosion in the whole Loess Pla- teau. Sheet erosion mainly occurs on Yuans or Liangs and the gentle slope surface of Maos, but on Yuan surface or boundary zone between gentle slope of Liangs and Maos and valley slope, piping erosion often develops, further downwards the gravitational erosion occurs and on the smooth valley slope the gully erosion occurs. Scouring erosion often takes place at the bottom of the gully and valley (side erosion also takes place if the valley bottom is deeply incised). All these erosion types can be trans- formed with changes of the intensity of erosion. This transformation of erosion types is another important regularity of soil erosion in the Loess Plateau. Knowing this regularity well, we can use this knowledge for pre- venting and controlling soil erosion.

Factors Controll ing the Occurrence and Development of Soil Erosion in the Loess Plateau

Among soil erosion processes in the Loess Plateau, water erosion is of the highest importance, for it is widely distributed and causes the most serious damage onsite as well as by silting up the course of the Yellow River.

There are four main factors dominating the occurrence and development of the soil erosion in the Loess Plateau, namely, 1. rainfall; 2. topography; 3. cover degree of vegetation; 4. soil. Different combinations of these four factors cause soil erosion of various type and amount.

GeoJournal 24.2/1991 199

rationalization degree of land use ]

4. soil character

i soil structure content of stable aggregates in water

~- chemical component of soil resistance to shear stress of soil

erodibility (capability of anti-erosion)

~no aE-b phenominon of soil erosion

l gravitational erosion I

a>b soil erosion of various types[

sheet erosion ~- piping erosion ~ - gully erosion

~ - scouring erosion

[migration of eroded materials [

J J

accumuIatiOnof gully andar thevalleybOttom transported into the river

Tab 2 Basic mode of the soil erosion in the Loess Plateau (waterflow erosion)

Rainfall: The runoff is closely related to the rainfall intensity and amount. However, the quantitiy and velocity of surface runoff are also influenced by soil moisture and permeability, which control the infiltration rate.

Topography is a basic factor controlling velocity and direction of surface runoff. The main parameters of topog- raphy are slope gradient and slope length. The topographic factors control the runoff by increasing or reducing the erosion capacity.

In the process of soil erosion, both rainfall and topog- raphy create an erosion capability, which can also be called erosivity.

Vegetation may reduce the force of the surface runoff and change its direction or block it, therefore, vegetation plays the role of protecting the soil from erosion. Arable lands may produce the same effect if reasonably used. But in areas where the vegetation is seriously destroyed or the arable land is not reasonably utilized, grave consequences can occur.

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Soil characteristics

Soil characteristics are the main factor of resistence against erosion. This resistence of the soil is depending on soil structure, soil texture, chemical properties, content of the organic matters and resistence to sheer stress. Of them, soil structure is the most important. The number and sta- bility of aggregates plays the key role. The more stable aggregates a soil has, the higher is its structure stability.

The vulnerability of the soil against erosion, also called erodibility, is dependent upon the vegetation and the resistence of the soil.

The occurrence and degree of soil erosion are depend- ing upon the different combinations of the above-men- tioned two factors: erosivity and erodibility. The interaction of these factors have been elaborated in detail in Tab 2, which shows the basic guidelines of soil erosion in the Loess Plateau of China.

References

Zhang Zonghu: Loess in China. Geojournal 4, 6, 525-540 (1980)

Zhang Zonghu: Geomorphologic Map of the Loess Plateau in China. Geological Publishing House China 1986.

Selby, M. I.: Slopes and Weathering, Human Activity and Environ- mental Processes 183-203 (1987)

Imeson, A. C.: Soil Erosion and Conservation. Human Activity and Environmental Processes 329-347 (1987)

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