Wind Erosion Research in China: Past, Present and Future

Peijun Shi , Ping Yan *,Yi Yuan , Mark A. Nearing

China Center of Desert Research at Beijing Normal University, Beijing 100875, China; Key Laboratory of Environmental Change and Natural Disaster, Ministry of Education of China Beijing

100875, China. National Soil Erosion Research Lab., USDA-ARS, Purdue University, Indiana, USA.

Abstract Wind erosion is one of most important processes associated with land degradation and desertification

in the arid, semiarid and portions of sub-humid regions of China. The total land area experiencing wind erosion is approximately 160.74×104 km2, which is 16.7% of the national territory. Wind erosion is recognized as a great threat to land utilization and sustainable social and economic development. Documentation of wind erosion and its negative impacts in China dates back over 2000 years. Since the 1950s Chinese scientists have carried out an integrated investigation of the principal lands susceptible to wind erosion, and made many laboratory tests and field observations with respect to the stabilization and utilization of soil in desert areas. Since the late 1970s there has been an increased concern around the world regarding land desertification caused by climatic changes and human activities. Hence, wind erosion, one of the main processes of desertification, has attracted the attention of Chinese scientists to an even greater extent. Studies have been conducted to investigate the mechanics, causes, and control techniques related to wind erosion using wind tunnel simulation tests and field observations in typical areas. Some encouraging accomplishments have been made. In this paper we summarize the main research results on wind erosion that have come to light in China in recent decades, and put forward some perspectives and suggestions beneficial for dealing with problems in both research and control practices of wind erosion in China.

Key words: Arid land, China, Desertification, Dust storm , Semiarid land, Soil erosion, Wind erosion

1. Introduction

Wind erosion is one of most serious environmental problems in the arid, semiarid and dry sub-humid areas around the world. Accelerated soil erosion by wind results in both on-site and off-site damage. On-site damage includes the depletion of nutrients, organic matter, and fine particulate matter. In addition, wind erosion causes considerable crop damage, especially to young seedlings, and increased farm maintenance costs. Off-site damage includes adverse health impacts caused by dust storms over vast areas, and harm to transportation, communication and irrigation infrastructure. Areas most subject

* Corresponding author. Institute of Resources Science, Beijing Normal University, 19 Xinjiekouwai Street, Beijing, 100875, China. Tel.: +86-10-62206087; fax: +86-10-62207162. E-mail: yping@bnu.edu.cn

to wind erosion around the world are in the United States and Canada in North America; in drier portions of Argentina, Bolivia, and Peru in South America; in both European and Asiatic parts of Russia; in China, India, and Pakistan and much of the Middle East in Asia; north and south of the equator in Africa; and in Australia (Skidmore, 1986a). It is estimated that the degraded area caused by wind erosion amounts to 5.05 ×106 km2, accounting for 46.4% of the global degraded land (UNEP and ISRIC, 1990). Wind erosion is also one of most important processes among the land degradation or desertification in the arid, semiarid and a portion of the sub-humid regions of China (Chen et al., 1994). The total land area experiencing wind erosion is approximately 160.74×104 km2, or 16.7% of the national territory (Ci and Wu, 1997), where wind erosion is recognized as a great threat to the land utilization and sustainable social and economic developments.

Documentation of wind erosion and its disastrous consequences in China dates back over 2000 years. “Earth rain” and “sandy rain” recorded in history were actually the rain with dust resulting from wind erosion (Zhang, 1984). Ban Gu (32-92 A.D.), a historian in Dong Han Dynasty, has the first recorded description of the landform of Yardang in Lop Nor, and later a historically famous geographer in the Wei Dynasty, Li Daoyuan (466-527 A.D.), defined the formation of Yardang as “rills cut by water and subsequently blown by wind” (Xia, 1987). Many local administrators in Qing Dynasty organized local people to take practical measures to protect farmlands and irrigation canals from blown sand (Dong et al., 2000). After investigation in the Loess Plateau of China during 1866-1872, Richthofen attributed the thick loess deposition in that region to the wind erosion of the Gobi and other sandy deserts to the northwest (Pye and Tsoar, 1990). Subsequent to his journey in Central Asia (1889-1902), Hedin noted that the Yardang in the Lop Nor area had been eroded 6 m deep by wind in a period of 1,600 years, from which can be deduced that the average wind erosion rate was 4 mm yr-1 (Hedin, 1905). During the 1930s and 1940s Chinese scientists began to focus on the problems of wind erosion and its increasing harm to agricultural production in arid and semiarid areas (Chen et al., 1994). Since China’s reunification scientists have carried out an integrated investigation of the principal deserts and sandy regions over the whole country, and conducted many laboratory tests and field observations with respect to utilization and stabilization in desert areas. From late 1970s, wind erosion has attracted the attention of Chinese experts to an even greater extent because of the growing global concern regarding land desertification caused by climatic changes and human activities. Attempts have been made to understand the general conditions, mechanics, causes, and control techniques of wind erosion using wind tunnel simulation experiments and field observations in typical areas. Some encouraging accomplishments have been obtained.

In this paper, the authors present an outline of the main research results from China related to wind erosion, and put forward some prospects and suggestions intended to address some of the problems in both research and control practices of wind erosion in China.

2. Progress in research of wind erosion in China

Chinese scientists have paid a great amount of attention to wind erosion and regard it as a key aeolian process for understanding and managing land desertification and dust storm disasters in arid and semiarid regions. After a great deal of research using wind tunnel tests and field investigations in recent years, China has made great progress with aspects related to the dynamics, influencing factors,

measurement, estimation, and control techniques of wind erosion, as well as with other relevant aspects of aeolian geomorphology, land desertification, evolution of desert environments, and dust storms.

2.1 Aeolian geomorphology and land desertification In China, wind erosion study has been conducted within the context of desert research, and in

particular aeolian geomorphology and land desertification. In the 1950s and 1960s, the Desert Control Team of the Chinese Academy of Sciences (CAS) completed a large-scale and integrated survey of each desert and sandy region around the county, and established six desert experimental stations of in five northwestern provinces and Inner Mongolia. The natural conditions and main characteristics of ten main deserts and sandy lands in China were investigated thoroughly (Fig. 1). After systematical research on the formation of desert dunes, laws of blowing sand, desert land utilization, sandstorm disaster prevention and similar topics, Chinese scientists published a series of research papers and books. Among these, three representative works were Deserts in China by Zhu et al. (1978), Studies of the Geomorphology of Wind-drift Sand in the Taklimakan Desert by Zhu et al. (1981) and Aeolian Geomorphology by Wu (1987).

Figure. 1 A map of the distribution of deserts and desertification areas in Northern China ( from Zhu et al., 1989).

After the 1980s, the emphasis of desert science in China gradually transformed into desertification research, which also promoted the study of wind erosion. The main achievements of this desertification research may be listed as：(1) understanding the types and main characteristics of desertified land in arid, semiarid and sub-humid zones of northern China; (2) presenting the status, distribution and harmful impacts of desertification; (3) understanding desertification processes, and particularly the clarification of the relative contributions of natural vs. anthropogenic factors; (4) establishing an indicator system and desertification monitoring and mapping using large-scale remote sensing and field

investigations (Fig. 1); and (5) experimental research and demonstrations of land rehabilitation in different desertified areas under various natural conditions (Zhu and Liu, 1981; Zhu et al., 1989; Ding et al., 1998; Wang and Wu, 1999).

2.2 Wind erosion dynamics Aeolian processes involve the mobilization, transport, and deposition of surface material by the wind.

Among these processes, wind erosion includes interactions such as deflation and abrasion between airflow or two-phase, gas-solid flow and surface material. Hence, the physics and hydromechanics of blowing sand are the basic theoretical underpinnings used to explore the dynamics of wind erosion, which include force analyses of sand grains, modes of blowing sand movement, the flow structure of blowing sand, and sand transport models.

The forces that act on a soil particle include exterior and internal forces. External forces acting on the soil particle include the frontal drag and lifting forces caused by the wind and the impact forces caused by saltating particles when they fall back to the ground. Internal forces include gravity, attractive force (including electrostatic force between particles and universal gravitation), water-film adhesive forces, and biological adhesive forces (Fig. 2). By using both normal photography and a high-speed cinecamera in a wind tunnel experiment, Ling and Wu (1980) and He and Gao (1988) observed the dynamics of blowing sand movement, and using the photographic data they deduced the probabilities of some of the chief forces acting on single grains and discussed their effects on particle motion. From experiments with high-speed, multi-flash photography, a high-speed cinecamera, and an instrument that identifies motion parameters, Liu (1995) analyzed the forces on particles with different movement modes and developed hydrodynamic models of various stages of blown sand movement.

Figure. 2 Force analysis of soil surface particles undergoing wind erosion.

Particle movement by the wind is described by the three modes of suspension, saltation, and creep. Chinese researchers have focused many studies on particle saltation. Ling and Wu (1980) summarized the trajectory features of saltating grains, and analyzed the statistics of lift-off angle and impact angle. With the aid of high-speed, multi-flash photography, Zou et al. (1992) measured saltation trajectories, speed, lift-off angle and impact angle under various surface conditions, and demonstrated that there existed a linear relationship between lift-off angle and impact angle. Liu (1995) determined that in addition to these three essential modes of particle movement and prior to saltation, a particle experiences vibration, rolling and sliding.

B F

Wind direction

Impact direction

F

A

mg

C

The blowing sand flow structure, and in particular the distribution of particle concentrations with height above the surface, is an important indicator determining particle flux, and it also impacts the engineering design of wind erosion control. Liu (1960) is the first researcher in China who obtained a distribution equation describing suspended particle concentration with height under different airflow stabilizations and its relation to the average wind velocity. In accordance with the results obtained by field determination and wind tunnel tests, Wu (1989) concluded that: (1) the sand concentration decreases exponentially with height; (2) with wind velocity increasing, sand concentration in the higher

layer increases relative to that in the lower layer; (3) at the same velocity, with the total sand concentration increasing, the sand concentration in the lower layer (the first layer) increases relative to that in the high layer; (4) the critical value of blown sand flow structure (＝Q2－10/Q0－1) can be used to identify whether the surface experienced dominantly erosion, transportation, or deposition:＜1 means super-saturated flow susceptible to deposition on the surface, ＞1 means non-saturation flows and susceptibility to erosion, and ＝1 means saturated flows susceptible to transportation. Ma (1988) further analyzed three main factors influencing blown sand structure, i.e. wind velocity, total sand concentration, and surface type. Zou et al. (1992) pointed out by statistics and probability analysis that the mathematical function of blown sand distribution profiles follows logarithmic regularity. Furthermore, Chen et al. (1995) explored the distribution of grain size parameters in the blown sand flow.

Particle flux is a key to blown sand physics. For a long time, many researchers in China and other countries have brought forward a large number of theoretical models and empirical formulas of particle flux. In China, except for one general theoretical model of particle flux based on aerodynamic principles (Liu, 1960), most research has focused on empirical equations relating sand transportation rates and wind velocity using data from field bservations and wind tunnel experiments (Zou and Dong, 1993; Dong et al., 1995).

2.3 Factors influencing wind erosion Wind erosion occurs a result of wind momentum acting on the soil surface, often composed of sand,

under specific environmental conditions. Both natural and human factors, as well as their interactions, influence the process and rates of erosion. Natural factors include climate, soil, vegetation and landscape, and human factors include land reclamation, overgrazing, over-cutting for firewood and misuse of water resources (Table 1). In recent years there have been many experimental studies in China focusing on individual factors affecting wind erosion, and there is a common agreement that over-cultivation and trampling by livestock are main reasons for human-derived accelerated wind erosion.

Table 1. Some key factors influencing wind erosion.

Climate Soil Vegetation Landform Human activity

Type (±) Coverage

(＋)

Surface roughness (±)

Slope (±)

Over-cultivation (－)

Over-grazing

Wind speed (－) Wind direction

(±) Turbulence (－) Precipitation (＋) Evaporation (－) Air temperature

Soil type (±) Particle

composition (±) Soil structure (±)Organic matter

(±) Calcium

Note: (＋ ), (－ ) and (±) mean the wind erosion becomes weak, heavy and uncertain as the factor increasing.

2.3.1 Climate The influence of climate on wind erosion depends not only on wind velocity but also on precipitation

and temperature that, in turn, determine evaporation. Chepil et al. (1962) considered these climatic factors as the dominant ones relative to determining wind erosion rates, and introduced a combined wind erosion climatic factor C, which was later used for the Wind Erosion Equation (WEQ) (Woodruff and Siddoway, 1965). Later, to increase its accuracy of estimation, Food and Agriculture Organization

dETP

PETPuC

i

ii

i⎟⎟⎠

⎞⎜⎜⎝

⎛ −= ∑

=

12

1

3

1001

modified the Chepil’s formula into (FAO, 1979; Skidmore, 1986b):

(1)

Where C is the wind erosion climatic factor, u (m s-1) is mean monthly wind speed at 2 m height,

ETPi (mm) is potential monthly evaporation amount, Pi (mm) is monthly precipitation, and d is the number of days in the month concerned.

From the above formula (1), Dong and Kang (1994) selected meteorological data from 233 stations covering parts of 12 provinces of northern China to calculate the wind erosion climatic factor in arid and semiarid China. Figure 3 illustrates the iso-C value map. The annual C value in arid and semiarid areas in China ranges from 10 to 150. The C value map indicates there are six wind erosion areas with more than 100 C value, which are Junggar Basin, Qaidam Basin, Alxa Gobi, northern Inner Mongolia Plateau, Horgin Steppe, and Taklimakan Desert.

Figure. 3 The distribution of the wind erosion climatic factor (C Value) in the arid

and semiarid areas of China (modified after Dong and Kang 1994)

2.3.2 Soil Soil characteristics affecting its erodibility are texture, density, structural stability and water content.

Among these, the mechanical composition of soil particles is considered as the dominating factor, and it is commonly agreed that particles smaller than 0.25 mm and larger than 0.08 mm are the most easily eroded by wind. In relation to the mechanical composition of the soil as it influences wind erosion, Zhang et al. (2001) completed a distributional map of soil texture in China. Dong et al. (2000) classified the soil subject to wind erosion in the arid and semiarid areas of China into seven primary types according to material composition (Fig. 4), i.e., Gobi desert, sandy desert, loess deposits, residuum, flood deposits, salinized deposits, and irrigation deposits, and hypothesized that the sandy desert, desert/loess transitional zone, and flood deposits along abandoned river channels are the most

easily eroded by wind. Through wind tunnel tests, Liu (1999a) determined the natural erodibility of the major soil types in the agro-pasture zone of northern China, indicating that natural erodibility of the zonal soils has a tendency to decrease from the Calcic Ustic Isohumisols in dry steppe to Calcic Orthic Aridisols, Haplic Orthic Aridisols in desert steppe and Pachic Ustic Isohumisols in typical steppe. Chen (1991) hypothesized that soil structure, degree of compaction and vegetation coverage are the main determining factors of soil resistance to wind erosion, and found out that the threshold velocities of soil particles depend on the average diameter of soil aggregates instead of the grain size of single particles. After conducting experiments in a wind tunnel, Dong and Li (1998) discovered that the relationship for wind erodibility of aeolian sand as a function of its grain size follows a discontinuous function, with 0.09 mm sand being the most susceptible to wind erosion. The erodibility of aeolian sand can be divided into 3 categories: difficult to erode at >0.7 mm and <0.05 mm; moderately erodible at 0.7-0.4 mm and 0.075-0.05 mm; and most erodible at 0.4-0.075 mm. With similar grain size, a mixture of sizes is more susceptible to wind erosion than is a uniformly sized material.

2.3.3 Vegetation Vegetation reduces the wind velocity at the soil surface and also generally increases the soil

erodibility. Many research results confirm that the relationship between vegetation coverage and wind erosion rate is an exponential function, i.e., with the increase of vegetation coverage the wind erosion rate decreases exponentially. The measurements of threshold velocity and wind erosion in wind tunnel tests under various vegetation conditions showed that the threshold velocity increases with vegetation coverage, and that wind erosion rate decreases sharply as vegetation coverage increases (Fig. 5) (Dong et al., 1987; Hu et al., 1991; Liu et al., 1992). Recently, referencing the model developed by Wasson and Nanninga (1986), Huang et al. (2001) established a quantitative relationship between vegetation cover and sand transport flux in Mu Su Sandy Land. With this model and field observations, they derived the effective vegetation coverage under different wind conditions, and estimated that when vegetation coverage reaches 40-50% the wind erosion could be reduced effectively and controlled at a tolerable level.

2.3.4 Landform factors

It was shown by wind tunnel simulation (Dong 1994; Li 1999) that there are two effects of slope on wind erosion. Firstly, the wind velocity increases with slope along the up-slope direction of a landform and strengthens the wind erosion on the slope, and secondly, the threshold velocity of grain increases

with slope and weakens the wind erosion. However, in the usual circumstances with slope less than 25o, wind erosion increases generally with slope along the up-slope direction of a landform (Dong 1994).

0

20

40

0-2%

5-10% 5-10% 10-15% 10-15%15-25% 20-30%

Gobi

0

20

40

2-4%

45-55% 40-50%

0-2%

0-2% 1-2%

Desert

0

40

80

20-25%

70-80%

30-40%

15-20%

25-35%

5-15%1-5%

20-25%

Loess

Frac

tiona

l per

cent

age

(%)

0

20

40

2-4%1-2%

15-25%

0-1% 0-4%

40-50%

30-35%

Residuum

0

20

40

Flood deposits

Figure. 4 A diagram of grain size composition of some typical soil materials in the arid and semiarid

areas of China (data from Zhu et al., 1978, 1989).

Figure. 5 Relationship between vegetation coverage and wind erosion in wind tunnel experiments ( data from Dong et al., 1995).

2.3.5 Anthropogenic factors Northern China, especially the semiarid agro-pasture zone, has a fragile ecological system sensitive

to human disturbance. As population increases, excessive and destructive human activities such as cutivation, overgrazing, over-cutting for firewood, and misuse of water resources have destroyed the natural vegetative cover and hence accelerated wind erosion. Comparative wind tunnel experiments have shown that wind erosion may be accelerated more than a factor of 10 by cultivation, a factor of 1.14 by overgrazing, and a factor of 22.8 by over-cutting (Dong et al., 1987; Hu et al., 1991; Liu et al., 1992). Yan (2000) estimated using 137Cs measurements that in Gonghe Basin of Qinghai Province the annual wind erosion rate of dry farmland is three times greater than that of adjacent grassland, and that during cultivation wind erosion may increase to a factor of 5 to 8 over the adjacent grassland. As a whole, the increased wind erosion caused by human factors on average accounts for approximately 78% of the total wind erosion (Liu et al., 1992; Wang and Wu, 1999).

Table 2. Effects of human factors on wind erosion rates as determined using wind tunnel

experiments.

Human activity factors Experimental wind

speed (m/s)

Factor of increase

over adjacentSource

9.4-32.6 14.8Cultivation 16.43-23.03 11.3

Overgrazing 6.7-32.6 1.14

Cutting for firewood 26.5 22.8

Dong et al., 1987; Dong

et al., 1995; Liu et al., 1992

Table 3. Summary of wind erosion rates in some area in China derived by different methods.

Location Climatic type

Land type Wind erosion rate

Method Source

Hulun Beir, Inner

MogoliaSub-humid Sandy land 156.00 Erosion pins Ma, 1981

Naiman, Inner

M li

Dry

b h idSandy land 80.00 Erosion pins Xu et al.,

1993

0 10 20 30 40 50 60 70 80 90 1006

8

10

12

Threshold wind volecity

Vegetation coverage(%)

Win

d vo

leci

ty (m

s-1)

0

50

100

150

Wind erosion rate (15 m s-1 wind speed)

Win

d er

osio

n ra

te (k

g m

-2m

in-1)

Horqin, Inner

M li

Dry

b h idSandy land 174.00-349.5

0

Erosion pins

d i

Zhao et al.,

1988Siziwang, Inner

Mogolia

Dry

sub-humid Farmland 335.00

Erosion

profilometer

Zhu and Liu,

1981 Daxing, Beijing Dry Farmland 13.30 WEQ Yan, 1991

Xiajing, Shangdong Dry

b h idSandy land 21.00 Erosion pins Zhao, 1992

Youyu, Shanxi Dry Loess farmland 13.73 Erosion pins Kong et al.,

Shengmu, Shananxi Dry

sub-humid Loess farmland 18.87

Statistic

model of wind Dong, 1998

Shanxi-Shananxi-

Inner Mogolia Region

Dry

sub-humid

Sandy

land/Loess 15.90

Sand

samplers and Liu, 1999b

Houshan Region, Semiarid Farmland/Grass 14.40-41.10 Grain size Dong and

Gonghe Basin.

Qinghai Semiarid Farmland/Grass

land/Sand dune7.44-43.68 137Cs tracing Yan, 2000

Gonghe Basin.

Qinghai Semiarid

Four types of

desertified land

157.00-1510.

00Wind tunnel

Dong et al.,

1993 Mid-south Region

of Qinghai-Tibet Plateau Semiarid

Farmland/Grass

land/Sand dune 22.62-69.43 137Cs tracing

Golmud, Qinghai Arid Dune land 84.14 137Cs tracing

Yan et al,.

2001

Lop Nor, Xinjiang Arid Yadan 60.00 Erosion

id lHedin, 1905

Kuerle, Xinjiang Arid Farmland/Grass 31.71-59.85 137Cs tracing Pu et al.,

2.4 Wind erosion measurements Attempts have been made in recent years to measure wind erosion rates in the arid and semiarid

areas in China. Methods used include: a) investigation of some obvious wind erosion signs and residues; b) a wind erosion profilometer; c) field observation using erosion pins; d) simulation experiments in wind tunnels; e) comparison of grain size distributions of wind eroded soil with that of original soil; f) sand collection by sampling; g) mapping and remote sensing; h) 137Cs tracing; and i) wind erosion modeling (Table 3). Table 3 summarizes the estimated wind erosion rate in some typical sampling sites and areas. It can be concluded that effected by different scales of measuring period and site, the wind erosion rate by wind tunnel tests, pins and profilometer are clearly overestimated, while the others concentrate generally on a range from 10 t/ha a to 80 t/ha a. The most severe erosion takes place in sand dune land and the farmland also has higher wind erosion rate.

2.5 Wind erosion estimation models In order to obtain an accurate wind erosion rate, and to evaluate various control measures, scientists

of many countries have developed various kinds of wind erosion models. Principal models used today include the Wind Erosion Equation (WEQ) (Woodruff and Siddoway 1965), the Revised Wind Erosion Equation (RWEQ) (Fryrear et al., 1996), the Wind Erosion Prediction System (WEPS) (Hagen, 1991), and the Simulation Model of Daily Wind Erosion Loss (Cole et al., 1983). Comparatively, there has been relatively little research conducted on wind erosion models in China, and at present there exists no comprehensive model that can be used across China to estimate wind erosion. Dong (1998) published a statistical model of wind erosion derived from wind tunnel simulation as following:

( ){∫ ∫ ∫ −++=

t x y

Q 32 0001.00021.00441.00413.190.3 θθθ

( ) ( )[ ]} dtdydxtyxFdHSVV DRCR ⋅⋅×⋅ − ,,/102.8 28252

(2)

Where Q is wind erosion rate, V is wind velocity, H is relative air humidity, d is average diameter of soil particle, F is soil crushing strength, VCR is vegetation coverage, SDR is surface artificial destruction ratio, and θ is angle of surface slope. Through the model, Dong (1998) calculated the wind erosion rate in the Liudaogou watershed, Shenmu County, Shaanxi Province as 18.8 t ha-1 yr-1 (Table 3). A different type of wind erosion model was developed by Wang et al. (2001) based on stochastic theories of wind erosion. This model calculates stochastic probability distributions, expectations, and variations for certain types of erodible soil particles at a given point in time.

2.6 Classification of wind erosion magnitudes China contains a large amount of area with arid and semiarid climates in which the wind erosion

intensity depends on varied conditions of climate, geomorphology, soil, vegetation and land use. Various attempts have been made to classify wind erosion land into categories based on those criteria. Qi and Gan (1991) divided the wind erosion area of the Loess Plateau into: (1) a severe wind-eroded zone in the mid-temperate desert steppe and north part of warm-temperate steppe with wind erosion rates 50-100 t ha-1 yr-1; (2) a moderate wind-eroded zone in the middle part of the warm-temperate steppe with wind erosion rates of 20-50 t ha-1 yr-1；(3) a slightly wind-eroded zone in the southern part of the warm-temperate steppe with wind erosion rates less than 20 t ha-1 yr-1.

Table 4. Classification of wind erosion magnitude.

Zhao et al. (1989) and Ministry of Water Resources Grade

Zachar (1982)

Erosion rate (m3 ha-1 a-1) Erosion rate (t km-2 a-1) Vegetation coverage

Weak <0.5 <200 > 70% Slight 0.5-5 200-2500 70-50%

Moderate 5-15 2500-5000 50-30% Severe 15-50 5000-8000 30-10%

50-200 8000-15000 10-1% Catastrophic >200 >15000 < 1%

Using relative eroded depth, vegetation coverage, and eroded area percentage to define an index,

Zhao et al. (1989) classified the wind erosion magnitude in Inner Mongolia (Table 4). Using remote sensing data (MSS/TM) combined with field survey, Zhao et al. (1989) compiled a “Map of Wind Erosion in Inner Mongolia” (1:1,000,000 scale). Also，Ministry of Water Resources (MWR) of China (1997) established a standard for classification and gradation of soil erosion, in which the magnitude of wind erosion was divided into six grades: weak, slight, moderate, severe, very severe and catastrophic. Under this standard, the MWR compiled a “Map of Wind Erosion in China” (1:4,000,000 scale). In 2000, the Monitoring Center of Soil and Water Conservation of the MWR and Institute of Remote Sensing Application of the Chinese Academy of Sciences completed a new edition of the “Map of Wind Erosion in China” (1:4,000,000 scale). According to those two maps, the area of wind erosion in China increased from 1,880,000 km2 at the end of 1980s to 1,910,000 km2 at the end of 1990s, which represents a net increase of 1.6 %.

2.7 Dust storms Northern China belongs to a part of the larger dust storm region of Central Asia, which has one of

the most frequent occurrences of dust storm of anywhere in the world (Pye, 1987; Xia and Yang, 1996). Both in a geologic timeframe and in the period of recent human activity, Northern China is one of main regions of the world where severe dust storms occur and which acts as the source of “dust-rain” (Zhang, 1984). In recent years, following the warming and drying of the climate and the intensity of human activity, the area of land desertification has increased and the eco-environment has deteriorated sharply in Northern China. This has resulted in the frequent occurrence of dust storm disasters and an accompanying enormous loss of life and property of the people in the local and surrounding regions. In the spring of 2000, particularly, there were more than 10 days of blowing sand consisting of heavy airborne dust and drifting sand. Their occurrence was early in the year, the area influenced and intensity were exceptionally large (Ye et al., 2001), and they severely endangered communications, transportation, the atmospheric environment, life, and living conditions. These dust storms have garnered significant attention and a call for action across China.

Available research shows that in China dust storms are distributed in the northwest, north-central, and northeast regions, but especially in northwest region. There are two types of classifications of dust storms in China (Fang et al., 1997). The first is based on the number of dust storm days as measured through ground meteorological monitoring, and classifies regions into four categories: influenced, slight, frequent, and highly frequent areas. According to this classification, the Tarim Basin, the north part of the Tibetan Plateau, the Alxa Plateau, and the Ordos Plateau are classified into the highly frequent category. The greater part of China falls into one of the four categories for dust storm impact, including parts of 18 provinces, municipalities and autonomous regions. The second classification, using the frequency of strong and especially strong dust storms over the last 40 years, identifies three frequently occurring regions, incoluding the Hexi Corridor and Hetao Region (with its center in the town of Minqin), the Hetian region in the Xinjiang Uigur Autonomous Region, and the Turpan region of the Xinjiang Uigur Autonomous Region (Fig. 6).

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:

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BeijingBeijingBeijingBeijingBeijingBeijingBeijingBeijingBeijing

ShijiazhuangShijiazhuangShijiazhuangShijiazhuangShijiazhuangShijiazhuangShijiazhuangShijiazhuangShijiazhuang

HuhhotHuhhotHuhhotHuhhotHuhhotHuhhotHuhhotHuhhotHuhhot

TaiyuanTaiyuanTaiyuanTaiyuanTaiyuanTaiyuanTaiyuanTaiyuanTaiyuan

S

Fr

evere duststorm path

equent area of duststorm

Susceptible area of duststorm

XiningXiningXiningXiningXiningXiningXiningXiningXining

YinchuanYinchuanYinchuanYinchuanYinchuanYinchuanYinchuanYinchuanYinchuan

North PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathNorth PathUrumqiUrumqiUrumqiUrumqiUrumqiUrumqiUrumqiUrumqiUrumqi

West pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest pathWest path

orthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Pathorthwest Path

LanzhouLanzhouLanzhouLanzhouLanzhouLanzhouLanzhouLanzhouLanzhou

Xi'anXi'anXi'anXi'anXi'anXi'anXi'anXi'anXi'an

NNNNNNNNNNNNNNNNNNNNNNNNN

0 300 600

Figure. 6 The distribution of severe dust storms in Northern China (modified from Fang et al.,

1997).

400 600 800 1000 1200 1400 1600 18000

2

4

6

8

10

Duststorm frequence of 10 years Average value of 50 years

Freq

uenc

e

A.D.

Figure 7 indicates the occurrence frequency of dust storms in North China since 300 A.D. (Shi et

al., 2000). According to this figure, the frequency began to increase rapidly in about 1100 A.D. In the most recent 1000 years, there have been five periods of high occurrence frequency, including 1060-1090 A.D.,1160-1270 A.D., 1470-1500 A.D., 1610-1700 A.D., and 1820-1890 A.D. During the most recent 50 years the occurrence frequency of dust storms has been related to regional climate change. The data derived from 100 meteorological stations in North China from 1951 to 1997 shows that dust storms increased in such regions as the northeast part of the Tibetan Plateau, the Gonghe Basin in the west part of the Qaidam Basin, the Qilian Mountains region, and the territory between China and Mongolia. In the other regions the frequency of dust storms decreased (Zhang and Lu, 1999; Yang et al., 1998). The occurrence frequency of strong and extreme dust storms has been generally increasing since the 1950s. The mean number of occurrences per 10 years was 0.5 in the 1950s, 0.8 in the 1960s, 1.3 in the 1970s, 2.3 in the 1980s, and 1.4 in the 1990s (Xia and Yang 1996). In the year 2000, the number of occurrences was 14.

Figure. 7 Frequence of dust storms since 300 A.D.in northern China (data from National Meteorologic Bereau of China).

2.8 Wind erosion control The people in the arid and semiarid areas in China have recognized the importance of controlling

wind erosion for a long time. Since the 1950s, the Chinese Government has been conscious of the necessity to control wind erosion to ensure economic development in the whole nation. Measures adopted in wind erosion control involve: vegetation, engineering, chemical methods, and land management practices.

Vegetation steps include forestation and protecting and recovering natural vegetation. China has conducted shelter forest construction in north China over the last two decades and continues today. Air seeding was successfully used to increase grass growth and stabilize shifting sand dunes in semiarid areas. This project is reported to have increased the plant coverage from 5.1% to 9.0% in the Northern

China (Liu, 2000). In desert areas where the natural conditions are not suitable for vegetative control measures, it is

necessary to introduce the engineering methods. These methods are mainly used to protect transportation lines crossing the desert and have been practiced on the Shapotou Section of Baotou-Lanzhou Railway crossing the Tengger Desert, the Naiman Section of Jining-Tongliao Railway crossing the Otindaq and Korqin Deserts, the Yunmen Section of Lanzhou-Xinjiang Railway, and the Desert Highway in the Taklimakan Desert. The engineering forms used in China include fences, straw check boards, sand transport boards, feather-like sand conducting fence arrays, and sand separating ditches (Zhu et al., 1978). Some chemical materials is also used in loose shifting sand to add bonding agents and form a non-erodible crust on the soil. Because of their high cost and detrimental effect on the environment, sand fixing chemicals have been limited to only small areas (Hu, 1997).

China recognizes that changes in land management practices are necessary to control wind erosion. There are two aspects concerning land management practices that China is following, or needs to follow, to control wind erosion. First, the old style of land-use are being adjusted and optimized. China has paid a great deal of attention to the poor use of land that has constituted the main driving force that has accelerated wind erosion and land desertification in recent decades. After launching a policy of strictly prohibiting the cutting of natural forest in the upper regions of Yangtze River and Yellow River in 1998, the Chinese central government made the decision to return farmland to forest and grassland in critical parts of these western regions. By the year 2000, 380,000 hectares of farmland and 470,000 hectares of wasteland have been converted to forest and grassland (Wu, 2001). Secondly, with regard to the use of farmland, land management practices to reduce wind erosion include strip cropping, rotations, crop residue management, deep tillage, ridge tillage and zero tillage. However, changes have been slow to come. The implementation of those practices has been largely ignored except for areas in the Gonghe Basin of Qinghai Province (Jin et al., 1989). Recently, conservative tillage has been introduced for wind erosion control in the Shanxi and Hebei provinces (Shangguan and Chen, 1995; Xu et al., 2001).

3. Future needs in China for Wind Erosion Research and Control 3.1 Strategies for wind erosion control In recent decades, wind erosion control in China has progressed, and wind erosion rates have

been kept within limits in some regions. Because of global climate change, the climate is moving toward further warming and drying in North China (Wang et al., 2001; Ye et al., 2001). In the last 50 years, the average annual temperature increased by 0.5-2.0 , and the precipitation has become less ℃

(Shi et al., 2001), resulting in the reduction of soil moisture during the winter and spring. Together with the weather change, the increased intensity of human activities has caused the continual expansion of desertified land (Wang et al., 2001). For these reasons wind erosion control will continue to be a large problem for China into the next century.

In the past, wind erosion has been addressed in local regions. In the future, a national strategy of wind erosion control must be taken using prevention as the dominant measure. In the grassland and

desert zone of North China, the grassland and farming-pastoral zone and Tibet Plateau, the burden of pasture must be lightened, and grazing must be forbidden in order to protect the grassland. Areas of good water and soil conditions, where sustainable production can be maintained, must be identified and managed to maintain production with sufficient protection against wind erosion.

In order for wind erosion to be controlled in the long term, China must recognize the economic value of intact ecological systems. Recovery of vegetation for erosion control should be regarded as a long-term, social, public benefit, and a new compensation system must be established which takes into account the ecological benefit of wind erosion prevention and control to offset costs to the local land users.

3.2 Scientific problems for wind erosion research Only if the dynamic theory of two-phase boundary layers is developed can the interaction of the

wind with the earth’s surface, design principles of wind engineering, and transportation and sedimentation process of sand be related. Some important points that need to be addressed include: (1) an understanding of the boundary conditions of the bottom layer in high altitude aerosols; (2) a theoretical model of the dynamics for two-phase boundary layers; (3) an understanding of the mechanisms of dune development; (4) development of common principles for the design of wind engineering structures; (5) a model that links wind engineering and aeolian geomorphology.

A key problem of the study of wind erosion is quantitative assessment. Accurate wind erosion estimates and their regional distributions constitute critical information necessary for prevention and control, and are critically important for related study fields such as aeolian geomorphology, dust storms, loess deposition, and land desertification. The main scientific problems include: (1) a need for accurate and standard methods for measuring wind erosion (see Table 4); (2) comprehensive, reliable data on wind erosion for the whole country; and (3) a wind erosion prediction model or models applicable to the whole of China.

Soil loss tolerance (T-value) is defined as “the maximal level of soil erosion that will permit a high level of crop productivity to be sustained economically and indefinitely” (Wishmeier and Smith 1978; Schmidt et al., 1982). Such a definition may not be satisfactory for the problems of wind erosion in China, including prevention of desertification and devastating dust storms. A new look must be taken for setting soil loss tolerance levels for wind erosion in China that incorporates off-site problems that affect society as a whole, land use sustainability, and both local and national economics.

4. Conclusions

China has accomplished a great deal relative to wind erosion problems including completion of an aeolian geomorphology map and wind erosion map of China; establishment of a preliminary index system to evaluate wind erosion; improvements in techniques of wind erosion measurement; and development of some regional methods of wind erosion control. Nonetheless, the potential problems of wind erosion for China in the future remain enormous. The massive dust storms which blanketed China in 2000 are testament to the huge impact that wind erosion has in store for the future and what remains to be done. A few of the monumental tasks that remain to be completed range from basic science to major policy matters for the Chinese people. They include the development of a comprehensive understanding of the dynamics of blown sand in boundary layer, a wind erosion prediction model for

China, a nationwide assessment of wind erosion rates using consistent techniques, definition and determination of wind erosion tolerance, and national policy decisions that place wind erosion control as a national priority. The future of wind erosion control in China must focus on the strategy of using prevention as the dominant measure combined with land management and engineering control practices; wholesale adjustment of land use patterns; and establishing a national compensation system to develop the ecological capital associated with wind erosion control practices.

China must also increase its international cooperation on wind erosion research and control, understanding and introducing the achievements and successful experiences from other countries with wind erosion models and advanced erosion control techniques. China needs also to strengthen its human resource pool with training necessary to meet the national requirements for wind erosion control.

Furthermore, China has a vast semiarid area where wind commonly interacts with water, displaying a complexity of fluvio-aeolian processes. In particular, the agro-pasture zone of north China exhibits this wind-water interaction. It is important that both wind and water erosion research should be enhanced, not only for purposes of solving erosion problems in China, but also for the advancement of erosion research around world (Tang, 2000; El-Baz et al., 2000).

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 40001001), the National Basic Research Priorities Programme of China (G2000018604) and the Key Technologies Research and Development Programme of the Tenth Five-year Plan of China (2002BA517A10).

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FROM: Keynote’s Paper of 12th International Soil Conservation, Beijing, China, May 26-31, 2002