Zobeck, Baddock, Van Pelt 2013 Geomorph chapter

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11.20 Anthropogenic EnvironmentsTM Zobeck and MC Baddock, USDA-ARS, Wind Erosion and Water Conservation Research Unit, Lubbock, TX, USARS Van Pelt, USDA-ARS, Wind Erosion and Water Conservation Research Unit, Big Spring, TX, USA

Published by Elsevier Inc.

11.20.1 Introduction 395

11.20.2 Human-Induced Wind Erosion – A Global Perspective 396 11.20.3 Anthropogenic Factors that Influence Wind Erosion 399 11.20.3.1 Tillage Effects 400 11.20.3.2 Effects of Vegetation 401 11.20.4 Environmental Effects of Wind Erosion 402 11.20.5 Techniques for Studying Wind Erosion 404 11.20.5.1 Field Studies 404 11.20.5.2 Field Equipment to Estimate Wind Erosion 405 11.20.5.3 Modeling Wind Erosion 406 11.20.5.4 An Indirect Method to Estimate Wind Erosion 407 11.20.6 Control of Anthropogenic Wind Erosion 407 11.20.7 Future Outlook and Perspectives 408 References 408

Zo

en

Ba

Di

Tre

GlossaryAUSLEM Australian landscape-scale wind erosion model

developed for rangeland conditions.

Microrelief/microtopography Small-scale differences in

the elevations of the soil surface on the order of

millimeters to centimeters.

PM10 Particulate matter with an aerodynamic diameter of

less than 10 mm.

beck, T.M., Baddock, M.C., Van Pelt, R.S., 2013. Anthropogenic

vironments. In: Shroder, J. (Editor in Chief), Lancaster, N., Sherman, D.J.,

as, A.C.W. (Eds.), Treatise on Geomorphology. Academic Press, San

ego, CA, vol. 11, Aeolian Geomorphology, pp. 395–413.

atise on Geomorphology, Volume 11 http://dx.doi.org/10.1016/B978-0-12-374

Safire An automated, high frequency, electronic saltation

impact sensor to detect particles using piezoelectric sensing

elements.

SENSIT An automated, high frequency, electronic

saltation impact sensor to detect particles using

piezoelectric sensing elements.

Wenglor Relatively inexpensive, commercially available

photoelectric laser-based electronic saltation sensor.

Abstract

Anthropogenic aeolian environments are those where wind erosion – principally mineral dust production – is related to

human influences. This chapter begins by citing regions where anthropogenic factors have resulted in significant winderosion. It details the major human-related controls on aeolian activity, such as changes to soil surface erodibility, especially

by tillage, and alteration of vegetation characteristics. The environmental consequences of wind erosion are classifiable as

on-site and off-site effects. Field techniques used for studying anthropogenic wind erosion are also discussed, as well as the

models used to estimate soil loss and methods for the control of erosion on agricultural land.

The US Department of Agriculture offers its programs to all

eligible persons regardless of race, color, age, sex, or national

origin, and is an equal opportunity employer. Mention of

trade names or commercial products is solely for the purpose

of providing specific information and does not imply recom-

mendation or endorsement by the USDA-ARS.

11.20.1 Introduction

Aerosols or airborne dust can originate from sources un-

related to anthropogenic activity but may also be initiated or

exacerbated by anthropogenic actions. Anthropogenic dust

refers to dust activity (emission and suppression) that is pre-

sent due to human-related activity (Zender et al., 2004).

Zender et al. (2004) proposed two categories of dust activity.

Anthropogenic dust of the ‘First Kind’ consists of dust pro-

duction and emissions due to human activities that directly

modify or disturb the land surface and thereby alter soil

erodibility. This disturbance includes activities that mechan-

ically inject dust into the atmosphere, independent of wind

speed, as well as disturbance that changes the soil surface to

make it more susceptible to wind erosion. Anthropogenic dust

of the ‘Second Kind’ represents changes in dust emissions and

deposition due to effects of climate change. The work de-

scribed in this chapter is restricted to anthropogenic dust of

the ‘First Kind’ with a primary focus on wind erosion caused

by agricultural activities. A survey of 16 present-climate global

dust budget estimates revealed annual dust emissions ranging

739-6.00313-4 395

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396 Anthropogenic Environments

from 358 to approximately 3000 Tg yr�1 (Zender et al., 2004).

A more recent survey of 13 estimates of present-climate global

dust emissions provides a range from 500 to approximately

3320 Tg yr�1 (Shao et al., 2011). Of this, approximately 11%

has been attributed to an anthropogenic origin, mainly in-

dustrial processes (Satheesh and Moorthy, 2005). Tegen et al.

(2004) estimated the contribution of agricultural areas to the

atmospheric dust load by calibrating a dust-source model with

emission indices derived from dust-storm observations. Their

results indicated that agricultural areas contribute o10% of

the global dust load. The uncertainties in these scale models,

however, is demonstrated by the fact that Mahowald et al.

(2004) found that dust storm frequencies could be explained

by an anthropogenic contribution of anywhere between 0 and

50% of total dust in the atmosphere.

Even though the amount of dust emissions attributed to

anthropogenic factors is a relatively small portion of the total

global dust load, major environmental catastrophes have

been attributed to aeolian activity enhanced by human activ-

ity. Perhaps the most memorable example of anthropogenic-

induced wind erosion is the occurrence of the Dust Bowl era

in USA from 1931–39 (Baumhardt, 2003). During this period,

the southern Great Plains of USA suffered from drought and a

severe economic depression. This decade was preceded by

favorable annual precipitation more than 100 mm above the

average, farm mechanization with intensive tillage, and stable

European demand for wheat (Triticum aestivium L.) (Baum-

hardt, 2003; Egan, 2006). Extensive areas that had been suc-

cessfully farmed for three decades became ravished by wind

and drought (Figure 1) creating what became known as the

American Dust Bowl (Stewart et al., 2010). The entire area

affected was approximately 40 million hectares (Figure 2).

Wind erosion on rangeland and cropland reached an annual

peak of approximately 20 million hectares (Hurt, 1981).

Concern over the severe wind erosion experienced during

the American Dust Bowl and other forms of erosion led to the

development of the Soil Conservation Service (now called the

Natural Resources Conservation Service) in the US Depart-

ment of Agriculture in 1935 (Helms, 2010). Although

great progress at controlling wind erosion has been made

Figure 1 Dust storm at Goodwell, Oklahoma, USA on June 4, 1937. Photohttp://www.weru.ksu.edu/new_weru/multimedia/dustbowl/dustbowlpics.html

(USDA-NRCS, 2007), wind erosion is still a problem in some

areas (Figure 3).

Another case of human-induced wind erosion occurs in the

central Asian Aral Sea basin where water began to be diverted

from the sea’s feeder rivers, the Amu Darya and Syr Darya, for

irrigation purposes in the early 1960s. The average total dis-

charge into the Aral Sea between 1911 and 1960 was around

55 km3, but by the period 2001 to 2005, it had dropped to

9 km3 with the diverted water supporting 8 million hectares

under irrigation. The consequence has been a reduction in

surface water area by 74% (some 50 000 km2) (Micklin, 2006)

and exposure of the highly erodible lake and delta sediments,

plus increased agricultural land cover, which has significantly

increased dust emission potential (Argaman et al., 2006). The

dustiness of the Aral Sea made it the most apparent human

impact in a study of global aerosol ‘hot spots’ (Prospero et al.,

2002), and severe health implications have been linked to

toxins (including pesticide loadings) in the blown dust (Wiggs

et al., 2003)

Another famous instance of lake dessication by human

activity, which has significantly promoted dust activity,

was seen at Mono Lake and Owens Lake in California.

Here, interbasin water transfer for the development of

Los Angeles revealed the readily wind erodible dry lake sur-

faces (as much as 65 km2 at Mono Lake since the late 1940s,

and 280 km2 at Owens Lake since 1926). The effect of the

hydrological changes on groundwater-dependent vegetation

also has been pronounced (Elmore et al., 2008). The con-

sequence of water extraction from these lakes has created two

of the largest single sources of fugitive dust in North America

(Gill, 1996).

11.20.2 Human-Induced Wind Erosion – A GlobalPerspective

The principal regions with human-induced wind erosion in

North America occur in USA, Canada, and Mexico. Wind

erosion in USA is a significant problem in agricultural mineral

soils, in irrigated arid and semiarid dryland farming regions,

taken by Mrs. Emma Love of Goodwell, Oklahoma. Available at

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Denver

The Dust Bowel

TopekaKansas

Oklahoma

AustinTexasNew Mexico

Santa Fe Oklahoma city

Colorado

Figure 2 Area most affected by the American Dust Bowl. Reproduced from Hines, R.R., 2010. Theme Eight: Rural America and the GreatDepression. Available at http://www.alamo.edu/pac/faculty/rhines/1302Theme8.htm. Accessed May 2011.

104° W 102° W 100° W

Oklahoma 36° N

34° N

32° N

100° W102° W

200 km0

Midland

Lubbock

New Mexcio

Amarillo

Texas

104° W

32° N

34° N

36° N

Figure 3 Image from NASA’s Moderate Resolution Imaging Spectroradiometer of a large blowing dust event associated with bare fields innorthwest Texas on the afternoon of 1 January 2006. The plume was approximately 1.0� 105 km2, and visibility at 1 p.m. at LubbockInternational Airport was 1.2 km. Red dots indicate active fires. Image from NASA MODIS Rapidfire system.

Anthropogenic Environments 397

and in some humid areas with very sandy or organic soils.

Approximately 700 million metric tons of soils are eroded by

wind in USA annually (Figure 4; USDA-NRCS, 2007). In

Canada, wind erosion most frequently occurs on sandy soils

of the Canadian Prairies, on Prince Edward Island, in parts of

southern Ontario, and in some areas of peat soils (Wolfe,

2001; Agric. and Agri-Food Canada, 2007). A study of reports

from 31 weather stations from 1977 to 1987 found that the

central part of farm land in Saskatchewan was an area of high

dust storm frequency (Wheaton and Chakravarti, 1990).

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Wind and water erosion on cropland, 2003

Legend

Major rivers

Federal land

Map ID: m10281Data source:2003 National Resources InventoryU.S. Department of Agriculture, Natural Resources Conservation ServiceResource Inventory and Assessment Division, Wash., DC July 2008

Scale0 100 200 400

Kilometers

Albers equal area projection

Each blue dot represents 10000 tons per yearof water (sheet and rill) erosion.Total of 970.5 million tons per year.

Each red dot represents 100000 tons per yearof wind erosion.Total of 776.4 million tons per year.

Note: The 2003 National Resources Inventory does not include data forAlaska, Hawaii, and the U.S. territories.

Figure 4 Estimated average annual wind and water erosion on cropland in USA estimated in the 2003 National Resources Inventory(reproduced from USDA-NRCS, 2007. National Resources Inventory 2003. USDA, Natural Resources Conservation Service. Available at http://www.nrcs.usda.gov/technical/NRI/2007/nri07erosion.html (accessed May 2011)).

398 Anthropogenic Environments

Although it has not been possible to accurately measure the

extent or severity of wind erosion in Canada (Sparrow, 1984),

the annual soil loss due to wind erosion in the southern

Prairie Provinces of Canada has been estimated to be one third

greater than that due to water erosion (Wolfe, 2001). Semiarid

dryland farming regions often produce limited crop residues,

have poorly structured soils, and are prone to strong winds. In

addition, they may have little surface roughness (microrelief)

due to excessive tillage. In USA, these areas include states lo-

cated in the Great Plains, the Columbia Plateau of west central

Washington, southeastern Idaho, and northern Montana.

Michigan, Florida, New York, Wisconsin, the boot heel region

of Missouri, and southern Texas are more humid regions with

significant wind erosion-prone soils. In Mexico, cultivated

lands in the northern Chihuahuan Desert (e.g., Rio Casas

Grandes valley) have been confirmed as contributing sources

for the major dust storms originating in that region. For five

significant dust events occurring across 2002 and 2003, 23%

of the sources identified by remote sensing analysis were

characterized as agricultural land (Rivera Rivera et al., 2010).

As a specific example of human influence on dust storm fre-

quency, water demands on Lake Texcoco near Mexico City

meant that the lake was dry by the late 1950s and accounted

for an estimated 40% of the metropolitan area’s dust storms in

1971. The dust was reduced to zero by 1984 due to concerted

irrigating and revegetation efforts (Goudie and Middleton,

1992), although the lake area and nearby agricultural land are

again now recognized as dust sources affecting Mexico City

(Dıaz-Nigenda et al., 2010).

The primary regions of human-induced wind erosion in

South America occur in the drier areas of the Del Plata Basin,

including the semiarid zones of the Gran Chaco and Pampas

biomes (Viglizzo and Frank, 2006), other dry-farmed crop-

land in Argentina, the Bolivian High Plains, the Paraguayan

Chaco (Buschiazzo, 2006), and in Patagonia (Del Valle, et al.,

1998). Argentina experiences the largest regions disturbed by

wind erosion in South America, where it has affected 9% of

the country’s area (Chisari et al., 1996). Wind erosion is an

especially important cause of soil degradation in the semiarid

Argentinean Pampas (SAP) (Buschiazzo et al., 1999;

Buschiazzo, 2006), where the potential wind erosion rate was

estimated to vary from 24 Mg ha�1 yr�1 in the northeastern

part to 179 Mg ha�1 yr�1 in the southwestern part of the La

Pampa province (Michelena and Irutia, 1995). Of an

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Anthropogenic Environments 399

estimated 565 000 ha affected by wind erosion in the SAP,

more than 60% had moderate to heavy wind erosion risks

(Covas and Glave, 1988 from Buschiazzo, 2006). Buschiazzo

and Taylor (1993) have shown that in some areas wind ero-

sion has transformed the soils from Mollisols into Inceptisols

due to the loss of organic matter and decreased thickness of

the surface horizon.

With the large extent of drylands in northern China and

the considerable population interacting with this environ-

ment, there has been substantial research applied to under-

standing the human influence on wind erosion in this region.

An estimated 91 million hectares have been adversely affected

by wind erosion over the past 50 years throughout China, with

nearly 7 million hectares undergoing encroachment by mobile

dunes, whose mobilization is commonly caused by over-

grazing of vegetation cover (Zhao et al., 2006).

The number of dust storms observed in northern China has

shown an overall decline recently, especially since the mid-

1980s, despite an increase in desertification attributed to an-

thropogenic activity (Zhang et al., 2003). Zhang et al. (2003)

reported estimates that the desert area of China has increased

by 2–7% over the past 43 years. Their study shows that the

influence of humans can be particularly notable since the most

heavily human-affected areas, including the Horqin sandy land

and Mu Us desert, have shown overall dust increases in the past

20 years. Mapping of desertified land in the Chaidm Basin re-

vealed a near doubling in the degraded area from 1959 to 1994,

and this region has clearly shown an increasing dustiness trend

contrary to the northern Chinese trend (Wang et al., 2004).

There does, however, remain some debate on the significance of

human sources in degraded grasslands or cultivated lands of

northern China compared to the contribution of natural dust-

bearing areas (Wang et al., 2004).

To counter the unintended consequences of enhancing

wind erosion, considerable effort has been made in China to

reduce aeolian activity: for example, The Great Green Wall

(Parungo et al., 1994). There have also been many examples of

attempts to stabilize sand dune movement as well, for instance,

to protect transport routes using checkerboards of vegetation

(Qiu et al., 2004). Elsewhere in central Asia, the last 25 years

has seen a drying of the Hamoun wetlands in the Sistan basin

of southeast Iran and southwest Afghanistan. Water extraction

due to expanded irrigation and upstream damming of in-

flowing rivers (e.g., the Helmand) has been exacerbated by a

severe recent drought and these dried wetlands are now a sig-

nificant regional dust source (Walker et al., 2009).

In Africa, natural dust emissions from the Sahara have the

greatest global magnitude, and understanding the role of

humans in aerosol emission, especially in the Sahel region,

has been a major aim. One recent study on sediment cores

from the ocean under the North African dust corridor has

linked a rise in dust deposition dated at the start of the

nineteenth century to the beginning of commercial agriculture

in the Sahel region (Mulitza et al., 2010). Moulin and

Chiapello (2006) found that background dust levels over the

Atlantic had increased by a factor of two since the mid-1960s,

based on independent dust concentration and dust optical

thickness measurements. They suggested that after accounting

for the effects of climate, anthropogenic soil degradation as-

sociated with Sahelian population growth was the cause of

increased dust activity. From a ground study on the develop-

ment of nabkha dunes in the Inland Delta region of Mali, the

emergence of these forms on abandoned fields was attributed

directly to enhanced wind erosion on local cultivated lands,

especially during droughts (Nickling and Wolfe, 1994).

Anthropogenic pressure can also affect sand dune dy-

namics. In the dominantly inactive Kalahari dunefield of

southern Africa, adequate vegetation cover is the principal

cause of dune stability for periods when the modern wind

strengths are sufficient for sand transport. Reduction of vege-

tation as a result of grazing pressure has led to several local-

ized increases in surface activity of linear dunes in the

southwest Kalahari (Thomas and Twyman, 2004) (Figure 5).

Of significance in the shifts in dune activity attributable to

humans is the short time scale that the anthropogenic effect

can operate, in contrast to the longer term changes associated

with climate change (Thomas and Leason, 2005).

Much of the semiarid lands in Australia used for cropping

and grazing have a high wind erosion risk, which is strongly

increased by drought (McTainsh and Leys, 1993). Agriculture

has been a significant factor in making the Mallee region of

northwest Victoria and southwest New South Wales a persistent

dust source area where there are on average at least 5 dust days a

year (McTainsh and Pitblado, 1987) and an aerosol signature

just recognizable at the global level (Prospero et al., 2002). The

original clearing of vegetation and cropping on the fine soils of

the Mallee region encouraged wind erosion, where inappropri-

ate land management under subsequent cultivation has further

increased wind-borne soil loss (Leys, 1990). Leys (1999) high-

lights that much of arid Australia produces considerably less

dust than certain higher rainfall regions where agriculture is

prevalent and the erosion there is ‘management-induced’. Since

the 1960s, a significant change has occurred in the vegetation

type in semiarid regions of Australia, with the increased emer-

gence of woody weeds not edible by livestock. A consequence of

this has been more persistent ground cover in many grazing

lands susceptible to wind erosion, a change that has been

used to partly explain a reduction in broad-scale wind erosion

frequency (McTainsh and Leys, 1993).

Wind-borne soil loss also has been accelerated by human

land use in semiarid regions of Europe (Lopez et al., 1998;

Gomes et al., 2003), whereas the occurrence of localized wind

erosion in the more humid areas of the continent is almost

entirely human-induced through cultivation of light soils

(Barring et al., 2003; Funk et al., 2008). Dust emission in

Lower Saxony, Germany was observed to be 6.6 times greater

due to tillage compared to erosion of untilled soils (Goossens

et al., 2001). In another study, Riksen and de Graaff (2001)

reported that approximately 3.5 million hectares of sandy,

peaty, or loess soil types are vulnerable to wind erosion as

arable land in northern Europe. In these areas, they estimated

that the highest on-site costs caused by wind erosion on crop

loss and necessary resowing was as much as 500 Euros per

hectare once every 5 years.

11.20.3 Anthropogenic Factors that Influence WindErosion

The principal anthropogenic factors affecting wind erosion

relate to the soil surface erodibility and vegetation

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Figure 5 Heavy grazing on linear dunes in the southwest Kalahari Desert, Botswana. Photo by Paolo D’Odorico.

400 Anthropogenic Environments

characteristics. A recent review of soil erodibility dynamics and

its representation for wind erosion and dust emission models

on agricultural and rangeland surfaces has been provided by

Webb and Strong (2011). As mentioned earlier, there are

stunning examples of how human activity has exposed for-

merly vegetated (Dust Bowl region and the USSR Virgin Lands

Scheme; Goudie and Middleton, 1992, 2006) or wet areas

(Aral Sea, Owens Lake; Gill, 1996) to the ravages of the wind

and led to accelerated wind erosion. Besides agricultural

production and land drainage, wind erosion can also be

accelerated due to fire and overgrazing, both potentially human-

induced, and other anthropogenic activity. Wind erosion is

accelerated after fire on erodible soils due to the great re-

duction in protective vegetation on the soil surface (Zobeck

et al., 1989; Stout and Zobeck, 1998; Whicker et al., 2002;

Vermeire et al., 2005; Sankey et al., 2010). Overgrazing reduces

vegetation on the soil surface as well and also disrupts pro-

tective biotic soil crusts by trampling (Karnieli and Tsoar,

1995). Although overgrazing can produce accelerated wind

erosion, traditional grazing may produce less erosion than

abandoned tilled land (McTainsh and Leys, 1993; Qi et al.,

2008). Biotic crusts, produced by algae, lichens, and other

microorganisms, are a dominant feature on arid and semiarid

landscapes and produce very stable surfaces unless disturbed

(Belnap and Gillette, 1997; Belnap et al., 2009). In a com-

parison of crusted and foot or vehicular disturbed surfaces,

Belnap and Gillette (1997) found friction threshold velocity

much higher, and thus susceptibility to wind erosion was

much lower, for undisturbed crusts than disturbed crusts.

Wheeled vehicles also increase the erodibility of natural

surfaces (Goossens and Buck, 2009) and vehicular traffic on

unpaved roads may lead to dust bearing the contaminants of

local industrial activities (Meza-Figueroa et al., 2009; Sinha

and Banerjee, 1997). Recreational activities such as off-road

vehicle (ORV) driving can also disturb the soil surface and

cause accelerated wind erosion. A recent study of the effects of

ORV traffic on 16 arid soil types in Nevada showed that sur-

face disturbance by ORV traffic increased dust emissions and

surface erodibility initially but that winnowing and com-

paction of sandy soil types resulted in decreased rates of dust

emission and increases in the threshold wind speed required

to entrain particles (Goossens and Buck, 2009). Other non-

agricultural anthropogenic factors that have been studied

include the impact of military maneuvers (Gillies et al., 2010)

and mining operations (McKenna Neuman et al., 2009;

Brotons et al., 2010). Although agricultural and grazing lands,

roads, recreational areas, and other land use contribute to the

overall anthropogenic dust load, this chapter will focus mainly

on wind erosion caused by agricultural activities.

11.20.3.1 Tillage Effects

Tillage refers to the physical manipulation of the soils usually

performed during agricultural operations. Tillage is used to

prepare the soil for planting a crop, for weed control, to in-

corporate chemicals, to disturb a surface crust in order to

improve crop establishment, promote water infiltration,

and offer erosion control. A wide variety of tools are used

during the tillage process. Tillage generally breaks the soil into

aggregates (naturally occurring groups of individual soil par-

ticles) or clods of varying sizes; buries residues and redistrib-

utes organic matter; mixes the soil, commonly burying loose

Page 7

Figure 6 Saltating sand grains filled the furrows (orientedroughness) during a dust storm in Big Spring, Texas. The ridges willprotect part of the surface when winds are blowing in a directionperpendicular to the ridges.

Saltatingparticledirection

Abradedsurface

Sheltered surface

Figure 7 Schematic representation of a ridged field. Part of the fieldis sheltered from abrasion by saltating grains. Reproduced fromZobeck, T.M., 1991. Soil properties affecting wind erosion. Journal ofSoil and Water Conservation 46(2), 112–118, with permission fromSoil and Water Conservation.

Figure 8 Compact rotary sieve used to determine dry aggregate sizedistribution.

Anthropogenic Environments 401

erodible material resting on the soil surface; and modifies the

surface topography.

These tools produce a soil surface microrelief or micro-

topography that may have a random pattern or roughness that

is oriented in the direction of tillage (Zobeck, 1991). Random

roughness is created by the random arrangements of clods and

plant residues on the surface and oriented roughness is pro-

duced as the tools form ridges or tractor tires create patterns in

specific directions. Most tillage practices produce oriented and

random roughness. Wind erosion is sensitive to the effects of

both oriented (Fryrear, 1984; Bielders et al., 2000) and ran-

dom (Fryrear, 1984; Zhang et al., 2004) roughness. Soil sur-

face microrelief can be further modified by climatic factors

such as rainfall (Zobeck and Popham, 1997) and wind caus-

ing blowing particles to abrade clods or bury surfaces during

deposition (Figure 6).

Microrelief affects wind erosion by modifying the aero-

dynamic roughness (z0) and the fraction of the soil sheltered

from the effects of saltating particles. Several methods have

been used to describe microrelief. The ridge-height-to-spacing

ratio was related to soil loss in the classic USDA wind erosion

equation (Woodruff and Siddoway, 1965). The aerodynamic

roughness, random roughness expressed as the standard de-

viation of measurements of the soil surface, and ridge height

and spacing are used for wind erosion prediction in the more

recent USDA Wind Erosion Prediction System (WEPS) (USDA-

ARS, 2007). The cumulative shelter angle distribution (CSAD)

has been proposed as a method to estimate the fraction of the

soil surface susceptible to abrasion by saltating particles

(Potter et al., 1990). The CSAD is based on the notion that

erosion is predominantly produced as saltating soil particles

strike the soil surface (also called sandblasting), dislodge other

erodible particles, and cause more particles to blow (Zobeck,

1991). Part of the soil will be protected by upwind ob-

structions produced by tillage ridges, nonerodible clods, etc.

(Figure 7). The CSAD is used to estimate the fraction of the

soil surface susceptible to abrasion.

The dry aggregate size distribution (DASD) refers to the

relative amounts of air-dry aggregates or clods, on a mass

basis, by size class, present on the soil surface (Zobeck et al.,

2003a). A rotary sieve (Figure 8) developed 50 years ago is

still a standard automated method to determine the DASD

(Chepil, 1962; Zobeck, 1991). Hand sieving using flat sieves

can be used but the results are not as reliable due to lack of

control of sieving conditions such as shaking rate and inten-

sity, etc. Sieving of dry aggregates has been used to determine

the percentage of nonerodible material, defined as the fraction

of aggregates 40.84 mm diameter, used by the US Depart-

ment of Agriculture to estimate potential wind erosion of

agricultural land (Table 1). The DASD may also be described

by a variety of distributions including logarithmic, fractal, and

Weibull distributions with associated parameters (Zobeck

et al., 2003a).

11.20.3.2 Effects of Vegetation

Perhaps the greatest surface stabilizing influence comes from

vegetation and crop residues. The value of crop residues for

controlling erosion has been recognized for at least six decades

(Chepil, 1944) and the use of residues is integral to many

well-established wind erosion amelioration techniques, such

as straw checkerboards (e.g., Qiu et al., 2004). Residue pro-

tects the ground by offering elements that prevent saltating

particles from cascading and by increasing the roughness

Page 8

Table 1 Soil wind erodibility as determined by percentage ofnonerodible soil (40.84 mm diameter)

Aggregates 40.84 mm diameter (%)

Tensa Units

0 1 2 3 4 5 6 7 8 9

Mg ha�1

0 – 694 560 493 437 403 381 358 336 31410 300 293 287 280 271 262 253 244 237 22820 220 213 206 202 197 193 186 181 177 17030 166 161 159 155 150 146 141 139 134 13040 125 121 116 114 112 108 105 101 96 9250 85 81 74 69 65 60 56 54 52 4960 47 45 43 40 38 36 36 34 31 2970 27 25 22 18 16 13 9 7 7 480 4 – – – – – – – – –

aColumns and rows represent the percentage of nonerodible aggregates. For example,

to find 33% go to the nonerodible aggregates tens row at 30 and units column at 3

(30þ 3¼33) to find 155 Mg ha�1.

Source: Adapted from the USDA, National Agronomy Manual (USDA-NRCS. 2002.

National agronomy manual [Online]. Third ed. USDA, Natural Resources Conservation

Service. Available at http://directives.sc.egov.usda.gov/Open-

NonWebContent.aspx?content=17894.wba (accessed May 2011)).

100806040

Percent soil cover

2000.0

0.2

0.4

0.6

Soi

l los

s ra

tio

0.8

1.0

Figure 9 Effect of soil cover on wind erosion soil loss ratio.Reproduced from Fryrear, D.W., 1984. Soil ridges-clods and winderosion. Transactions of the American Society of AgriculturalEngineers 27(2), 445–448, with permission from ASABE.

402 Anthropogenic Environments

height in much the same way as tillage-induced roughness.

Some wind erosion models for agricultural lands treat all crop

residues as a standardized equivalent protection of flat small

grain residues (Woodruff and Siddoway, 1965; Bilbro and

Fryrear, 1985). Standing residues and growing crops provide

greater protection than flat residues because they absorb much

of the shear stress in the boundary layer (Skidmore, 1994) and

thus keep it from impacting the soil surface. Standing residue

displaces the effective roughness height by a zero plane dis-

placement height that depends on the height, density, and

stiffness of the vegetation (Oke, 1978).

The effects of vegetation and crop residues on reducing soil

loss is estimated using the soil loss ratio (SLR), an index

calculated by dividing the amount of soil loss from a residue-

covered or vegetated soil surface by the amount of soil loss

from a similar bare soil surface (Figure 9). The SLR decreases

rapidly from 1.0 for a bare, unprotected surface to a value of

approximately 0.2 for 40% of the soil covered by residue or

vegetation (Fryrear, 1984).

Another description of plant canopy or residue used by

predictive models for standing vegetation is the plant silhou-

ette through which wind must pass. A strong relationship has

been observed between silhouette and the SLR (Bilbro and

Fryrear, 1994). However, isolated roughness elements or very

sparse residue may actually increase erosion by compressing

airflow and creating localized high wind velocities that exceed

threshold (Sterk, 2000).

Although rangelands and pastures are vegetated surfaces,

native plant communities in arid and semiarid environments

do not always protect the soil surface from the erosive forces of

wind (Li et al., 2008). Livestock foraging and trampling creates

disturbance of the vegetation (Muminov et al., 2010), biological

crusts (Neff et al., 2005), and physical crusts (Baddock et al.,

2011), resulting in increased erosion and dust emissions.

Damage to vegetation results in greater surface shear stress from

higher wind speeds impacting the surface (Grantham et al.,

2001). Fire is another destabilizing influence as it removes

vegetation and damages soil microbial crusts, and also results in

increased shear force reaching a bare, unprotected soil surface.

Fire-induced soil water repellency from plant pyrolysis products

has been shown to decrease the threshold wind velocity re-

quired to initiate particle movement (Ravi et al., 2006, 2009).

11.20.4 Environmental Effects of Wind Erosion

Wind erosion produces a number of environmental concerns.

The often deleterious environmental effects of wind erosion

may be divided most conveniently into on-site effects, those

affecting the eroding soil surface and structures on and ad-

jacent to the eroding surface, and off-site effects, those af-

fecting environments at some distance from the eroding

surface. As wind erosion results in the winnowing and release

of fugitive dust from the source, most of the off-site effects are

the result of deposition of this dust lost from the source and

the two effects are linked by wind trajectory and time.

Wind erosion removes the finer, more chemically active

components of the soil, especially nutrients affecting plant

growth (Figure 10) (Lyles, 1975; Zobeck and Fryrear, 1986;

Stetler et al., 1994; Sterk et al., 1996; Neff et al., 2005; Van Pelt

and Zobeck, 2007). For example, cotton (Gossypium hirsutum

L.) lint weights and kenaf (Hibiscus cannibinus L.) stem weights

were reduced by 40% and sorghum (Sorghum bicolor (L.)

Moench) grain yields were reduced 58% in a study of a se-

verely wind eroded field in west Texas (Zobeck and Bilbro,

2001). In addition to soil fertility degradation, the dis-

proportionate loss of soil organic carbon (Zobeck and Fryrear,

1986; Neff et al., 2005; Van Pelt and Zobeck, 2007) and soil

fines may affect soil water infiltration and holding capacity,

further affecting soil productivity in semiarid regions (Lyles

and Tatarko, 1986). Unfortunately, recovery of the soil to a

quasioriginal condition may be very slow in arid and semiarid

environments (Fernandez et al., 2008).

Page 9

Anthropogenic Environments 403

In source fields, saltating soil particles may sandblast crop

plants and can seriously damage seedling stands (Figure 11)

(Skidmore, 1966; Armbrust, 1968; Fryrear and Downes,

1975). A partially damaged stand often requires the producer

to make economically risky decisions regarding replanting

(Fryrear, 1973; Riksen and de Graaff, 2001). This risky de-

cision is further complicated by recent findings that, for cer-

tain crops and certain growth stages, sandblast injury may

result in increased growth rates in surviving plants (Baker,

2007). According to Farmer (1993), the deposition of wind-

blown soils on crops decreases their value and hinders pro-

cessing. In certain parts of the world, however, agronomic and

other ecosystems depend on the nutrients inputs from de-

posited dust (Sterk et al., 1996; Avila et al., 1998; Rajot and

Valentin, 2001; Neff et al., 2008).

Most of the sediment mobilized by wind erosion is de-

posited near the field of origin (Hagen et al., 2007). De-

position of wind-eroded sand along field margins, especially

along weedy fence lines and in drainage ditches, results in

costly and often recurring maintenance tasks for land owners

and government authorities (Figure 12). The wind-eroded soil

that is not deposited near the source field is transported

as suspended load and may suddenly and seriously affect

Figure 11 Examples of cotton plant damage after exposure to sand abrasiFigure 1 in Baker, J.T., 2007. Cotton seedling abrasion and recovery from w

N NO3 P K SO4 Ca Cu Fe Mg Mn Mo Zn c0

2

4

6

Enr

ichm

ent r

atio

s

8

10

Figure 10 Enrichment ratios (concentration in dust/concentration insurface soil) of plant nutrients in dust collected over the comparativesource field. Reproduced from Van Pelt, R.S., Zobeck, T.M., 2007.Chemical constituents of fugitive dust. Environmental Monitoring andAssessment 130, 3–16, with permission from Springer.

visibility within the source region. Such outbreaks may reduce

visibilities to less than 10 m (Skidmore, 1994). During a single

dust storm in June 2006 near Lubbock, Texas, 21 vehicles were

reported involved in 6 different accidents sending 23 people

to local hospitals and resulting in one death (Blackburn,

2006). The proximity of highly active dust sources impacting

visibility was a major cause of one of the worst multiple-

vehicle accidents ever in USA. In just over 1 h in November

1991, 33 collisions and the involvement of 164 vehicles led to

151 injuries and 17 dead on Interstate 5 in the agricultural San

Joaquin Valley of California (Pauley et al., 1996).

Dust in suspended load may be lifted to tens of kilometers

and transported in the prevailing winds hundreds or even

thousands of kilometers downwind. There are sources of dust

on every mid-latitude continent, and transport pathways to

deposition areas in the oceans and to other continents have

been documented (Prospero et al., 2002). Dust from distant

disturbed desert lands of the Colorado Plateau, USA, has led

to increased dust deposition on mountain snow and a short-

ening of snow cover durations by around a month in the San

Juan Mountains of the plateau (Painter et al., 2007). Neff et al.

(2008) found that dust and associated K, Mg, Ca, N, and P

deposition into Colorado alpine lakes increased five-fold in

on for 0, 5, 10, 20, 30, and 40 min, left to right. Reproduced fromind blown sand. Agronomy Journal 99, 556–561.

Figure 12 Deposited soil ridge on downwind edge of a farm fieldthat contains several buried levels of fences.

Page 10

404 Anthropogenic Environments

the early twentieth century due to the expansion of livestock

grazing on the surrounding bajadas.

Indeed, human activity has been linked with increased wind

erosion, dust emissions, and dust deposition in many locations.

Dust is an important agent for transporting soil parent material

and plant nutrients (Rajot and Valentin, 2001; Reynolds et al.,

2006a, b; Goldstein et al., 2008) and many studies have cited

the deleterious affect dust has on human health (Engelstaedter

et al., 2006; Anuforom, 2007; Mahowald et al., 2007). A recent

clearing of a forested region in tropical Mexico resulted in in-

creased erosion and deposition of sediment and trace metals to

a local crater lake (Ruiz-Fernandez et al., 2007). Construction

sites are temporary and yet can be a significant local source of

wind-eroded dust (Keating, 2003). Caravacho et al. (2001)

found that clay content was the best predictor of PM10 (par-

ticulate matter with an aerodynamic diameter of less than

10 mm) emissions from agricultural activities, construction sites,

and unpaved roads. Military activity, especially with tracked

vehicles, increases wind erosion and fugitive dust emission by

damaging vegetation, soil crusts, and soil aggregates (Grantham

et al., 2001; Van Donk et al., 2003). However, Orts et al. (2007)

found that polyacrylamide mixed with aluminum chlorohy-

drate and cross-linked polyacrylic acid super absorbent in a

6:1:1 ratio, respectively, reduces wind erosion and fugitive dust

clouds from construction sites and helicopter landing pads.

As stated before, the characteristics of fugitive dust are de-

termined from the surface from which they were entrained.

Wind erosion of fallow fields and dust from unpaved roads near

agricultural areas may contain fertilizer and pesticide residues

(DeSutter et al., 1998; Larney et al., 1999). Dredged sediments

from industrial harbors pose a hazard from the chemical con-

taminants they contain (Hagen et al., 2009). Surface mining

operations, including overburden removal, ore removal, and

material processing, contribute significantly to atmospheric

aerosols (Ghose and Majee, 2000; Petavratzi et al., 2007). Utah

subalpine lake sediment cores have been used to trace the his-

tory of industry (Reynolds et al., 2010). The cores show in-

creased levels of specific elements associated with local ore

bodies from mining in the 1870s and increased N and P levels

in post 1950s sediments, coincident with the time in which

mineral fertilizers were increasingly used (Reynolds et al., 2010).

The erosive forces of wind and water have spread mine and

smelter tailings beyond their original extent (Pierzynski et al.,

2002; Lottermoser and Ashley, 2006) resulting in chemical

and radiological impact on surrounding soils. Ore and ma-

terials storage piles have been studied to determine the shape

that results in the least fugitive dust emissions (Badr and

Harion, 2005, 2007). In addition, a recent computer model

has been developed to predict periodic dust emission rates of

flat storage piles based on locally fluctuating wind fields (Kon

et al., 2007). Toxic elements in the tailings are mobilized

by the wind and pose a hazard to environmental and human

health (Moreno-Brotons et al., 2010). Generally the con-

taminant concentrations in the tailings are small, but leachate

impoundments and efflorescent salts result in highly concen-

trated and easily erodible surfaces that pose a threat to human

health and vegetation establishment (Meza-Figueroa et al.,

2009). Additionally, abandoned mining sites may, through

surface instability and wind erodibility, become difficult to

restore to climax vegetation (Campbell et al., 2002).

Diversion of water for agriculture and municipal uses has

resulted in dry lakebeds and playa surfaces with shallow water

tables (Gill, 1996). Much like the mine impoundments dis-

cussed above, these secondary anthropogenic environments

are commonly temporarily covered with easily erodible ef-

florescent salts resulting from the weathering of local geo-

logical materials and the surface evaporation of shallow

ground water (Gillette et al., 2001; Breit et al., 2008). Fully dry

playas with hard crusts emit little dust (Baddock et al., 2011),

but the seasonally wet playas release salt-rich dust that impacts

radiative forcing and human and environmental health

(Reynolds et al., 2007).

11.20.5 Techniques for Studying Wind Erosion

11.20.5.1 Field Studies

The study of wind erosion on human utilized fields and

landscapes is difficult, time-consuming and arguably more

complicated than studies of native lands due to the significant

and frequent temporal changes in the soil surface properties

and vegetation. A significant number of studies, however, have

been conducted for various reasons throughout the world.

These field studies have provided a wealth of data on erosion

processes and rates, plus the fundamental controls produced

by soil properties, vegetation cover, and meteorological con-

ditions. A review of field studies indicates the aim of more

recent fieldwork has been to help inform erosion modeling

attempts. For instance, a multdisciplinary team of scientists

has worked since 1993 to provide new insight and infor-

mation about wind erosion and dust emissions from farm

fields in the Columbia Plateau region of Washington, Oregon,

and Idaho (CPPP, 2010). This agricultural region is subject to

seasonal dust storms and the USEPA National Air Quality

Standard for PM10 is often exceeded (Sharratt and Lauer,

2006). Field studies have been used to continuously monitor

wind erosion and dust emissions in the region (Kjelgaard

et al., 2004a) and have provided evidence for direct suspen-

sion of dust where saltation is not a major mechanism of dust

emissions (Kjelgaard et al., 2004b). Time-integrated PM10

concentrations were well correlated with horizontal total mass

transport. A geographic information system and WEPS (Wind

Erosion Prediction System) were successfully used to scale up

from the field to the county scale in a wind erosion assessment

of Adams County, Washington (Feng and Sharratt, 2007a, b).

Studies with the Columbia Plateau were also conducted to test

modeling simulations of the Single-event Wind Erosion

Evaluation Program (SWEEP) (Feng and Sharratt, 2009) and

WEPS (Feng and Sharratt, 2007a, b).

Wind erosion of a bare agricultural field has been studied

in Big Spring, Texas for most years since 1989 in an effort to

gather basic wind erosion process data and validate erosion

models. The field instrumentation used during the 1990s has

been described by Fryrear et al. (1991). More recent work on

this field to evaluate dust emission rates and erosion threshold

has been described by Zobeck and Van Pelt (2006). Similar

fields, as those described by Fryrear et al. (1991) but some

with residue present, were also established on 18 other

locations in USA in an effort to validate the Revised Wind

Erosion Equation (RWEQ) (Fryrear et al., 2000). The same

Page 11

Figure 13 Big Spring Number Eight (BSNE) sampling cluster.

Anthropogenic Environments 405

field sediment sampling procedure was used in southern

Alberta, Canada to evaluate WEPS (Larney et al., 1995) and to

quantify nutrient content of transported sediment (Larney

et al., 1998). More fundamental process work on an eroding

agricultural field near Wollforth, Texas described the effect of

field length and other site features on details of the saltation

mass flux through time (Stout and Zobeck, 1996). More re-

cently, a field study to compare measured and predicted

sediment transport and factors that affect soil wind erodibility

was conducted on a winter wheat field near Burlington, Col-

orado, USA (Van Donk and Skidmore, 2003).

Recent efforts to study wind erosion in South America in-

cluded work on two bare soils in Argentina (Buschiazzo et al.,

1999b), followed by a study of the effect of tillage and fertility

on the wind erosion quantity and quality (Buschiazzo

et al., 2007). Fieldwork into soil loss and an assessment

of factors that affect the susceptibility to wind erosion in

the Khanasser Valley of Syria was described by Masri et al.

(2003). In the Sahel of southwest Niger, field wind erosion

studies were conducted to determine the effect of low amounts

(Sterk and Spaan, 1997) and randomly placed (Sterk, 2000)

flat residue cover to control wind erosion. More recent work in

the same region quantified storm-based erosion/deposition

patterns and the effect of topography, erosion and deposition

on crop growth and yields (Sterk et al., 2004). From South

Africa, the degree of wind erosion from agricultural fields in

the Free State Province during winter and spring conditions

has been studied with field monitoring (Wiggs and Holmes,

2011).

Field studies on agricultural land in Europe have included

the WEELS (Wind Erosion on European Light Soils) project

conducted in northwestern Europe in 1997–2000 and fi-

nanced by the EU Environment and Climate Programme,

(Warren, 2002; Sterk and Warren, 2003). This study was

conducted to collect data to run a wind erosion model on

three 5� 5 km supersites throughout northwestern Europe,

sample the sites for 137Cs to compare estimates of erosion of

soil loss over a 30þ year period, and measure wind erosion

on small plots to further validate the model. The work fol-

lowed a more limited but coordinated European research

project on Wind Erosion and Loss of SOil Nutrients in semi-

arid Spain (WELSONS) carried out from 1996–99 to under-

stand and predict the impacts of land-use change and

management on soil degradation by wind erosion (Lopez,

et al., 1998; Gomes et al., 2003). Sediment transport and soil

properties related to wind erosion were measured in the field

with instruments such as saltiphones and Modified Wilson

and Cooke (MWAC) sediment catchers as discussed below.

Reviews of past wind erosion research in China have been

presented by Shi et al. (2004). An erosion profile meter or

erosion pins were used to study wind erosion on farmland in

Inner Mongolia (Zhu and Liu, 1981; Zhao et al., 1989) and in

Shanxi Province (Kong et al., 1990). A study of dust de-

position and nutrient movement using dust deposition sam-

plers on a sandy corn (Zea mays L.) field in the Horquin Sandy

Land of eastern Inner Mongolia revealed potential loss of

sediment and plant nutrients during the spring of the year (Li

et al., 2004). Simple aeolian dust samplers were used to study

wind erosion from five landscape types, including farmland,

in the inland basin of the Heihe River of north-western China

(Wang et al., 2004). A dried lake and degraded grassland

produced more than 33 times the dust flux as the bare sandy

farmland tested. However, wind tunnel studies of agricultural

land suggested that cultivated soils may erode at 10 to 15 times

that of uncultivated soils (Dong et al., 1987).

11.20.5.2 Field Equipment to Estimate Wind Erosion

Although field studies of wind erosion under natural wind

conditions in agricultural fields present the most ideal con-

ditions, such studies are unpredictable and provide no control

of wind conditions. Portable wind tunnels offer the advantage

of testing natural surfaces in the field, but they must be care-

fully designed to ensure that a logarithmic boundary layer is

formed and that wind erosion processes may develop without

interference from the tunnel structures. Field portable wind

tunnels have been used for decades to study wind erosion of

agricultural fields. Van Pelt et al. (2010) have provided a his-

tory of portable wind tunnels. Recent studies using this

method include a tunnel used in the Ebro Basin of Spain to

assess wind erosion susceptibility of agricultural land in this

region, including tilled and abandoned fields (Fister and Ries,

2009) and a new tunnel used to assess dust emissions on a

variety of surface conditions and management systems (Van

Pelt et al., 2010).

A wide variety of equipment has been devised to collect

samples of saltating particles and dust in the field or to de-

termine saltation threshold (Zobeck, 2009). One of the most

widely used passive (requires no external suction) saltation/

dust sampler is the BSNE, Big Spring Number Eight

(Figure 13) (Fryrear, 1986). Normally, several samplers are

mounted on a pole to collect samples as height varies. The

horizontal mass flux is determined by integrating an equation

that describes the relationship of sampler height versus

Page 12

Figure 14 Modified Wilson and Cooke (MWAC) sampling clusterPhoto by Geert Sterk.

406 Anthropogenic Environments

horizontal mass per unit time (Zobeck et al., 2003b). The

Modified Wilson and Cooke (MWAC) sampler (Wilson and

Cooke, 1980) is another popular passive saltation/dust sam-

pler constructed of plastic or glass bottles fitted with tubes to

allow capture of dust (Figure 14). Examples of other types of

saltation and dust samplers and a discussion on how to de-

termine horizontal mass flux have been given by Zobeck et al.

(2003b).

Several types of sensors have also been developed to

measure particles moving in saltation using automated high

frequency, electronic sensors. The SENSIT (Stockton and Gill-

ette, 1990) and Safire (Baas, 2004) detect particles using

piezoelectric sensing elements. These sensors have circular

sensing regions that detect particles from any direction. The

Saltiphone is an acoustic sensor that detects particle impacting

on a 200 mm2 diameter microphone mounted in a tube with a

wind vane that works to point the tube into the wind (Spaan

and Van Den Abeele, 1991). Van Pelt et al. (2009) have de-

scribed the range of use and limitations of each of these sen-

sors. A photoelectric laser-based device to make high frequency

measurements of saltation has been described by Mikami et al.

(2005). Another relatively inexpensive, commercially available

photoelectric laser-based device (Wenglors) has been de-

scribed by Hugenholtz and Barchyn (2011). The Wenglor was

found to be insensitive to particle momentum but provided

low measurement variability and the consistent design may

improve comparisons of results between investigations. These

types of sensors can be used to detect the onset and duration of

erosion and to determine the threshold of particle movement

using measurements of saltation activity plus the mean and

standard deviation of wind speed (Stout, 2004).

11.20.5.3 Modeling Wind Erosion

With soil loss being highly sensitive for agricultural lands, and

with recognition that different treatments and conditions of

the land can have a major effect on the rates of erosion, a

number of models have been developed to quantify wind

erosion from surfaces at scales applicable to the field, the

fundamental unit of land management. Models for predicting

the wind-borne soil loss associated with different soil types,

cropping/grazing systems, and tillage regimes, all under par-

ticular climatic conditions, are designed to assist land man-

agement decision making and promote the sustainability of

agriculture in terms of wind erosion. Furthermore, the accur-

acy of modeling outcomes against measured sediment trans-

port from field areas offers an assessment of how well erosion

processes and their controls are understood. However, Leys

(1999) pointed out that the complexity of wind erosion pro-

cesses on human-affected land is typically higher than for sand

dunes, given the wider variety of sediment textures, greater

temporal and spatial changes in soil properties and vegetation,

as well as the irregular modifications to surface erodibility

caused by tillage. Many detailed reviews and explanations of

soil erosion models are offered (e.g., Leys, 1999; Zobeck et al.,

2003b; Webb and McGowan, 2009), and only a brief coverage

of models assessing wind erosion up to the landscape scale is

provided here.

To quantify soil loss, the factors that influence erosion

need to be adequately represented in any model. Early wind

erosion models were highly empirical, beginning with the

Wind Erosion Equation (WEQ) (Woodruff and Siddoway,

1965), but the nature of the inputs meant their use was largely

restricted to where they were developed and where

suitable data were available (Van Pelt and Zobeck, 2004).

Aside from improvements in describing soil erodibility, such

as the inclusion of a term for soil crusting, a development

within the Revised Wind Erosion Equation (Fryrear et al.,

2000) was the shortening of the model’s time scale, away from

annual values to calculations of soil loss on an erosion event

basis. The required inputs also related to the soil and weather

conditions associated with the event of interest, rather than

longer term averages.

There has been an evolution to more process-based

models such as the Wind Erosion Prediction System (WEPS)

(Hagen, 1991), which incorporates friction velocity and the

conditions to exceed a threshold for erosion to occur. Sub-

models for weather, soil, hydrology, crop growth, and de-

composition plus tillage are all incorporated. The model has

been used to estimate long-term soil erosion rates in agri-

cultural fields and its accuracy varies with location (Hagen,

2004; Feng and Sharratt, 2007a, b). Application of WEPS to a

scale larger than a homogenous field, and the accommodation

of different land surfaces, requires different subareas to be

calculated as separate spatial entities (Hagen, 1991). The Texas

Tech Erosion Analysis Model (TEAM) was also developed using

mathematical expressions that describe specific physical pro-

cesses involved in erosion on a field (Gregory et al., 2004). It

has a nonsuspended plus a suspended sediment component,

an important field length term but with a drawback in that it

does not handle changes in erodibility following soil wetting.

Another physically based approach was established in the

Wind Erosion Assessment Model (WEAM), originally de-

veloped for predicting sand drift and dust emission in Aus-

tralia (Shao et al., 1996). Applicable to the field, its capability

in modeling erosion over a larger scale has also been

demonstrated (e.g., Shao and Leslie, 1997). Designed to

Page 13

Anthropogenic Environments 407

specifically tackle the spatial and temporal changes that are

commonly lacking in land erodibility modeling, a recent

modeling attempt called AUSLEM has been developed for

rangelands of western Queensland, Australia (see Webb et al.,

2009). Operating at the landscape scale (103 km2), AUSLEM

includes empirical relationships for a series of controlling

variables and produces a novel, daily-resolved outcome re-

lated to the continuum of land erodibility rather than a

quantified soil loss. The effect of animal stocking rates on

erodibility is reflected in the model’s grass cover input factor

within its component submodels.

A European Union-funded project, the Wind Erosion on

European Light Soils study (WEELS), designed and imple-

mented a tool enabling predictions of long-term soil loss

under varying land uses and for different climates (Bohner

et al., 2003). This model describes wind erosion risks in terms

of erosion hours and wind-induced soil loss.

11.20.5.4 An Indirect Method to Estimate Wind Erosion

In spite of the ability to predict erosion rates by modeling, few

quantitative estimates or direct measurements of past erosion

rates exist. Above-ground nuclear weapons testing during the

1950s and 1960s resulted in the release of anthropogenic

radionuclides (AR) including 90Sr, 137Cs, and 239þ 240Pu into

the atmosphere. These AR were deposited on the earth’s sur-

face as wet and dry deposition termed fallout. The first

measurable quantities of fallout began in 1954 and sub-

sequent pulses in 1958 and 1963–64 account for almost all

the AR fallout received to date (Ritchie and McHenry, 1990).

The Nuclear Test Ban Treaty of 1963 effectively halted at-

mospheric inputs of AR, the atmospheric levels of AR fell

rapidly, and by the late 1970s, AR fallout had essentially ended

(Walling and He, 1999). Polyvalent AR such as 137Cs and239þ 240Pu are tightly adsorbed onto the clay and organic

fraction of the soil (Everett et al., 2008) and only move with

the soil particle to which they are attached. Thus decreases in

AR activity on the landscape indicate erosion has occurred and

increases of AR activity indicate deposition.

For several decades, 137Cs has been used to estimate pat-

terns and rates of soil redistribution due to water movement

(Ritchie, 1998). Fewer studies have investigated the use of137Cs to estimate the patterns or rates of soil redistribution by

wind. In North America, one of the first of these studies made

the assumption that the erosion estimated by 137Cs activity

decrease on a field knoll might be due to wind action (de Jong

et al., 1982). Subsequently, 137Cs has been used to estimate

rates and patterns of soil redistribution by wind in North

America (Coppinger et al., 1991; Ritchie et al., 2003), Europe

(Chappell and Warren, 2003), Africa (Chappell et al., 1998),

Australia (Harper and Gilkes, 1994), and Asia (Ping et al.,

2001). Recently, the use of models to estimate soil redistri-

bution rates from water movement has been validated to

estimate redistribution rates by wind (Van Pelt et al., 2007).

Figure 15 Wind strips provide protection to soil and crops nearBozeman, Montana. Photo by Ron Nichols, USDA-NRCS (http://photogallery.nrcs.usda.gov/Detail.asp).

11.20.6 Control of Anthropogenic Wind Erosion

The Best Management Practice (BMP) to prevent erosion by

wind on agricultural land is to limit the contact of the wind

with the soil surface by maintaining an effective cover of

vegetation or managed residues. The increasing use of no-till

and conservation tillage practices has resulted in more effec-

tive surface cover during the period between growing crops.

Further, advances in harvesting equipment such as finger

headers on small grain combines have led to improvements in

the postharvest standing heights of crop residues. Unfortu-

nately, semiarid regions with marginal crop growth and de-

ciduous crops such as cotton or sunflowers may produce

insufficient silhouette to protect the surface adequately. Fi-

nally, in developing economies and for emerging bio-fuel in-

dustries, soil surface protection may not compete with the

alternative use values of crop residues.

Vegetation can also be used to reduce the length of the field

in the direction of wind erosion, thus reducing sediment

emission and transport. Vegetative barriers protect the soil

surface by sheltering an area from the wind on the lee side of

the barrier. A barrier will protect or shelter an area from wind

erosion to a distance of approximately ten times the height of

the barrier (USDA-NRCS, 2002). Vegetative barriers include

annual or perennial plants. Annual barriers may consist of a

taller annual crop or residue (Bilbro and Fryrear, 1997; Bilbro

and Stout, 1999) protecting an adjacent area newly planted to

a sensitive vegetable crop (strip cropping system). Such a strip

cropping system is shown in Figure 15. Perennial barriers

include windbreaks composed of several rows of trees bor-

dering an erodible agricultural field. To be most effective,

vegetative barriers should be planted perpendicular to the

direction of the wind that prevails during the most critical

periods of wind erosivity. Artificial materials have also been

used to reduce wind erosion, especially slat fences (Bilbro and

Fryrear, 1997; Lee and Park, 1999). The effectiveness of bar-

riers at reducing wind erosion is a function of optical density

and shape-drag coefficient (Bilbro and Stout, 1999) and has

been successfully modeled (Schwartz et al., 1997).

In areas where it is difficult to maintain adequate residue

on the soil surface other practices such as emergency tillage

may be employed. Tilling the surface in the direction per-

pendicular to the prevailing wind direction increases the

aerodynamic roughness and provides microrelief to protect a

Page 14

408 Anthropogenic Environments

portion of the surface as described earlier (Figure 7). Tilling of

finer textured soils can be used to produce nonerodible dur-

able clods that will offer some protection from abrasion until

they are broken down due to rainfall or other weathering

processes. In sandy soils with very fragile or no aggregates,

intense rainfall may promote the development of a weak

surface crust that commonly has loose erodible material de-

posited on it (Zobeck, 1991). This loose erodible material will

readily blow during intense winds. Tillage implements such as

the rotary hoe and sand fighter may be used to disrupt the

crust, produce a rough surface, and bury the loose erodible

material. This technique may be necessary after each intense

rainfall until the crop emerges and grows enough to create

roughness and protect the soil surface. When irrigation is

applied to a dry soil, a reduction in wind erosion can be ex-

pected (USDA-NRCS, 2002). However, irrigation or wetting

the soil offers very short-term mitigation of wind erosion on

sandy soils. These soils dry very rapidly and may erode shortly

after wetting them.

11.20.7 Future Outlook and Perspectives

Fugitive dust has a negative impact on air quality. Non-

attainment of air quality parameters happens several times per

year in wheat-growing regions of Washington (Sharratt and

Lauer, 2006). Although the US Clean Air Act exempted farm-

ing from dust control rules, the San Joaquin Air Quality

Management Districts are moving closer to regulating dust

emissions from agriculture (Engle, 2004). A part of the US

National Food Security Act of 1985 sets a compliance con-

dition of maintaining soil loss at under 11 Mg ha�1 for farmers

seeking government payments and subsidies. Although it is

difficult to place a realistic value on the soil lost to erosion, the

cost of maintaining compliance may dissuade producers from

seeking compliance and further punitive measures to promote

conservation have been suggested (Bunn, 1997, 1998, 1999).

Clearly, improved land management practices have been

linked to a reduction in dust activity (Todhunter and Cihacek,

1999; Stout and Lee, 2003). In Germany, the government is

advising farmers to limit field lengths between wind barriers

for wind erodible soils (Gunreben, 2005). Practices that utilize

the newest agricultural technology, protect natural resources,

and provide a safe, sustainable food source throughout the

world are needed.

From the findings of numerous field and laboratory ex-

periments, and following improvements in modeling erosion

first at the field scale, and then into the landscape or regional

level, there has been considerable appreciation of and ad-

vancement in our understanding of human impacts on ae-

olian processes. Dealing with the anthropogenic contribution

at a larger scale, particularly in global dust modeling, is a

relatively new, but highly significant issue. Considerable un-

certainty still exists regarding the human contribution to dust

loading through land use and the subsequent changes to

surface erodibility (Mahowald et al., 2004). Datasets with

good spatial and temporal resolution that will enable quality

assessments of land use location, extent, and change, espe-

cially in semiarid areas, will be a key requirement in the future.

Further understanding and representation of the spatio

temporal aspects of soil erodibility caused by human activities

can provide improvements but remains a serious challenge in

the development of process-based models (Webb and Strong,

2011). Advances in airborne and satellite remote sensing

provide tools to allow the collection of surface data, including

soil erodibility, with high spatial and temporal resolution.

However, challenges remain in using remote sensing techni-

ques to develop algorithms that provide realistic estimates of

dynamic soil erodibility parameters that make improvements

in global models.

Currently, studies commonly attribute an anthropogenic

component to detected increases in dust loading over time, or,

in order to account for discrepancies between models and

observations (Ginoux et al., 2010). Determining the changes

in wind erosion caused directly by humans as opposed to the

modulations in dustiness that are a result of climatic change

represents an important aim. Improvements of models at all

scales will continue. Furthermore, recalling Zender et al.’s

(2004) Second Kind of anthropogenic dust, the changes in

dust emission caused by human-induced climate change will

require even more research attention in the future.

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Biographical Sketch

Ted M Zobeck is a Research Soil Scientist in the Wind Erosion and Water Conservation Research Unit, Agricultural

Research Service, US Department of Agriculture at Lubbock, TX. Dr. Zobeck has worked on research in aeolian

processes and wind erosion science, surface soil properties related to wind erosion, and soil productivity and

management for more than 25 years. He is currently the President of the International Society for Aeolian

Research and Co-Editor-in-Chief of the Elsevier journal Aeolian Research.

Scott Van Pelt was raised in the Dallas-Ft. Worth suburbs and he was accustomed to the incursions of dust into

this humid area and the remarks of ‘Here comes West Texas!’. After graduating high school, he earned a BS in

Biology and a MS in Plant Ecology and Systematics at the University of New Mexico in 1978 and 1983, re-

spectively. He earned a PhD in Soil Physics from New Mexico State University in 1990. He has worked for several

consulting firms in New Mexico and currently lives in Big Spring, Texas, where he is employed as a Soil Scientist by

the Wind Erosion and Water Conservation Research Unit of the USDA Agricultural Research Service.

Matthew Baddock is an aeolian geomorphologist who has worked in different drylands on three continents. He

employs both field measurement and remote sensing techniques to study wind-related processes and landforms.

His PhD (University of Leicester) was on sand dune dynamics, and in postdoctoral work at Loughborough

University, USDA-ARS, and University of Virginia, he has investigated the controls on dust emission and wind

erosion at a variety of spatial and temporal scales.