JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 93, NO. D10, PAGES 12,645-12,662, OCTOBER 20, 1988
ThresholdFriction Velocitiesfor Dust Productionfor Agricultural Soils
DALE A. GILLETFE
Geophysical
Monitoringfor ClimaticChange,Air Resources
Laboratories,
NOAA,Boulder,Colorado
Threshold
velocitiesfor agricultural
soilsweremeasured
for a widevarietyof conditions
in ord'erto quantify
a modelof dustemissions
for the UnitedStates.Thesemeasurements
supplement
thresholdvelocitiesfor arid
and semiaridsoils (Gillette, et al., 1980, Gillette, et al., 1982). The model will be used in precipitation
acid/basebalancestudies.The soilswere organizedaccordingto surfacetexture,organicmattercontent,and
calciumcarbonatecomposition.They were furtherorganizedby the physicalsurfacestates:smooth-loose,
cloddy,andcrested.Sandysoilswerefoundto havethelowestthreshold
velocitiesandweretheleastaffected
by wetting(precipitation).
Loamysoilswerefoundto havethehighestthreshold
velocitiesandwerethemost
affectedby precipitation
wetting.
1.
INTRODUCTION
Recent interest has been expressedin evaluationof dust
emissionfrom open sourcessuch as agriculturalsoils. Dust
emittedby wind erosionof agriculturalsoilsis of importanceon
a largegeographicalscalefor two nationallydiscussed
problems
in the United States: (1) the acidfloase
balanceof atmospheric
precipitation,and(2) soil erosion.
The alkaline componentsof soil-deriveddust are basesthat
may be important in neutralizing acidic precursors in
precipitation.This possibilityis being pursuedby the National
Acid PrecipitationAssessmentProgram (NAPAP). Aerosols
derived from wind erosion of soil, which are transportedgreat
distances,
representa long-termsoil loss. The NationalResource
Inventory (NRI) of the Soil ConservationServiceof the U.S.
Departmentof Agriculture(USDA), is actively assessingsuch
long-termlossof soil.
For estimation of dust production and wind erosion, one
model relies heavily on values of thresholdfriction velocities
[Gillette, 1986]. Threshold friction velocity for wind erosion
correspondsto the minimum wind stressneeded to overcome
forcesholdingsoil particlesin place. Thresholdfrictionvelocity
is relatedsimply to the thresholdwind stress:the squareof the
saltsin desertsoils, the presenceof "desertpavement"in many
desertsoils, and other large differencesin vegetationand land
use.
It is the purposeof thispaperto applythe knowledgeobtained
in the studiesof idealizedsurfaces,alongwith the more empirical
approachtakenin the studyof desertsoils,in orderto determine
thresholdfriction velocitiesfor dustproductionfrom agricultural
soils.
2. SCIENTIFIC STRATEGY
The organizationof the soil samplesto be classified for
thresholdfriction velocity was chosenaccordingto surfacesoil
texture and further modified by carbonateand organic matter
content. This choice was made becauseit is the organization
usedby the NRI nationalsoil inventory. It alsowas the starting
point in the organizationframeworkused in the pioneeringsoil
erosionwork of Chepil and Woodruff[1963]. A moderatedegree
of homogeneityin soil physical properties,such as threshold
frictionvelocity, is expectedwithin a given soil textureclassfor
the same carbonateand organicmatter soil composition. The
source of variation would be the exact differences in mixtures of
sand,silt, and clay within a given soil textureclassas well as the
threshold
frictionvelocityu.t timesthedensityof theair equals differencesin the size distributionsof the particlesmakingup the
the thresholdwind stress. Although friction velocities at the
thresholdof erosionhave been determinedexperimentallyfor
idealized surfacesby several investigators[Baghold, 1941;
Chepil, 1945;Arvidson,1972; Greeley,et al., 1974;Hess, 1973;
Iversen, et al., 1973; Ryan, 1964; Sagan and Pollack, 1969;
Woodet al., 1974, Phillips, 1980;Ishihara andIwagaki, 1952],
theseidealizedsurfacesonly slightlyresemblethe complexityof
the natural earth surfaces. A wide variety of particle sizes,
inhomogeneous
particlecomposition,effectsof wetting/drying
sand, silt, and clay fractions. Aggregationsof surface soil
textureshave been used to define "wind erodibility groups"
(WEGs) by Chepil and Woodruff[1963]. A recentdefinitionof
WEGs used by the Soil ConservationService for NRI work is
given in Table 1. These wind erodibility groupingsof soil
textures assign the most erodible designationto sand textures
(WEG 1) and the leasterodibledesignations
to loams(WEG 8).
Included
in WEG
8 are a
number
of textures located in the
middle of the texture triangle shown in Figure 1. Typical
and freezing/thawing,
vegetation,and aggregationof particles percentages
of loosesoil materialsmallerthan0.84 mm are given
complicatethe physical circumstances
in agriculturalsoils. in Table 1.
Threshold friction velocities have been measured for desert soils
A classification
of the 1982 NRI
of all erodible soils of the
by Gilletteet al. [1980, 1982]. Althoughsomeof the friction
velocitiesmay be applicablein this study, large differences
betweendesertand agriculturalsoil thresholdvelocitiesexist
becauseof workingof the landin agriculture,
largeprecipitation
differences,
the existenceof usuallygreaterquantitiesof soluble
48 contiguousUnited Statesshowsthe percentagedistribution(of
area) of all surface soil textures that were judged to have
potential wind erosion by local USDA soil scientists. This
inventoryshownin Figure 1 showsno erodiblesoil area for the
soil textures "silt" and "sandyclay." Figure 1 also showsthat
most of the United Statesis coveredby soil with the following
erodibletextures:sand,loamy sand,clay loam, silty clay loam,
This paper is not subjectto U.S. copyright. Publishedin 1988 by
sandyloam, silt loam, andloam.
the AmericanGeophysicalUnion.
Our
Papernumber88JD03081.
initial
soil classification
shown in Figure 1.
12,645
uses the surface
soil textures
These texture classifications are
12,646
GILLETTE:THRESHOLD
FRICTIONVELOCITIESFORAGRICULTURALDUSTEMISSION
TABLE 1. Wind ErodibilityGroupversusSoil Texture,Percentage
of Dry Aggregates
SmallerThan 0.84 mm, and Wind ErodibilityIndex
WEG
PercentDry
Aggregates
<0.84 mm
Soil Texture of SurfaceLayer
very fine sand,fine sand,sand,
WEI,*
T/(Ac* Yr)
93-99
160-310
90
134
75
86
75
86
60
56
55
48
50
38
<20
0
75
86
or coarse sand
loamy very fine sand,loamy fine
sand,loamy sand,loamy coarse
sand,or sapricorganicsoilmaterials
very fine sandyloam,fine sandy
loam, sandyloam or coarsesandyloam
clay, silty clay, noncalcarious
clay loam, or silty clayloam
with more than 35% clay content
noncalcareous loam or silt loam
with lessthan 20% clay content,
or sandyclay loam, sandyclay,
or hemic organicsoil materials
noncalcareous
loam or silt loam
with more than 20% clay content,
or noncalcareous
clay loam with
lessthat 35% clay content
silt, noncalcareous
silty clay loam
with lessthan 35% clay content
or fibric organicsoil material
soilsnot susceptible
to wind
erosion because of coarse suface
4L
fragmentsor wetness
calcareousloam, silt loam, clay
loam, or silty clay loam.
WEG, wind erodibilitygroup;WEI, wind erodibilityindex.
*Data courtesyof USDA Wind ErosionLaboratory.
supplementedby four calcareous(rich in calcium carbonate)
textureslisted as WEG 4L in Table 1' loam, silt loam, clay loam
andsilty clay loam. In our secondaryclassification
clay andsilty
clay soilshavinglessthan 3% organicmatterare alsoseparated
from clay and silty clay soils having more than 3% organic
matter for reasonsthat will be given in section4. Silt and sandy
clay texturesare ignored,since zero area of thosetexturesis
shownin the inventoryof erodiblesoils as given by the NRI in
Figure 1.
A further subdivisionin the organizationfor thresholdfriction
velocity work is basedon the frictionvelocityReynoldsnumber,
B.
B= (u.,Dp)4
motion at thresholdvelocity are virtually the same. The length
scalewas best establishedwhen the size distributionhad a sharp
peak; sucha peak was observedfor only a small fractionof the
soils. In many casesthe mass size distributionshoweda very
gentle maximum, and in several cases,multiple modes were
found.
Gillette [1984] reviewed the literature for natural threshold
velocitiesand found a minimum Reynoldsnumber of 2.6 for all
reportedvaluesfor the observationsof Lettau andLettau (1978).
The thresholdfriction speedparameterA for idealizedsoil (e.g.,
monodisperse
particles) is almostconstantfor Reynoldsnumbers
largerthan 1 [Iversen,et al., 1976]. A is definedas
A = u.t(ovgD•/o)m
(1)
where
where
u.t threshold
frictionvelocity;
ov particledensity;
Dv asizescale
oftheerodible
particles;
v
the kinematicviscosityof the air.
In laboratorystudiesusing monodisperse
particleshaving the
• air density;
g the acceleration
of gravity.
The thresholdfriction velocity would be expectedto be
same
density,
theparameter
Dj,hasbeentaken
tobethediameter approximatelyproportionalto the squareroot of Dp' in this case
of the testedparticles.For natural soils, however,particle sizes
rangeover severalordersof magnitude. In addition,naturalsoils
form aggregates,the size distributionsof which change as a
functionof wetting,disturbance,etc. Gillette et al. [1980] found
that a useful length scale for erodingsoils is the mode of the
mass size distributionof loose aggregatespresenton the surface
of the soil. They found that the size distribution of those
aggregatesavailable for erosion and those particles set into
the mode of the size distributionof looseparticleson the soil
surface.Indeed,thisrelationshipwasfoundto be themostuseful
predictorof thresholdfriction velocity for naturaldesertsoils,
accounting
for 55% of the variancefor 72 degreesof freedom
[Gilletteet al., 1980]. Althoughthis relationshipwas the most
usefulpredictor
of u.e a largescatter
of datawouldbe expected
becauseof the rather inexact descriptionof the natural dry
aggregate
sizedistributionby a singleparameter.Thuswe useda
GILLETTE:THRESHOLDFRICTIONVELOCITIESFORAGRICULTURALDUST EMISSION
I
i
I
x
12,647
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GILLETTE:THRESHOLD
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FORAGRICULTURAL
DUSTEMISSION
rough but useful classification of surface description 3.2. Soil Descriptions
corresponding
to informationon dry aggregate
sizedistribution:
Several soils representativeof each textural group were
"smooth-loose"
corresponds
to a disaggregated
soil having a chosenin agriculturalareasof the United Stateshavingwind
small mode of the size distribution; "cloddy" correspondsto a erosionproblems.The soiltextureswerelocatedby soilmapsor
soilhavingcoarseaggregates;
and"crusted"
corresponds
to a soil by approximate
soiltexturedetermination
in thefield. Later,all
havinga significantsurfacecrust.
soilswereclassifiedin a soillaboratoryfor soiltexture,andsome
Soilcrustsandaggregates
of soilswerefoundto increaseu.t mapping-or field-determined
textureswerefoundto be slightly
[Gillette et al., 1980]. To quantify the strengthof crustsin inaccurate. For example, severalof our "silt loam" soils,
resisting erosion and in maintaining themselvesagainst according
to mappingclassification,
wereactually"loam"soils,
mechanicaldisturbance,modulusof rupture,M, was determined. asdetermined
by laboratorydeterminations.
Modulus of ruptureis definedas
Locationsand details,such as descriptionsof the land and
croppingfor eachsoil sample,are givenin Table2. All soil
M = 3FI•2WT2)
(2) samplesweretestedfor in situconditions.The windtunnelwas
placeddirectlyoverthesoilto be tested,sothattheexposed
soil
where
wasashomogeneous
aspossible.
F
impressedforceat whichthe crustbreaks;
Masspercentages
andsizedistributions
of thetestsoilswere
L
lengthbetweensupportsof the crust;
determined after water-soluble material, calcium carbonate, and
T
is thickness of crust;
W
width of the crust.
organicmaterial were removed. The pipettemethodand
sedigraph
methodwereusedto determine
the sizedistributions.
Theresulting
compositions
of sampled
soilsin Table2 areshown
The stabilityof aggregates
of soilswas testedby measuring
in Figure 1.
the percentageof aggregates
larger than a given size (1 mm)
Other components
of the test soils are given in Table 3.
remaininglargerthanthatsizefollowing2 min of rathervigorous Solublematerialwasmeasuredgravimetricallyfrom a soil water
mechanicalsieveshaking.
extractof a suspension
formedby intermittent
stirringof soilin
water for 3 hours. The measurementwas checked(with good
3. METHODS
agreement)
against
specificconductance,
whichwasconverted
to
an estimateof solublematerialby using an empiricalformula.
3.1. ThresholdFriction VelocityDetermination
ThepH valueswereobtained
for a soil/water
ratio1:5,usinga
A portablewindtunneldescribed
by Gillette[1978]wasused Hach meter. Organic matter and carbonatematerial were
with an open-flooredtest sectionso that a variable-speed determinedby standardlaboratoryprocedures. Chemical
turbulentboundarylayer could be formed over a flat soil determinations
were made for air-dry samplesafter they were
containing
small-scale
roughness
elements.
The windtunnelused gentlydisaggregated
and passedthrougha 2-mm sieve.The
a two-dimensional5:1 contractionsection, with a honeycomb samplewasplacedin a plasticcentrifuge
tube,wherecarbonates
flow straightener
and a roughlyconicaldiffuserattachedto the and solublesaltswere removedwith 1 N NaOAc (pH=5), using
workingsectionin a configuration
similarto thatdescribed
by Jackson's[1975] centrifugewashingprocedure.The carbonate
Wooding[1968].Dimensions
of thecrosssection
of theworking andsalt-freesamplewasthentransferred
to a beaker,andorganic
sectionare 15.24 X 15.24 cm, and the length of the working
matterwasdestroyed
by treatment
withH202 anddispersed
with
section is 240 cm.
25 mL of Na4P207.
Stirring
andultrasonic
treatment
followed.
Wind speeddatawerecollectedat severalheightsabovethe
surfacemidwayacrosstheendof theworkingsection.The Pitot
Clayswereisolatedby sedimentation
afterremovalof thesand
fractionby wet sieving.
Soil moisturevaluesfor thesamplesare alsoreportedin Table
tube anemometerwas calibrated and correctedfor temperature
and pressurechanges. Data for the meanvelocityU versus 3. They are expressed
in termsof the soil moistureas a
height (wind profile data) were fitted to the functionfor percentage
of theoven-dry
soilmassobtained
after48 hoursof
aerodynamically
roughflow [seePriestley,1959]:
dryingat 105øC.The soilmoistures
aregivenfor thecondition
u=
o)
(3)
of the soil as it was found in the field.
For some samples,
measurements were made of the soil moisture at -15 bar tension
whereu, is frictionvelocity,z is heightabovethesurface,z0 is
roughness
height characteristic
of the surface,and k is Von
(roughlythe soil moistureat the wiltingpointof sunflowers).
Comparison
of thissoilmoisture
withtheobserved
soilmoistures
Karman'sconstant.The thresholdvelocityprofile was obtained
whencontinuous
movementof soilparticleswasfirstvisible.
Severalof our field samplingswere within about100 m of the
NOAA Boulder Atmospheric Observatory(BAO) in Erie,
Colorado. These soilswere monitoredclosely,and timeswhen
showsthat for all but one soil the moisturecontentwas equal to
or below thewilting point.
threshold
friction
Friction velocities
velocities
were
reached
were
recorded.
were obtained from wind fluctuation data
recorded
attheBAO:u. = {[u'w']]ø'5
where
thesquare
brackets
indicate mean quantity and u' and w' are fluctuationsof
horizontaland verticalwind speedfrom the mean. For several
Texassoil samplesa 6-m towerwas usedto collectwind data
duringwinderosionevents.Priestley's[1959]expression
(3) for
frictionvelocitywas usedto derivefrictionvelocitiesfrom the
recordedwind profiles.
Dominantmineralogyof theclayfractionwasdetermined
by
X ray diffraction,
usingthemethoddescribed
by Gilletteet al.
[1982]for threesamples.Claysfor samples
CA 1, CO 10, and
CO 9 weredominantly
mica andchlorite,smectites,
andmica
andchlorite,respectively.
Bulkdensity
wasmeasured
in situusingthemethod
of Robert
Grossman
(SoilConservation
Service,NationalSoil Laboratory,
personal
communication,
1985),in whichthevolume
of thesoil
is measured
alongwith its massin thefield.
In addition,
although
mostsamples
wereobtained
fromsoils
withoutvegetative
residue,somemeasurements
weremadeon
land that had vegetativeresidue. Table 4 lists these
GILLETTE:
TttRESHOLD
FRICTION
VELOCITIES
FORAGRICULTURAL
DUSTEMISSION
TABLE 2. Location,
Taxonomy,
andDescriptions
of SoilSamples
Soil
Sample
Location
Description
Comments
Sand Texture
TX 1
TX 2
TX 4
TX 5
TUN 1
Plains,Tex.
Brownfield, Tex.
Plains,Tex.
Plains,Tex.
Plains,Tex.
cottonfield
cultivatedfield
cottonfield
cultivatedfield
cultivatedfield
CA 1
PalmSprings,Calif.
fiver wash,bare
CA 2
CA 3
TX 10
Palm Desert,Calif.
Olancha,Calif.
Plains, Tex.
dune
uncultivated,bare
cultivatedfield
flat
flat
flat
flat
flat
flat
flat
cloddy,ridged
LoamySandTexture
TX 3
Bronco, Tex.
cultivatedfield
flat
TX 7
TX 8
TUN 2
Big Spring,Tex.
Big Spring,Tex.
Big Spring,Tex.
cottonfield
cultivated
field
cultivated
field
cloddy, ridged
cloddy,ridged
KS 1
CO 7
CO 11
GardenCity, Kans.
Greeley,Colo.
Nederland,Colo.
baresoil,uncultivated
baresoil,cultivated
bare soil, uncultivated
flat
crusted, flat
thin crusted soil
SandyLoam Texture
TUN 3
CO 12
NE 8
CO 10
Meade County, Kans.
Nederland, Colo.
Alliance, Nebr.
Erie, Colo.
Craig, Colo.
cultivatedfield
bare soil, uncukivated
cultivated field
cultivated field
bare soil, uncultivated
TX 6
Big Spring,Tex.
cottonfield
NE 6
NE 7*
Gering,Nebr.
Gering,Nebr.
winterwheatfield
winterwheatfield
CO 8
flat
flat, rocky
cloddy
thin crusted
crusted
Clay Texture
TX 9
NM 1
ND 7
TUN 4
Abemathy,Tex.
Las Cruces,N.M.
Fargo,N. Dak.
Abemathy,Tex.
dry, barelake
baresoil,rangeland
cultivatedfield
baresoil,rangeland
smallaggregates
crusted
smallaggregates
smallaggregates
Silty Clay Texture
ND 2
Fargo,N. Dak.
cukivatedfield
eroding
Clay Loam Texture
CO
CO
CO
CO
1
3
4
5
CO 6
KS 2
NM 2
TUN 6
Erie, Colo.
Erie, Colo.
Erie, Colo.
Erie, Colo.
winter wheat field
cultivated field
cultivated field
cultivated field
Longmont,Colo.
Liberal, Kans.
Las Cruces,N.M.
MeadeCounty,Kans.
cultivatedfield
cultivated field
baresoil, rangeland
cultivatedfield
flat, slightlycloddy
flat, smooth, loose
flat, cloddy
flat, wet field
crusted
crusted
flat, erodible
Silty Clay Loam Texture
ND 1
Mapleton,N. Dak.
cultivatedfield
small aggregates
Silt Loam Texture
ND 3
ND 4
NE 5
NE 3
Casleton,N. Dak.
Casleton,N. Dak.
Sidney,Nebr.
Sidney,Nebr.
baresoil,cultivated
baresoil,cultivated
cultivatedfield
cultivatedfield
flat, smallaggregates
flat, smallaggregates
cloddy,largeaggregates
cloddy,largeaggregates
SandyClay Loam Texture
TUN 5
Hale County,Tex.
cultivatedfield
agricultural
soil
12,649
12,650
GILLETTE:THRESHOLDFRICTIONVELOCITIESFORAGRICULTURALDUST EMISSION
TABLE 2.
(continued)
Soil
Sample
Location
Description
Comments
Loam Texture
CO 2
CO 9
Erie, Colo.
Pueblo, Colo.
cultivated field
bare soil, uncultivated
crusted
NE 1
NE 2
NE 4
KS 3
KS 4
NM 3
Sidney,Nebr.
Sidney,Nebr.
Sidney,Nebr.
GardenCity, Kans.
GardenCity, Kans.
Las Cruces,N.M.
cultivatedfield
cultivatedfield
cultivatedfield
cultivatedfield
cultivatedfield
bare soil, rangeland
clods,crustedandloose
clods,crustedandloose
clods,crustedandloose
clods,crustedandloose
clods,crustedandloose
crusted
CalcareousLoam,SiltyLoam,Silty ClayLoam,and Clay Loam
ND 5
ND 6
Casleton,N. Dak.
Jamestown,N. Dak.
cultivatedfield
cultivated field
*Soil containedshalefragments,
not reallycloddybutloosemixedwith gravel-sized
particles.
'•WEG 4L category.
measurements.The vegetativeresiduewas weighed after the
sampleswere oven dried for 24 hours.
Table 5 lists the threshold friction velocities for the test soils
for the three soil conditions;smooth-loose,crusted,and cloddy.
When thresholdvelocity was not reachedby our apparatus,the
maximum friction velocity is given using the "greater than"
symbolto indicatethat the actualthresholdfrictionvelocity is
higher.
The coarse-aggregate
size distributionwas determinedfor
smooth-looseand cloddysamplesby dry sieving. There were no
coarse aggregatesin the soil crusts. The soil sampleswere
carefully transportedto our laboratoryto avoid breakageof
aggregates.From the size distributionthe maxima (i.e. modes)
were obtainedfor eachtest (Table 5).
A measure
of how
resistant the size distribution
the 1-mm sieve.
The method used to obtain the initial size
distributionof dry aggregates
is a gentledry sieving,usingas
little shakingas possibleto separatethe soil. This method,
althoughrelative and certainlynot the same as the mechanical
stressesexperiencedby soil aggregatesin the field, gives a
measureof how stablethe aggregates
will be to suchmechanical
disturbances
assandblasting
by wind erosion.
Finally, the thicknessof the soil crust and modulus of
ruptureare alsogivenin Table 5. The modulusof rupturewas
TABLE 3. Compositionof Soil Samples
Moisture
%
Soil
Soil
Sample
Loose
Crusted
Cloddy
at -15 bar
Soluble
Soil
Soil
Soil
Tension
Salt,* %
Carbonate,
%
Organic
Matter, %
pH
Sand Texture
TX5
0.52
0.99
0.41
0.52
TUN 1
CA 1
0.3
0.06
TX1
TX2
TX4
CA2
CA3
1.3
0.1
0.23
0.05
0
0
8.1
0.04
0.43
0
3
0
0
7.7
9.9
1
1.74
0.41
0.89
7.6
7.3
0.5
TX10
Loamy Sand Texture
1.29
0.6
TX3
TX7
0.6
TX8
TUN
2
0.4
1.2
KS 1
CO 7
0.9
0.9
8.4
CO 11
1.64
3.5
was to
change is given by the stability measurements. These
measurements
(given in Table 5) show the percentageof soil
aggregatemasslargerthan 1 mm that survivesa periodof rather
vigorousdry sievingprovidedby a mechanicalsieve-shakerfor
GILLETTE:THRESHOLD
FRICTIONVELOCITIESFORAGRICULTURALDUSTEMISSION
12,651
TABLE 3. (continued)
Moisture
%
Soil
Soil
Sample
Loose
Soil
Crusted
Soil
Cloddy
at - 15 bar
Soil
Tension
Soluble
Salt,* %
Carbonate
%
Organic
Matter, %
pH
SandyLoam Texture
TUN 3
CO 12
3.0
1.57
3.3
NE 8
CO 8
4.3
4.2
CO 10
1.79
1.57
5.9
9.1
7.1
1
0.89
3.24
1
5
O.78
0.29
7.6
0.96
5.73
7.8
1.79
0.75
TX6
NE 6
NE 7
7.6
2.8
1.6
6.1
1.6
7.7
Clay Texture
TX9
NM
8
8
6.6
2.99
2.51
3.23
3.23
2.4
2.2
1
ND 7
TUN
4
11.6
19
0.98
7.1
Silty Clay Texture
2.58
7.43
ND 2
5.08
7.8
Clay Loam Texture
10.6
CO 1
CO 3
(air dry/onsat.)•'
(air dry/onsat.)?
(air dry/onsat.)'•
(air dry/onsat.)?
CO 4
CO 5
CO 6
KS 2
NM 2
TUN 6
1.5
ND
4.88
1.5
10.6
0
0
1.05
6.9
9.9
11.4
13.4
11.1
0
0
0
0
0
2
2
2
1.01
1.03
0.81
1.1
7.3
7.3
0
1.14
6
0.67
5.79
7.4
0
0
2.18
2
4.32
1.51
6.1
5.8
7.6
7.8
1
1.01
7.4
9.5
0.67
1.7
7.9
6.6
1.06
6.1
7.3
11
6.5
6.3
7.5
1.82
1.1
1.2
Silty Clay Loam Texture
1
Silt Loam Texture
NE
NE
ND
ND
5
3
3
4
5.69
6
6.4
2.77
2.25
2.56
6.4
4.41
14.2
13.7
1.45
2.42
SandyClay Loam Texture
TUN
5
2.4
2.2
Loam Texture
CO 2
9.9
CO 9
9.9
4.5
NE 1
NE 2
NE 4
4.8
5.3
6.5
KS 3
KS 4
NM 3
2.3
3
3.32
2.31
4.1
8.8
3.27
3.47
3.08
0.04
12.2
11.7
10
0
0
2
11.6
8
3
0
1.55
2.08
1.24
7.5
6.4
2.41
Calcareous
Loam,SiltLoam,SiltyClayLoam,ClayLoam
ND 5
ND 6
2.6
1.54
*No datafor samples
judgedto havenegligiblesolublesalt.
'•Indicates1 mm dry layer on a saturatedwet soil.
$WEG 4L category
6.59
15.52
5.94
1.29
7.9
8
12,652
GILLETTE:
THREStIOLD
FRICTIONVELOCITIES
FORAGRICULTURAL
DUSTEMISSION
down for the sandyclay loam andsilt loam textures. Sincethese
texturesrepresentonly three samplesin Table 6, however,it is
possible that our samples are biased to more aggregated
conditions.For cloddysoilsour valuesin Table 6 for percentage
TABLE 4. VegetativeResiduefor Soil Samples
Soil Type
Soil Sample
Residue
TX 1
TX 2
TX 4
TX5
26.7
8.3
91.6
19.1
Loamy sand
TX 3
TX7
TX 8
3.67
161
39
Sandyloam
TX 6
161
Sand
of soil mass smaller than 1 mm are much lower than the values
given in Table 1. This may simply show a concernby the
authorsof Table 1 for more highly erodiblesoils,hencesoils of
the smooth-loose classification.
Table 6 showsthat the mean modulus of rupture for soil
crustsis very low for sandtexturesandmuchhigherfor all other
textures. Crustshaving moduli of rupturelarger than 0.5 and
thicknessesof 1 cm or more are efficient in preventingwind
erosion for most winds.
Stabilityof dry aggregates
largerthan 1 mm was fairly high
(>40%) for all texturesexceptsilty clay, silty clay loam, andthe
Clay loam
CO 1
33.6
calcareoussoils making up WEG 4L (see Table 1). Such clod
stabilityfor almostall soils texturesmakes the clod-destructive
All samples
notmentioned
in thistablehadnegligiblevegetative
residue. propertiesof rainfall very important in wind erosion, since
cloddysoilsarefairly resistantto wind erosion.
With increasingerodibility, accordingto soil texture, the
used as a measure of the hardness of crust or its resistance to
thresholdfrictionvelocitiesof Table 5 generallydecreaseas the
change.It wasdeterminedby themethoddescribed
by Richards WEG erosion indices decrease. Thus sand is the most erodible
[1953] and was obtainedafter carefully transportinga field crust soil texture for both Tables 1 and 6. The texturessand,loamy
to the laboratoryand cuttingit to the dimensionsspecifiedby sand,sandyloam, clay and silty clay are the most erodiblefor
Richards.
both Tables 1 and 6, whereasthe texturesclay loam, silty clay
loam, silt loam, and sandy clay loam are considerablyless
Clay
TX 9
2.9
4. THRESHOLDFRICTION VELOCITIESFOR SMOOTH-LOOSE,
CLODDY, AND CRUSTED SOILSVERSUS SOIL TEXTURES
erodible.
Table 6 summarizesthe threshold friction velocity data
given in Table 5 as mean friction •reshold velocities,mean
percentagesof soil aggregatessmaller than 1 mm, average
stabilityfor bothsmooth-loose
andcloddysamples,andmodulus
of rupture. In comparingthe percentages
of materialsmallerthan
0.84 mm in Table 1 with the percentageof masssmallerthan 1
mm in Table 6, there is generalagreementfor the smooth-loose
soils of Table 6 with Table 1 values. The agreementbreaks
The effect of compositionis extremely importantas seen in
two cases.First, the desertclay-texturedsoils,asinvestigated
by
Gillette et al. [1982] were crustedand extremely nonerodible.
The clay-texturedsoils investigatedhere were highly erodible
and were almost smooth-loosefollowing a dry winter in both
North Dakota and Texas. The principal differencein the clay
soils of the desert and of North Dakota/Texas seems to be the low
organiccontentin the desertclay soils and a higherthan 5%
organic content in the North Dakota/Texas clay soils. An
TABLE 5. Observed
Threshold
FrictionVelocitiesandOtherWindErosionParameters
for theSampledSoils
Soil
Sample
u,t,cms-1
Mode,
mm
Loose Crusted Cloddy
Percent
< 1mm
Loose Cloddy
Loose Cloddy
Stability,
%
Loose Cloddy
Crest
Thickness,
cm
Modulus
of Rupture,
bars
Sand Texture
TX1
TX2
TX4
TX5
TUN
1
CA 1
CA2
25
25
30
30
22
37
<0.42
<0.42
<0.42
<0.42
<0.42
<0.375
90
95
98.9
95.9
98.8
96.7
74.2
7.1
22.6
42.9
56.5
84.6
0
0.6
4O
67
75
CA3
TX10
<1
0.64
lOO
neg.
0
neg.
0.03
30
LoamySand Texture
TX3
TX7
TX8
TUN 2
KS 1
81.5
<0.42
22
85
22
42
62.7
51.7
60
42
9O
152
49
<0.42
0.3
75
53
22
91.5
73.1
0.29
(14.8)
crust
0.2
CO 7*
CO 11
0.43
63.2
rocks
0.7
GILLETTE:
THRESHOLD
FRICTION
VELOCITIES
FOR'
AGRICULTURAL
DUSTEMISSION
TABLE 5.
-1
Sample
Mode, mm
U,t, •Tn S
Soil
Loose Crusted Cloddy
(continued)
Percent<1 mm
Loose Cloddy
12,653
Loose Cloddy
Stability, %
Crust
Thickness
Modulus
of Rupture
bars
Loose Cloddy
SandyLoam Texture
TUN 3
CO 12
22
20
>90
NE 8
44
>63
132
CO 8
<0.47
0.43
69.8
1.7
<1
>89
CO 10
TX6
89.5
80.4
rocks
rocks
71
8.7
81.3
36.7
93.2
290
>60
NE 6
>336
>164
NE 7t
100
crest
78
0.22
62.4
0.75
0.6
0.04
crest
3
0.26
91
rocks
0.5
0.62
3
0.62
0.87
Clay Texture
TX 9
NM 1
ND 7
TUN 4
35-60
>204
> 150
> 150
> 100
68
0.75
>150
0.61
2.5
9.3
54
84.2
30.9
1.5
1.62
96.6
70.4
95.52
83, lab
0.89
3
2.5
20, lab
16.9, lab
Silty Clay Texture
ND 2
56
> 150
> 150,
0.43
12.8
94
wet
Clay Loam Texture
CO 1
> 109,
54
10.2
46.9
WW
CO 3
CO 4
CO 5
CO 6
89
61
57
61
KS 2
70
(89)
(61)
(57)
(61)
>109
120
NM 2
TUN 6
0.43
0.61
0.43
0.43
0.43
72.7
3.4
49.3
57.3
99.7
16
100
46.1
23.3
67.6
>100
> 100
0.982
87.7
38.5
70.7
43.2
0.5
0.12
O.68
21.8
0.92
2
56.6
0.34
Silty Clay Loam Texture
ND 1
64
>150,
>150
0.61
81.8
12.4
3
wet
Silt Loam Texture
NE 5
NE 3
ND 3
ND 4
124
92
>200
> 159
>302
>138
>200
260
1.2
10
10
54
25.9
20
15.4
6.1
75.8
54
72.3
82.8
36.3
51.5
1
3
2.27
2
0.45
0.9
1.01
59.6
84.5
2
0.78
0.3
3
3
3
0.62
0.38
0.99
0.73
1.25
2
0.22
1.14
SandyClayLoamTexture
TUN 5
>100
26.7
Loam Texture
CO 2
CO 9
NE 1
NE 2
NE 4
KS 3
KS 4
NM 3
>109
261
>200
>258
>200
91
75
72
86.5
67
>197
>222
>250
25
0.75
10
6
1.7
0.3
0.15
16
26
10
13.1
39.6
24.4
63.6
78.1
48
7.3
13.7
10.8
61.6
69
83.9
64.3
83.2
60.5
62.2
0.72
0.83
88
62.9
> 100
Calcareous
Loam,SiltLoam,SiltyClayLoam,ClayLoam
ND 5
ND 6
71.2
(71.2)
> 100,
11.1
15
wet
Figuresin parentheses
indicate1mm drylayeron a saturated
wet soil;WW standsfor winterwheat(live plantsin soil);neg,negligible.
*Threshold estimated fromwind
records more than 2 km from site.
•'Soilcontained
shalefragments--not
reallycloddybutloosemixedwithgravel-sized
particles.
$WEG 4L category.
O.45
12,654
GILLETTE:THRESHOLD
FRICTIONVELOCITIES
FORAGRICULTURAL
DUSTEMISSION
TABLE 6. MeanThreshold
FrictionVelocitiesfor ThreeSoilConditions,
Stability,Modulusof Rupture,
and Other Wind Erosion Parameters,for Various Textures
u,t,cms-t
Percent
< 1mm
Texture
Loose Crusted Cloddy
Sand
28.2
Loamy sand
Sandyloam
Clay
Silty clay
Clay loam
Silty clay
33.6
28.7
54.3
56
67.6
64
65.7
75
103
290
>200
85
105
> 150
120
> 150
> 109
>150
>200
Loose Cloddy
Average
Stability,
%
Modulus
of Rupture,
bars
97.06
30
48
0.03
74
70.4
57.6
94
72.3
81.8
53
35.9
5.5
62.3
87.1
86.4
12.8
45
12.9
0.5
0.42
0.75
23
10.8
18.6
0.38
loam
Silt loam
108
Sandyclay
> 100
26.7
60.7
0.8
72.1
0.78
70.6
13.1
0.34
loam
Loam
Calcareous
78.3
71.2
> 150
> 100
> 150
> 100
50.7
11.2
0.66
loam, silt
loam, silty
clay loam,
clay loam
exampleof a desertclay soil is NM 1; examplesof North Dakota
andTexasclay soilsare ND 7 andTX 9. A wider rangingstudy
of the disaggregation
of clay soilsis under way (R. Breuninger,
D.A. Gillette, andR. Kihl, manuscriptin preparation,1988).
A secondimportantcompositionaldifferencewas pointedout
by the work of Chepil and Woodruff[1963]. In establishingthe
WEG 4L classification, they recognized the effect of high
calciumcarbonatepercentagein increasingwind erodibility. The
expectedeffect would be to lower thresholdfriction velocities.
cloddy soils. The following selected observationson the
responseof soil thresholdfriction velocity to precipitationare
organized by surface soil texture classification. Observations
were not made for responseof silty clay and silty clay loam and
for the calcareous soil textures, because of limited travel to
samplingsitesfor thosetextures.
Indeed, soils ND 3 and ND 5 were located within 2 km of each
other; both soils were similarly farmed, were flat, and had an
approximately3-cm-thick crust. One soil (ND 5) was rich in
calciumcarbonate,however,andeffervescedwhen a dropof acid
was put onto its surface. Both soilswere classifiedas silt loam
soils. Close-upphotographsof the surfacesof both soils show
that the more calcareoussoil (B in Figure 2) has a fine, loose
surface appearancecompared with the smootherconsolidated
surface of the less calcareous soil (A in Figure 2). This
difference in the microstructureof the thin surfacelayer caused
the large difference in thresholdvelocity of the two soils: the
calcareoussoil was erodiblefor friction velocitieslarger than 71
cms4, andthenoncalcareous
soildidnoterodefor thehighest
wind speeddevelopedby our wind tunnel. This differencein soil
propertiesis manifestedin the very thin, loose surface-layer,
approximately1 mm thick. In many cases,bulk soil properties
canbe very differentfrom the surface-layersoil properties.
5. THE EFFECT OF PRECIPITATION ON THRESHOLD
FRICTION VELOCITY
Precipitationis importantin wind erosion,sincefor almostall
soil textures it is observed that clods of soil "melt"
into a
flattenedsoil structurefollowingintenserainfall or meltingof a
sufficientquantityof snow. Thus precipitationcan modify
cloddy structureof a soil. However, mechanicaldisturbance
suchas sandblasting
may not, in view of the ratherhigh stability
Fig. 2. Close-up surfacephotographsof (a) clay loam soil, comof dry aggregatesto mechanicaldisturbancefor almost all soil paredwith a dime for scale,and (b) calcareousclay loam soil, comtexturesand the generally high thresholdfriction velocities for paredwith a dime,locatedwithina 2 km distahce
of eachother.
GILLETTE:
THRESHOLD
FRICTIONVELOCITIES
FORAGRICULTURAL
DUSTEMISSION
12,655
7.00
6.00
5.00
-
4.00
-
3.00
-
2.00
-
1.00
-
0.00
i
May Jun
i
Jul
I
Au•
i
I
Sep Oct
I
I
Nov De•
I
Jan
I
Feb MaP Apr May Jun
Jul
May 85- July 80
[]
MOD. RUP.
+
THICKNESS
•
RAIN
Fig. 3. Modulusof rupture,crustthickness,andmaximumprecipitationfor consecutive-day
stormsfor the periodof
samplingfor the NM 1 soil samplesversusday numberof sampling.
5.1. Sand Texture
5.4. Clay Texture
Wind erosionmeasurements
wereobtainedfor soil sampleTX
4 before and after an intense 3-cm rainfall.
The condition
before
and after the rainfall was smooth-loose, and the threshold
velocity did not change. On the other hand, observationsof
sand-texturedcloddyfields showedthat the effect of intenserains
Two clay soils,samplesTX 9 andNM 1, wereobservedprior
to and followingmajor precipitationevents. TX 9 was typicalof
clay soils that are rich in organicmatterand that break into small
erodible aggregatesafter sufficient drying. Thresholdvelocity
wasmeasured
at approximately
50 cm s4 priorto a 4-cm
was to "melt" the clod structure to a smooth-loose or weak,
rainstorm that soaked the soil for 2 days. The erodible clay
crustedflat soil having much lower thresholdfriction velocities material was thoroughlywetted and became a thick layer of
thanin the cloddystate.
water-saturated
mud. The precipitationevent was followed by a
long dry period. Within a month the thresholdfriction velocity
was60cms4 corresponding
toclaythathadcracked
into1- to
5.2. LoamySandTexture
2-mm-sizedaggregates.The aggregateswere erodedby winds at
that time but, becauseof their rather large size, did not become
suspendedand were depositedwithin a few tens of meters of
their origin. During the next severalmonthsof severedrought
and frequent winds higher than threshold,the clay aggregates
were mechanicallyworn down and thresholdfriction velocity
wasmeasuredat 35 cm s4.
was the same before and after rainfall.
NM 1 was typical of low organic compositiondesert clay
crusts. Our equipment did not generate sufficient friction
5.3. SandyLoam Texture
velocity to initiate erosion for this soil during any time. The
The NE 8 bare soil at Alliance,Nebraska,experiencedless relevant parameters of crust thickness and crust modulus of
moisture than normal from November through the date of the rupturewere plottedin Figure 3, alongwith the equivalentdepth
wind erosionevent,March 18, 1971. Althoughthe soil may have of maximumprecipitationduringthe time of sampling. The soil
shows some evidence of a decrease in soil crust thickness with
previouslybeen cloddy,copiousrain in October1970 (6.63 cm)
probablymelted the clods and formed a weak crust. Becauseof decreasedrainfall. The changeof modulusof rupture does not
sandblastingfrom an upwind depositof highly erodible sandy appear to be correlated with rainfall and may be randomly
Threshold friction velocity was measured before and after
erosion on soil sample TX 3. Wind erosion was stoppedby
approximately1 cm of rainfall, but resumedwithin 10 min after
rain cessation. Althoughunderlyingsoil was saturated,the top
1-mm layer of soil at the soil-air interface was dried by high
winds within minutesafter rainfall stopped. Thresholdvelocity
soil, this crust was destroyed producing the low threshold
variable.
velocity
(44cms4) onMarch18. Following
a fairlysevere
dust
stormthathadwindfrictionvelocities
largerthan70 cms4, a 5.5. Clay Loam
a crust
Soil samplesCO 4 and CO 5 were locatedwithin 1 km of
formed, the soil had a thresholdfriction velocity larger than 63
6-inch snowfall
ended erosion.
After
the snow melted
each other. In October 1985, threshold velocities for the two
200cms4, respectively.
cms4, whichprotected
thesoilfortheremainder
of itsbarren soilswere70cms4 andapproximately
state.
Soil sampleCO 4 had a smooth-loosesoil condition,and sample
12,656
GILLETTE:THRESHOLD
FRICTIONVELOCITIESFORAGRICULTURALDUSTEMISSION
350
.
03
300
-
b
:250 -
\
:200
..••
150
-
I00
-
5O
0
I
•
0
•
•
:2
CUMULATIVE
[]
NE 1
+
NE :2
o
•
4
NE 3
A
RAINFALL
NE 4
(cm)
x
CO 4
v
CO 5
Fig. 4. Thresholdfriction velocitiesversuscumulativeprecipitationfor initially smooth-loosesoil samplesNE 1, NE
2, NE 3, NE 4, and CO 4 and for initially cloddy soil sample CO 5, for October 1985 (no precipitation)through
February 1986 (6-cm cumulativeprecipitation).
CO 5 had a cloddy soil condition. Following the melting of a
snow layer on February 15, 1986 (6 cm cumulative
precipitation),high windscauseddrying andwind erosionon the
top layerof the water-soaked
soils. The thresholds
for bothsoils
wereapproximately
70 cm s'l, as shownin Figure4. The
melting of the snow had destroyedthe cloddystructureof soil
CO 5 but had not significantlychangedthe structureof soil CO
4. Had thehigh windsnot immediatelyfollowedthe meltingof
the snow coveringthe soils, both soils might have dried more
slowly, formingwind erosionresistantcrusts.
Soil sampleNM 2 was typicalof a desertflat crust. Its crust
thicknessand modulusof ruptureare plotted in Figure 5, along
with the maximum precipitationamount during the sampling
period. As with the NM 1 soil, modulusof rupturedoesnot
O.OO
5.00
-
4.00
-
3.00
:2.00
1.00-•
O'OO
I
May Jun
[]
MOD, RUP,
I
Jul
I
I
Aug Sop Oct Nov Dec Jan Feb Mar Apr May Jun
+
uay t•-
THICKNESS
Jury eo
•>
Jul
RAIN
Fig. 5. Modulus of rupture,crustthickness,and maximumprecipitationfor consecutive-daystormsfor the period of
samplingfor the NM 2 soil samplesversusdaynumberof sampling.
GILLETTE:
THRESHOLD
FRICTION
VELOCITIES
FOR
AGRICULTURAL
DUST
EMISSION
350
12,657
..
300
250
200
150
-
I00
-
50
-
0
,
0
œ
4
•
8
CUMULATIVE
[3
NE 1
+
NE œ
o
10
RAINFALL
12
14
1•
(cm)
NE 3
z•
NE 4
Fig. 6. Threshold
frictionvelocities
for initiallycloddysoilsamples
NE 1, NE 2, NE 3, andNE 4. The cloddiness
was destroyedby rainfall and soil crustswere formed.
correlate
withrainfallamount
although
crustthickness
is weakly rainfall and erosion-resistantcrusts were formed, while
correlated
withrainfallamount.Threshold
velocitywasnever smooth-loosestructure
(Figure4) wasaggregated
into a crust
exceeded for this soil.
thatwas very resistentto soil erosion. Fromour observations,
therainfallincreased
u.t for smooth-loose
soilsandmaintained
5.6. Silt Loam
largethreshold
velocitiesfor initiallycloddysoils. For these
Threshold
frictionvelocity
forsoilsample
NE 3 responded
to same soils, however, observations of severe wind erosion
cumulative
rainfall,asshown
in Figure4. Aftera slowdryingof followingwettingof the soilshavebeenmade. We feel thatthe
wettingof theloamsoilsis as
thewettedsoilthathadinitiallybeensmooth-loose,
following
a timingof thehighwindfollow'rag
4-cmrainfall,thesoildeveloped
a crustthatwasresistent
to wind
importantasfor clay loamsoils.
Crustthicknesses
of soil samples
NE 1, NE 2, andNE 4 are
erosion.The precipitation
in thecaseshownin Figure6 thus
NE 3 (a siltloam).
caused
a greatincrease
of theinitialthreshold
frictionvelocity.It shownin Figure7, alongwithdataforsample
Moduli
of
rupture
for
the
same
soils
are
shown
in Figure8.
hasbeenobserved,
however,
thathighwindsfollowing
driving
rain and clod destructionof silt loam and loam soilscauseintense Figures7 and 8 show thickeningof the crusts,as rainfall
on initially uncrustedsoil, and decreaseof modulus
wind erosion (Tom Nightingale,Universityof Nebraska, accumulates
personalcommunication,1986). This was alsoobservedfor the of rupture. Modulusof rupturewas largerfor the clods
rainfallis equalto zeroin Figure8) thanit wasfor
clay loam soil sampleCO 5 by the authors. The rather (accumulated
old
crusts.
This
is probably
caused
by themorecompacted
soil
ambiguous
resultthatintense
precipitation
maybe followedby
of clodsthatareformedby moistsoilbeingbrought
intensewind erosionor by crustingandno erosionmay be condition
may be contrasted
differentiated
by thetimingof thestrong
dryingwindfollowing frombelowby plowing. Theseconditions
of crustthickness
andmodulus
of rupturewith
wetting:if the windoccurswhilethesurfacelayeris still wet, withthechanges
rainfall
of
the
NM
3
soil
sample
shown
in
Figure
9. TheNM 3
wind erosionis possible;if it occursafter the soil crusthas at
soil
sample
was
initially
crusted
and
flat
and
remained
thatway
leastpartiallyformedonthesurface,
winderosion
will probably
be prevented.
Whentheinitialsoilcondition
wascloddy,a change
towarda
throughoutthe observations.Thicknessof this desertcrustseems
tovarywithrainfall,altho
..ugh
thevariation
of modulus
of rupture
more crustedsoil was observed. That is, after 6-cm accumulated is not clearlycorrelatedwith rainfall.
rainmoistened
theclodson sampleNE 3, thecloddystructure
was destroyedand the soil becameflatter and more crusted. In
Figure6 thethreshold
velocity,
initiallyveryhighforthecloddy
condition,
remainedveryhighfor thecrustedsoil.
5.7. Loam Texture
5.8. BulkDensity
Bulkdensity,a soilparameter
suggested
by RobertGrossman
(Soil Conservation
Service,NationalSoil Laboratory)
to be
importantin researchon wind erosionof soil is shown for soil
samples
NE 1, NE 2, NE 3, andNE 4 for initiallysmooth-loose
Threshold
frictionvelocities
forsoilsamples
NE 1, NE 2, and soils(sametimeframeasFigure4) in Figure10andforinitially
NE 4 arealsoshownin Figure6, forinitiallycloddyconditions.cloddysoils(sametimeframeasFigure6) in Figure11.Initially
As withsoilsample
NE 3, cloddystructure
wasdestroyed
by smooth-loose
soilswere convertedinto crustedsoilsby the
12,658
GILLETTE:
THRESHOLD
FRICTION
VELOCITIES
FORAGRICULTURAL
DUSTEMISSION
4
-
3.5
3
-
0
•
1
CUMULATIVE
[]
NE 1
+
NE 2
o
RAINFALL
(cm)
NE :3
z•
NE 4
Fig. 7. Crustthicknessversusaccumulatedrainfall for soil samplesNE 1, NE 2, NE 3, andNE 4.
wetting and drying cycles associatedwith precipitation. As
shownin Figure 10, the changein soil bulk densityis not great.
For initially cloddy soils, however, large air pocketsbetween
clods resulted in lower initial bulk density. The flattening,
filling, and crust-formingprocesses
associated
with precipitation
effectivelyincreasedsoil bulk density,as seenin Figure 11. For
the samplesthat were collected,the predictivevalue for threshold
frictionvelocityof bulk densitywasnot seen.
5.9. Synthesis
of Observations
on Effects
of Precipitation
These observationson the effect of precipitationon threshold
frictionvelocitieswere synthesizedand are presentedin Table 7.
Table 7 gives rangesof thresholdvelocitiesbefore and after
precipitation,using the data of Table 5 for sand, loamy sand,
sandyloam, clay, loam, and silt loam. The precipitationeffects
for silty clay, silty clay loam, sandy loam, and for the four
4
CUMULATIVE
[]
NE1
+
NE 2
o
RAINFALL
NE 3
(cm)
zx
NE 4
Fig. 8. Modulusof rupturefor soil samplesNE 1, NE 2, NE 3, andNE 4 versusaccumulated
rainfall.
GILLETTE:
THRESHOLD
FRICTION
VELOCITIES
FOR
AGRICULTURAL
DUST
EMISSION
7.00
,
6.00
-
B.00
-
4.00
-
3.00
-
2.00
-
1.00
12,659
-
0.00
Ma/ Jun Jul Aug Sep Oct Nov Dec Jan Feb Mac Ape May Jun Jul
[]
MOD. RUP.
+
TH/CKNESS
o
RAIN
Fig.9. Modulus
of rupture,
crustthickness,
andmaximum
precipitation
for consecutive-day
storms
for theperiodof
samplingfor theNM 3 soil samplesversusthe daynumberof sampling.
calcareous
texturesof WEG 4L (calcareous
loam,silt loam,clay
loam, and silty clay loam) have been extrapolated. We felt
justifiedin lumpingsilt loamswith loamsoils,because
manyof
our sampledloam soils had beenmappedin the field as silty
difficultyin recognizingthe exacttexturein soil mappingand
becauseof similarsoil propertiesthat led to this difficultyin
recognition.
6. EFFECTOFVEGETATIONAND VEGETATIVERESIDUE
loam soils. Indeed, the propertiesof the silt loam soils and the
loamsoilsthatwe sampleddid not differ greatly.Likewise,some
ON THRESHOLD FRICTION VELOCrFY
of the clay-textured
soilsthatwe sampledweremappedas silty
clay soils. We felt justified in lurepingthem, becauseof the
The values for threshold friction velocities in section 5 all
applyto situationsof barrenor effectivelybarrensoil. The effect
œ
1.9
-
1.8
-
1.7
-
1.6
-
1.5
-
1.4
-
1.3
-
1.2
-
1.1 1
-
0.9
-
0.8
-
OJ
-
0.•
-
o
0.5
-
0.4
-
0.3
-
0.•
-
0.1
-
0
0
CUMULATIVE
[]
NE 1
+
NE 2
o
RAINFALL
NE 3
(cm)
•
NE 4
Fig. 10. Bulkdensity
of soilsamples
NE 1,NE 2, NE 3, andNE 4 versus
accumulated
precipitation,
in whichthe
initialcondition
of thesoilwassmooth-loose.
Thetimeframeis thesameasfor Figure4.
12,660
GILL•-TT•: THRESHOLDFRICTIONV•.LOCITI•.SFORA6mCULTURAL DUST EMISSION
1.8
1.7
1.6
-
2
4
6
•
CUMULATIVE
[]
NE 1
+
NE 2
o
10
RAINFALL
1,=
ø
14
16
(cm)
NE :3
A
NE 4
Fig. 11. Bulk densityof soil samplesNE 1, NE 2, NE 3, andNE 4 versusaccumulated
precipitation,in whichthe
initial condition of the soil was cloddy.
of vegetationor of vegetativeresidueis to absorbsomeof the the geometryof the residue:heightof stubble,areacovered,etc.
momentumthat otherwisewould have gone into the work of
Indeed, research being pursued by us on the effects of
moving soil particles.Theoreticalanalysesof this momentum nonerodiblesphericalparticles mixed with erodible spherical
partitioningand experimentalinvestigations[e.g. Marshall, particleshas emphasizedthe complexityand importanceof this
1971] have shown that the effect of live or dead vegetation area of research.
(nonerodibleelements)is virtually anotherthreshold. Because
Presenceof live vegetation usually precludesactive wind
low concentrations of surface material have almost no effect on
erosiondamage.Such a situationwas presentin sampleCO 1,
erosion, and concentrationsin excessof the amount needed to
wherewinter wheatplantseffectivelypreventedwind erosionfor
fully protectthesoilhaveno erosioneffect,andthedifference
in
frictionvelocitiesin excessof 90 cm s4.
In somecases,
surfaceconcentrationsof live or dead vegetationbetweenthese
however,suchas sampleNE 6, winter wheat was not sufficient
two statesis notlarge,we approximate
therathersteepprotective to preventwind erosiondamagecausedby sandincursionfrom
functionof surfaceconcentrationof live or deadvegetationas a
an upwind flat sandy area having a low friction threshold
stepfunction,that is, as a threshold.Presence
of the material velocity. Indeed, severalobservershave noted that wind erosion
abovea certainconcentration
stopswind erosion;presenceof the oftenspreadsinto agriculturalfields from upwinderodingareas,
vegetativematerialbelow the thresholdconcentration
doesnot suchas unpavedroads,barrow pits, or other agriculturalfields.
Several exampleshave been noted of erosiontaking place in
alterthe erosionphysicssubstantially.
For thepresent
studywe haveobserved
severalerodingfields fieldshavingsufficientcloddinessto preventerosion,or in fields
and have noted the following thresholdvegetativeresidue having sufficient vegetative residueto prevent erosion,by the
incursionof sandblastingsand from an upwind sourcehaving a
amounts
forspecific
crops:1500lb/acre
(168gin-2),
soybeans,
low
peanuts,
cotton;
750lb/acre
(84gm'2),
anygraincropincluding thresholdfriction velocity.
corn;andgrass,wheat,barley,hay,sorghum,
vegetable
crops.It
shouldbe notedthatnoneof thevegetativeresidues
measuredfor
7. EFFECT OF AGRICULTURAL PRACTICES
ON THRESHOLD FRICTION VELOCITY
the soil samplesin thispaperwas greaterthanthesethreshold
values. Indeed, the effect of the vegetative residue was nil.
Vegetativeresiduecover(grass)for soil sampleTX 4 was92%
of the threshold surface concentration but soil erosion fluxes
Agricultural methodshave been designedand developedto
minimize
wind
erosion and to conserve
soil moisture.
Those
methods, along with an evolution toward larger farms, more
which had much smaller surface concentrationsof vegetative powerful farm machinery, and an economy that avoids the
residue. Soil sampleTX 7 was near the thresholdfor cotton widespreadeconomicdepression,suchas that of the disastrous
residue,but againfully developederosionoccurred.For highly 1930's dustbowl help to avoidwidespreadand long term intense
erodibleloosesoilsobservedby us as havingvegetativeresidue wind erosion.Agriculturalpracticesthat alleviate wind erosion
exceedingthese thresholds,however,erosionwas effectively are as follows: (1) Fallowing of fields, (2) shelterbelts,(3) cover
prevented.
Observations
of vegetativeresiduein erodingand crops,(4) deepplowing,(5) irrigation,and(6) minimumfillage.
Fallowing offields. To conservesoil moisture,many fields
noneroding
fieldsaswell astheexperimental
resultsof Marshall
[1971] andof LylesandAllison[1976]pointto theimportance
of are left fallow (dormant),with vegetativeresidueprotectingits
were virtually the same as for soil samplesTX 1 and TX 2,
GILLETTE:TttRESHOLD
FRICTIONVELOCITIES
FORAGRICULTURAL
DUSTE/mSS•ON
12,661
TABLE7. Change
inThreshold
Velocity
u,.(cms-1)Caused
byIntense
Rainfall
orMelting
ofSnow
Followed
byDry•ing,
forThree
InitialSoilConditions
Loose, Erodible
Texture
Sand
Change
none
Cloddy,ResistsErosion
U,t
U,t
Before
After
20-30
20-30
U,t
Change
clods
Crusted,ResistsErosion
U, t
Before After
U, t
U,t
Change Before After
>100
20-30
none
30-40
30-40
>100
20-30
none
30-4
30-40
> 100
>70
none
>70
>70
>100
>100
none
>100
>100
>100
>100
none
>100
>100
>100
>100
none
>100
>100
>100
>100
none
>100
>100
> 100
> 100
melt
Loamysand
none
25-35
25-35
clods
melt
Sandyloam
crust
25-45
>70
forms
Clay*
crust
crust
40-70
200->70
forms
Silty clay
crust
loam
Silt loam
crust
40-70
200->70
60-90
150t
loam
Loam
crust
60-110
150t
melt,
crust
60-90
150't
forms
crust
melt,
crust
forms
Sandyclay
melt,
crust
forms
crust
melt,
crust
forms
Silty clay
melt,
melt,
crust
60-90
150•
melt
>100
>100
none
>100
>100
70
150•'
melt
> 1O0
> 1O0
none
> 1O0
> 1O0
forms
Calcareous
limey loam,
silt loam,
silty clay
crust
forms
loam,
clay loam
Valuesofu,tarein centimeters
persecond.
*Clay must be distinguished
betweenthat high in organics(mollisolic)or that low in organic(as in arid
regions).
•'High winds during the drying processcausedrying and erosionof a thin surfacelayer and much dust
production,while the remainingsoil is saturated
with water.
surface. In westernNebraska,for example,fallowing typically
takes place every third year. By vegetative decay and farm
tillage operations,the residue is gradually reduced. In some
cases, it is reduced to less than the amount needed for wind
erosionprotection.
Shelterbelts. Shelterbeltswere heavily planted during dust
bowl crises of the 1930's and 1950's in several states in order to
reduce the effect of wind by providing a shelteredarea, of a
width equivalentto about10 timesthe treeheights.They are still
widelyusedin suchstatesasNorth DakotaandNebraska.
Cover crops. A temporary"covercrop" is plantedin certain
areassuchas North Dakotaduringperiodsof high wind-erosion
hazard. This practicetakesadvantageof theprotectiveproperties
of live plantsin preventingwind erosion. The cover cropsare
subsequently
plowed under and fields are replantedfor the next
growingseason.
Deep plowing. In areasof sandysoils, suchas westTexas,
deepplowing is usedto bring up soil richer in clay andmoisture,
so that the high thresholdvelocitiesof cloddy soil reducewind
8. EFFE•
OF A PROLONGED DROUGHT ON THRESHOLD
FRICTION VELOCITY
The effect of a prolonged drought would be to reduce
vegetativeresiduefrom a given crop and reducesoil moisture
levels. With the same farming operations,less soil moisture
would result in reduced cloddiness of the soil. A combination
of
reducedcloddiness,lessvegetativeresidue,and reducedcrusting
of the soil would result in lower threshold friction velocities and,
for the samedistributionof wind speeds,increasedwind erosion.
9. CONCLUSIONS
We organized thresholdfriction velocities for agricultural
soils accordingto texture of the surfacesoil. Soil composition
wasusedto organizethe soilsfor clay andsilty clay textureswith
respectto organic-richandorganic-poorsoils. Calcareousloams,
silt loams,silty clay loamsand clay loams(soilsof the WEG 4L
classification)
were distinguished
from noncalcareous
soilsof the
same textural classes. Threshold
friction velocities increase from
a minimumfor sandtexturesto a maximumfor loamy soils. The
erosion.
effectsof wettingthe soilsby precipitationis minimal for the low
Irrigation. Irrigation prevents much wind erosion for
thresholdvelocity soils and high for the high thresholdvelocity
nonsandydry soils in areaswhere the water table has not been
soils. The timing of high, drying winds after thorough soil
depleted, such as western Kansas and western Nebraska.
wettingis critical for loamy soils. If the wind occurswhile the
Unfortunately,in certain areasthe aquiferis virtually depleted,
soil is still wet, wind erosionis quitelikely to occur;if it follows
andirrigationis no longereconomically
viable.
aftersurfacecrustinghastakenplace,wind erosionis avoided. A
Minimum tillage. Methodshave been developed,such that
summaryof resultsappearsin Tables6 and7.
residue from the previous crop is not plowed under. Such
practicesmaximize the protectiveeffect of standingvegetative
Acknowledgments.This work has been supportedby The National
residue.
Oceanic and AtmosphericAdministration(NOAA) as part of the
12,662
GILLETTE:THRESHOLD
FRICTIONVELOCITIESFORAGRICULTURALDUSTEMISSION
National Acid PrecipitationAssessmentProgram (NAPAP). Harold Hess, S. L., Martian winds and dust clouds,Planet. Space Sci., 21,
1549-1557, 1973.
Dregne of Texas Tech Universityhas been extremelyhelpful in many
discussions
of the problem. Gary Nordstromgraciouslyallowed us Ishihara,T., and Y. Iwagaki, On the effect of sandstormin controlling
accessto the National ResourcesInventory of the USDA. The author
the mouthof the Kiku River, Bull. 2, DisasterPrey. Res. Inst., Kyoto
gratefully acknowledgesthe contributionof many other researchers,
Univ., Kyoto,Japan,1952.
whosenamesare arrangedalphabetically.AlbertBedardvery generously Iversen,J. D., R. Greeley, J. B. Pollack, and B. R. White, Simulationof
the Martian aeolian phenomenain the atmosphericwind tunnel,
provided data from the NOAA Boulder AtmosphericObservatory.
Proceedingsof the SeventhConferenceon SpaceSimulation,NASA
CharlesFensterof the Universityof Nebraska
wasextremely
helpfulin
Spec.Publ., NASA SP-336,pp. 191-213, 1973.
locatingsitesin westernNebraska. Robert Gibbensobtainedsoil crusts
for usfromthreesitesfor over1 yearat theUSDAJomada
Experimental Iversen,J. D., J. Pollack,R. Greeley,and B. White, Saltationthresholdon
Mars: The effect of interparticleforce, surfaceroughness,
and low
Range. StevenHolzheykindlyagreedto do 20 soil samples
at the U.S.
atmospheric
density,Icarus,29, 381-393, 1976.
Departmentof Agricultur13Soil ConservationService(SCS) National
Soil Laboratoryin Lincoln,Nebraska. PatriciaJacobberger,
of the Jackson,M. L., Soil ChemicalAnalysis,Prentice-Hall,EnglewoodCliffs,
N.J., 1965. (Now availablefrom M. L. Jackson,Departmentof Soil
SmithsonianAir and SpaceMuseum, locatedsites for us in westem
Science,Universityof Wisconsin,Madison,Wisc.).
Kansas.Rolf Kihl performedhigh-qualitylaboratorycharacterizations
at
field
the Universityof ColoradoInstitutefor Arctic and Alpine Research. Lettau,K., and H. Lettau,Experimentaland micrometeorological
studiesof dunemigration,Exploringthe World's DriestClimate,Inst.
Tom Nightingaleprovidedus with valuableinformation
and sampling
for Environ.StudiesRep. 101, editedby H. LettauandK. Lettau,pp.
sitesat the Universityof NebraskaHigh PlainsExperimentStation.
110-147, Univ. of Wisc., Madison, 1978.
NormanProchnowlocatedsamplingsitesfor us in North Dakotaand
providedus with valuable suggestions
and discussions.C. Murrell Lyles,L. andAllisonB., Wind erosion:The protectiverole of simulated
standingstubble,Trans.Agric.Eng., 19, 61-64, 1976.
Thompsonprovidedthe authorswith inspirationaldiscussions
and
Marshall,J., Drag measurments
in roughnessarraysof varyingdensity
suggestions
andseveralsamplinglocationsin westTexas.
anddistribution.Agric Meteorol.,8, 269-292, 1971.
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(ReceivedFebruary 17, 1988;
revised June 14, 1988;
acceptedJune 14, 1988.)