Universal soil loss equation ; soil loss measurement

Universal soil loss equation, soil loss estimation, factors of USLE, its use and limitation, soil loss measurement by multi slot divisor and coshocton wheel sampler

1. Subject
Soil and Water Conservation Engineering
Topic
Soil Loss Estimation By Universal Soil Loss Equation
&
Soil Loss Measurement Techniques
by
Dr. Sanjay Singh Chouhan
Assistant Professor
JNKVV-College of Agriculture
Powarkheda, Hoshangabad
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2. Estimation of Soil Loss
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3. Estimation of Soil Loss
•The control of erosion is essential to maintain the productivity of soil and to improve or
maintain downstream water quality.
•Toestimatesoilerosion,empiricalandprocessbasedmodels(equations)areused.
•UniversalSoilLossEquation(USLE)isanempiricalequationusedtoestimateannualsoil
lossperunitarea.
•Estimatingsoillossisconsiderablymoredifficultthanestimatingrunoffasthereare
manyvariables,bothnaturalsuchassoilandrainfallandman-madesuchas
adoptedmanagementpractices.
•Thesoillossconsiderablydependsonthetypeoferosion.Asaresult,models,whether
empiricalorprocess-based,arenecessarilycomplexiftheyaretoincludetheeffectofall
thevariables.
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4. The Universal Soil Loss Equation (USLE)
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5. The Universal Soil Loss Equation (USLE)
•Thefiledsoillossestimationequationsdevelopmentbeganin1940inUSA.
•Basedonnearly10,000plotyearrunoffplotdata,WischmeierandSmithin1965
developedtheuniversalsoillossequation,whichwaslaterrefinedwithmorerecentdata
fromrunoffplots,rainfallsimulatorsandfieldexperiences.
•Itisthemostwidelyusedtoolforestimationofsoillossfromagriculturalwatershedsfor
planningerosioncontrolpractices.
•TheUSLEisanerosionpredictionmodelforestimatinglongtermaveragesofsoilerosion
fromsheetandrillerosionsfromaspecifiedlandunderspecifiedconditions.
•Itprovidesanestimateofthelong-termaverageannualsoillossfromsegmentsofarable
landundervariouscroppingconditions.
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6. •Theapplicationofthisestimateistoenablefarmersandsoilconservationadvisersto
selectcombinationsoflanduse,croppingpractice,andsoilconservationpractices,which
willkeepthesoillossdowntoanacceptablelevel.
•TodevelopedtheUSLE
➢40yearsdatawascollectedandanalysed
➢Studyplotoflength22.13mwith9%slopwasused.
➢Plotkeptincontinuesfellowcondition
➢Morethen2,50,000runoffeventat48researchstationin26statesofUSAwas
analysed.
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7. Theequation(USLE)ispresentedasbelow.
A = R x K x L x S x C x P
Where,
A=soillossperunitareainunittime,tha
-1
yr
-1
.
R=rainfallerosivityfactorwhichisthenumberofrainfallerosionindexunitsforaparticular
location.
K=soilerodibilityfactor-anumberwhichreflectsthesusceptibilityofasoiltypetoerosion.
L=slopelengthfactor,aratiowhichcomparesthesoillosswiththatfromafieldofspecified
lengthof22.6meters.
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8. S=slopesteepnessfactor,aratiowhichcomparesthesoillosswiththatfromafieldof
specifiedslopeof9%.
C=covermanagementfactor-aratiowhichcomparesthesoillosswiththatfromafield
underastandardtreatmentofcultivatedbarefallow.
P=supportpracticefactor-aratioofsoillosswithsupportpracticelikecontouring,strip
croppingorterracingtothatwithstraightrowfarmingupanddowntheslope.
ThefactorsL,S,CandPareeachdimensionlessratioswhichallowcomparisonofthe
siteforwhichsoillossisbeingestimatedwiththestandardconditionsofthedatabase.
Knowingthevaluesofrainfallerosivity,soilerodibilityandslopeonecancalculatethe
effectivenessofvariouserosioncontrolmeasureswiththepurposeofintroducinga
cultivationsysteminanareawithsoillosslimitedtotheacceptablevalue.
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9. Factors Associated With The USLE
1.RainfallErosivityFactor(R)
•Itreferstotherainfallerosionindex,whichexpressestheabilityofrainfalltoerodethe
soilparticlesfromanunprotectedfield.
•Itisanumericalvalueobtainedfromthelongfieldexperimentsthattheextentofsoil
lossfromabarrenfieldisdirectlyproportionaltotheproductoftworainfall
characteristics:kineticenergyofthestormandits30-minutemaximumintensity.
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10. 2.SoilErodibilityFactor(K)
•Thesoilerodibilityfactor(K)intheUSLErelatestotherateatwhichdifferentsoilserode.
•Undertheconditionsofequalslope,rainfall,vegetativecoverandsoilmanagement
practices,somesoilsmayerodemoreeasilythanothersduetoinherentsoilcharacteristics.
•ThedirectmeasurementofKonunitrunoffplotsreflectthecombinedeffectsofall
variablesthatsignificantlyinfluencetheeasewithwhichasoiliserodedortheparticular
slopeotherthan9%slope.
•Someofthesoilpropertieswhichaffectthesoillosstoalargeextentarethesoil
permeability,infiltrationrate,soiltexture,sizeandstabilityofsoilstructure,organiccontent
andsoildepth.
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11. Table.
Values of K for Several
Stations(Source: K.
Subramanya, 2008)
Basedonrunoffplotstudies,
thevaluesoferodibilityfactor
Khavebeendeterminedfor
useinUSLEfordifferentsoils
ofIndiaasreportedbySinghet
al.(1981).ValuesofKfor
severalstationsaregivenin
Table.
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Station SoilType
Computed
ValuesofK
Agra Loamysand,alluvial 0.07
Dehradun Dhulkotsilt,loam 0.15
Hyderabad Redchalkasandyloam 0.08
Kharagpur Soilsfromlateriterock 0.04
Kota Kotaclayloam 0.11
Ootakamund Laterite 0.04
RehmankheraLoam,alluvial 0.17
Vasad Sandyloam,alluvial 0.06

12. TopographicFactor(LS)
•Slopelengthfactor(L)istheratioofsoillossfromthefieldslopelengthunder
considerationtothatfromthe22.13mlengthplotsunderidenticalconditions.Theslope
lengthhasadirectrelationwiththesoilloss
•Steepnessoflandslopefactor(S)istheratioofsoillossfromthefieldslopegradientto
thatfromthe9%slopeunderotherwiseidenticalconditions.
•Theincreaseinsteepnessofsloperesultsintheincreaseinsoilerosionasthevelocityof
runoffincreaseswiththeincreaseinfieldslopeallowingmoresoiltobedetachedand
transportedalongwithsurfaceflow.
•ThetwofactorsLandSareusuallycombinedintoonefactorLScalledtopographic
factor.
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13. •This topographic factor is defined as the ratio of soil loss from a field having
specific steepness and length of slope (i.e., 9% slope and 22.13 m length) to
the soil loss from a continuous fallow land. The value of LS can be calculated
by using the formula given by Wischmeierand Smith (1962):
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Where,
L=fieldslopelengthinfeetand
S=percentlandslope.

14. CropManagementFactor(C)
•ThecropmanagementfactorCmaybedefinedastheexpectedratioofsoillossfroma
croppedlandunderspecificcroptothesoillossfromacontinuousfallowland,provided
thatthesoiltype,slopeandrainfallconditionsareidentical.
•Thesoilerosionisaffectedinmanywaysaccordingtothecropsandcroppingpractices,
suchasthekindofcrop,qualityofcover,rootgrowth,waterusebyplantsetc.
•Thevariationinrainfalldistributionwithintheyearalsoaffectsthecropmanagement
factor,whichaffectsthesoilloss.
•Thisfactorreflectsthecombinedeffectofvariouscropmanagementpractices.
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15. Table: Values of Crop Management Factors for Different Stations in India(Source: K Subramanya, 2008)
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Station Crop Soil Loss, t ha
-1
y
-1
Value of C
Agra Cultivated fallow 3.80 1.0
Bajra 2.34 0.61
Dichanhium annualtu 0.53 0.13
Dehradun Cultivated fallow 33.42 1.0
Cymbopogon grass 4.51 0.13
Strawberry 8.89 0.27
Hyderabad Cultivated fallow 5.00 1.0
Bajra 2.00 0.40

16. SupportPracticeFactor(P)
•Thisfactoristheratioofsoillosswithasupportpracticetothatwithstraightrowfarming
upanddowntheslope.
•Theconservationpracticeconsistsofmainlycontouring,terracingandstripcropping.
•Thesoillossvariesduetodifferentpracticesfollowed.
•FactorPfordifferentsupportpracticesforsomelocationsofIndiaispresentedinTable.
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17. Table: Different Values of
Support Practice Factor (P) for
Some Indian Locations.
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Station Practice Factor P
Dehradun Contour cultivation of maize0.74
Up and down cultivation1.00
Contour farming 0.68
Terracing and bunding in
agricultural watershed
0.03
Kanpur
Up and down cultivation of
Jowar
1.00
Contour cultivation of Jowar0.39
Ootacamund Potato up and down 1.00
Potato on contour 0.51

18. Use of USLE
There are threeimportant applications of the universal soil loss
equation. They are as follows:
1.It predicts the soil loss.
2.It helps in identification and selection of agricultural practices.
3.It provides the recommendations on crop management
practices to be used.
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19. Various Erosion Classes In India
Based on 21 observation points and 64
estimated erosion values of soil loss
obtained by the use of USLE at locations
spread over different regions of the
country, soil erosion rates have been
classified into 6 categories.
Areas falling under different classes of
erosion are shown in Table.
19
Range
(Tones/ha/year)
Erosion ClassArea (km
2
)
0-5 Slight 801,350
5-10 Moderate 1,405,640
10-20 High 805,030
20-40 Very high 160,050
40-80 Severe 83,300
>80 Very severe 31,895

20. Limitations of Universal Soil Loss Equation
Theequationinvolvestheprocedureforassigningthevaluesofdifferentassociatedfactors
onthebasisofpracticalconcept.Therefore,thereispossibilitytointroducesomeerrorsin
selectionoftheappropriatevalues,particularlythosebasedoncropconcept.
ThefollowingaresomeofthelimitationsoftheUSLE:
Empirical
•TheUSLEistotallyempiricalequation.Mathematically,itdoesnotillustratetheactual
soilerosionprocess.Thepossibilitytointroducepredictiveerrorsinthecalculationis
overcomebyusingempiricalcoefficients.
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21. PredictionofAverageAnnualSoilLoss
•Thisequationwasdevelopedmainlyonthebasisofaverageannualsoillossdata;henceits
applicabilityislimitedforestimationofonlyaverageannualsoillossofthegivenarea.This
equationcomputeslessvaluethanthemeasured,especiallywhentherainfalloccursathigh
intensity.
Non-computationofGullyErosion
•Thisequationisemployedforassessingthesheetandrillerosionsonlybutcannotbeusedfor
thepredictionofgullyerosion.Thegullyerosioncausedbyconcentratedwaterflowisnot
accountedbytheequationandyetitcancausegreateramountofsoilerosion.
Non-computationofSedimentDeposition
•Theequationestimatesonlysoilloss,butnotthesoildeposition.Thedepositionofsedimentat
thebottomofthechannelislessthanthetotalsoillosstakingplacefromtheentirewatershed.
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22. Soil Erosion Measurement
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23. JNKVV-College of Agriculture, Powarkheda 23
Soil loss Measurements Techniques
Measurement of Soil Loss By
1.Multi-Slot Divisor (MSD)
2.Coshocton Wheel Sampler (CWS)

24. Multi-Slot
Divisor
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25. Multi slot divisor assembles
following components:
1.Runoff collector or
Runoff tank
2.Stilling tank
3.Divisor box (Slot Divisor)
4.Collecting tank.
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26. Measurement of Soil Loss By Multi-Slot Divisor (MSD)
•Themulti-slotdivisorsWasdevelopedbyR.V.Geigformeasuringrunoffandsoillossfrom
runoffplotsorsmallareas.
•Itisanintegralcomponentofrunoffplot.Itsconstructionalfeatureconsistsofseveral
slotsofsamedimension.
•Theslotsarefittedattheendofdivisorbox.
•Thewaterflowfromrunoffplotleadstowardsthedivisor.
•Thedivisordividestheentirewaterflowintodifferentparts.
•Theuniformityinflowdivisiondependsonuniformityofflowvelocity.Asforaspossible
theflowvelocityshouldbeuniformindivisorbox.
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27. •Variationsinflowvelocityalsocauseunequaldistributionofsedimentconcentrationin
dividedwaterflow,asresultthereisintroductionoferrorinmeasurement.
•Inoperation,thewaterflowisallowedtopassthroughthedevice;andflowfromoneof
theslotsisdirectedintothecollectingtank;andwaterfromtheremainingslotsisallowed
todrainout.
•Thewatersamplesaretakenfromthecollectedwaterinthecollectingtankforanalysis
purposes.
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28. Collection of Water Sample From
Centre Slot of A Multi-slot Divisor
for Lab Analysis.
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29. Coshocton Wheel Sampler

30. JNKVV-College of Agriculture, Powarkheda 30
Coshocton Wheel Sampler

31. Measurement of Soil Loss By Coshocton Wheel Sampler (CWS)
•TheCoshoctonwheelsiltsampler(developedatanexperimentalstationataplace
knownasCoshoctoninUSA)canbeusedforcollectingwatersamplesfromtherunoff
passingthroughamulti-slotdivisorordirectlythroughameasuringdeviceliketheH-
flume.
•Thedeviceconsistsofawheelmadeofthincircularplatetowhicheightvanesare
attached.Asamplingheadwithanarrowopeningcalledslotismountedradiallyonthis
wheel.Theslotiskeptraisedslopingdowntowardstheoutsideofthewheel.
•Waterdischargefromthechannelfallsona-waterwheel,whichisinclinedslightly
thereforecausingthewheeltorotate.Anelevatedsamplingslotmountedonthewheel
extractsasampleofwater-sedimentmixturewiththerepresentativeproportion.
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32. •With each revolution of the wheel, the slot cuts across the jet from the flume and
extracts a small portion of the flow. The extracted portion is collected in the storage tank
through the conduit.
•The H-Flume and the Coshocton sampler in combination are used for runoff and
sediment estimation from small agricultural watersheds.
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33. Coshocton
Wheel
Sampler
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34. JNKVV-College of Agriculture, Powarkheda 34

35. Thanks for listening…..
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