一些英文水土保持资料

Whatissoilerosion?

Soilisnaturallyremovedbytheactionofwaterorwind:such'background'(or'geological')soilerosionhasbeenoccurringforsome450millionyears,sincethefirstlandplantsformedthefirstsoil.Evenbeforethis,naturalprocessesmovedlooserock,orregolith,offtheEarth'ssurface,justashashappenedontheplanetMars.

Ingeneral,backgrounderosionremovessoilatroughlythesamerateassoilisformed.But'accelerated'soilerosion—lossofsoilatamuchfasterratethanitisformed—isafarmorerecentproblem.Itisalwaysaresultofmankind'sunwiseactions,suchasovergrazingorunsuitablecultivationpractices.Theseleavethelandunprotectedandvulnerable.Then,duringtimesoferosiverainfallorwindstorms,soilmaybedetached,transported,and(possiblytravellingalongdistance)deposited.

Acceleratedsoilerosionbywaterorwindmayaffectbothagriculturalareasandthenaturalenvironment,andisoneofthemostwidespreadoftoday'senvironmentalproblems.Ithasimpactswhicharebothon-site(attheplacewherethesoilisdetached)andoff-site(wherevertheerodedsoilendsup).

Morerecentlystill,theuseofpowerfulagriculturalimplementshas,insomepartsoftheworld,ledtodamagingamountsofsoilmovingdownslopemerelyundertheactionofgravity:thisisso-calledtillageerosion.

Soilerosionisjustoneformofsoildegradation.Otherkindsofsoildegradationincludesalinisation,nutrientloss,andcompaction.

Erosionprocesses

Soilmaybedetachedandmovedbywater,windortillage.Thesethreehoweverdiffergreatlyintermsof:

whereandwhentheyoccur

whathappenstotheareathatisbeingeroded(on-siteimpacts)

howfartheerodedsoilismoved,and

ifthesoilismovedawayfromtheplacewhereitwaseroded,whathappensasaresult(off-siteimpacts).

Watererosion

Soilerosionbywateristheresultofraindetachingandtransportingvulnerablesoil,eitherdirectlybymeansofrainsplashorindirectlybyrillandgullyerosion.

Rainsplash

WaterandsoilsplashedfollowingasingleraindrqprOmpacwEPP

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Rainmaymovesoildirectly:thisisknownas'rainsplasherosion'(orjust'splasherosion').Spashisonlyeffectiveiftherainfallswithsufficientintensity.Ifitdoes,thenastheraindropshitbaresoil,theirkineticenergyisabletodetachandmovesoilparticlesashortdistance.

Becausesoilparticlescanonlybemovedafewcentimetresatmostbythisprocess,itseffectsaresolelyon-site.Althoughconsiderablequantitiesofsoilmaybemovedbyrainsplash,itisallmerelyredistributedbackoverthesurfaceofthesoil(onsteepslopes,however,therewillbeamodestnetdownslopemovementofsplashedsoil).Thusamoredescriptivetermmightbe'rainsplashredistribution'.

Becauserainsplashrequireshighrainfallintensities,itismosteffectiveunderconvectiverainstormsintheworldsequatorialregions.Rainsplashisrelativelyineffectivewhererainfallswithalowintensity(e.g.becausetherainfallisoffrontalorigin),suchasinthenorth-westoftheUSAorinnorthernEurope.

Rillandgullyerosion

Rainfallmayalsomovesoilindirectly,bymeansofrunoffinrills(smallchannels)orgullies(largerchannels,toobigtoberemovedbytillage).Inmanypartsoftheworld,rillandgullyerosionisthedominantformofwatererosion.

Thatfractionoftherainfallwhichdoesnotinfiltrate(soakinto)thesoilwillflowdownhillundertheactionofgravity;itisthenknownasrunofforoverlandflow.Runoffmayoccurfortworeasons.Firstly,ifrainarrivestooquickly(i.e.withtoohighanintensity)forittoinfiltrate:therunoffwhichresultsisthenknownasinfiltrationexcessrunoff,orHortonianrunoff.Secondly,runoffmayoccurifthesoilhasalreadyabsorbedallthewateritcanhold(i.e.becauseitisfullysaturated,orifthesoilisfrozen).assaturationexcessrunoff.

Diffuseoverlandflow.Notetheraindropimpacts(Photo:

)

Runoffwhichresultsfromthissituationisknown

Asrunoffmovesdownhill,itisatfirstathindiffusefilmofwaterwhichhaslostvirtuallyallthekineticenergywhichitpossessedasfallingrain.Thusitmovesonlyslowly,hasalowflowpower,andisgenerallyincapableofdetachingortransportingsoilparticles.

Themicrotopography(i.e.small-scalepatternofirregularities)ofthesoil'ssurfacetendstocausethisoverlandflowtoconcentrateincloseddepressions,whichslowlyfill:thisisknownas„detentionstorage'or„ponding'.Boththeflowingwater,andthewaterindetentionstorage,protectthesoilfromraindropimpact,sothatrainsplashredistributionusuallydecreasesovertimewithinastorm,asthedepthofsurfacewaterincreases.Thereare,however,complexinteractionsbetweenrainsplashandoverlandflow.

Streampower(Q)

Ref:SoiiSciSocAmerJ.65(2001),page954

Detachmentandtransportprocessesassociatedwithvariationsinraindropandflowenergies.et=criticalraindropenergytocauseerosion.LineA—ecpriortoflow(increasingthroughcrustdevelopment).LineB=ecwhenflowoccurs(increasingdropenergyusedtopenetrateJ7ow)・Qgg-criticalstreampowerfortransportingloosematerial.

—criticalstreampowerfordetachingsoilfromsurfaceofsoilmatrix.RD-ST=raindropdetachment, splash

transport.RD—RIFT=raindrop detachment,

raindropinducedflowtransport.RD—FT—raindropdetachment^flowtransport.FD—FT=flowdetachment,flowtransport

Adiagramillustratingthecomplexinteractionsbetweenrainsplash,anddetachmentandtransportbyoverlandflow.Clickforalargerversion.(Source:PeterKinnell,UniversityofCanberra,Australia)

Ifraincontinues,theincreasingdepthofwaterwilleventuallyovertopthemicrotopographicdepressions.Overlandflowthatisreleasedinthiswayislikelytoflowdownhillmorequicklyandingreaterquantities

(i.e.possess

moreflowpowerasaresultofitskineticenergy),andsomaybeabletobegintransportingandevendetachingsoilparticles.Whereitdoesso,thesoil'ssurfacewillbeloweredslightly.Loweredareasformpreferentialflowpathsforsubsequentflow,andtheseflowpathsareinturnerodedfurther.Eventually,thispositivefeedbackresultsinsmall,well-definedlinearconcentrationsofoverlandflow(„microrills'or„traces').

Inmanycases,individualmicrorillsbecomeineffectiveovertimeduetosedimentation.Asubset,however,growfurthertobecomerills;andasmallersubsetmaygoontodevelopintogullies.Thisprocessof„competition'betweenmicrorillsandrillsleadstotheself-organizedformationofnetworksoferosionalchannels(dendriticonnaturalsoilsurfaces;constrainedbythedirectionoftillageonagriculturalsoils),whichformefficientpathwaysfortheremovalofwaterfromhillslopes.Itisinsucherosionalchannelsthatwatererosionalsooperatesmosteffectivelytodetachandremovesoilbyitskineticenergy.Inmostsituationserosionbyconcentratedflowisthemainagentoferosionbywater.

Theflow-dominatederosionalchannelsareseparatedbyinterrillareaswherethedominantprocessesarerainsplashanddiffuseoverlandflow;however,boundariesbetweenrillandinterrillareasarebothill-definedandconstantlyshifting.

Insomecircumstancessubsurfaceflowmaybeimportantindeterminingwherechannelerosionwillbeginanddevelop(e.g.atthebaseofslopes,andinareasofverydeepsoilssuchastropicalsaprolites).Meltwaterfromthawingsnowoperatesinabroadlysimilarwaytorain-derivedoverlandflow,detachingandtransportingunfrozensoilinareasofconcentratedflow.Snowmelterosionis,though,lesswellstudiedandlesswellunderstood.

Largerills(possiblybigenoughtobecalledgullies?)onan

erodinghillslope.(Sourceofphoto:unknown)

Aserosionalchannelsincreaseinsize(i.e.growtobecomelargerillsandgullies),processessuchasgravitationalcollapseofchannelwallsandheadsincreaseinimportance.Runoffandsedimentfromrillsandgulliesmaybemovedintoditches,streamandrivers,andsotransportedwellawayfromthepointoforigin.However,sedimentmayalsobedepositedwithintherillorgully,orbeyondtherillorgullysconfinesinadepositionalfan,atlocationswherethegradientslackens.Hereitmaybestoredforavariableperiodoftime,possiblybeingreworkedbytillageactivity,untilasubsequenterosioneventisofsufficientsizetore-erodethestoredsediment.Itmaythenberedepositedfurtherdownstream,ormakeitswayintoapermanentwatercourseandthencetolakeorocean.

Winderosion

Tillageerosion

Soilerosioninthepast

Erosionofsoilbywaterandwindhasbeenoccurringnaturallysincethefirstlandplantsformedthefirstsoil,duringtheSilurianPeriod.Acceleratederosionis,fromageologicalperspective,ofveryrecentorigin;yetonahumantimescale,

acceleratederosionisold.Thereisconsiderablearchaeologicalevidencefrommanypartsoftheworldthatacceleratederosionbywater(inparticular)isoftenassociatedwithearlyagriculture.

Inascientificcontext,watererosionsassociationwithunwiseagriculturalpractices

wasfirstnotedwithinduringtheearlydecadesofthe20thcenturybypioneersofsoilconservationsuchasHughHammondBennettintheUSA,andsubsequentlybyworkersinotherpartsoftheglobe.

Duringtheperiodofcolonialism,theimposedadoptionofEuropeanagriculturalmethodsfrequentlyledtoacceleratederosionindevelopingcountries.There,theproblemoftencontinuestothepresentday.

AgullyinIndiana,USA,inthe1(S30srceofphoto:unknown)

Inthelastfewdecadesofthe20thcentury,therewasaworlwidemovetowardsintensiveagriculturaltechnologies.Thesefrequentlyleavethesoilbareduringtimesofheavyrainfall.Asaresult,previouslyproblem-freeareasoftheworld,suchasnorth-westEurope,begantoexperiencenotableincreasesinwatererosion.

Theextentofsoilerosion

Despitetheglobalnatureoftheproblemoferosionbywater,eventodaywedonothavegoodinformationregardingtheglobalextentoferosionbywater.Dataontheseverityoferosionisalsooftenlimited.

TheGLASODstudyestimatedthataround15percentoftheEarth'sice-freelandsurfaceisafflictedbyallformsoflanddegradation.Ofthis,acceleratedsoilerosionbywaterisresponsibleforabout56percentandwinderosionisresponsibleforabout28percent.

Thismeansthattheareaaffectedbywatererosionis,veryroughly,around11millionsquarekm.,andtheareaaffectedbywinderosionisaround5.5millionsquarekm.

Theareaaffectedbytillageerosioniscurrentlyunknown.

Becausesoilisformedslowly,itisessentiallyafiniteresource.Theseverityoftheglobalerosionproblemisonlynowbecomingwidelyappreciated.

TheGLASODestimateofgloballanddegradation:notethatthisincludesallformsofsoildegradation,notjusterosion.(FromUNEP-GRID)

Erosionlittleandlarge

Oneofthethingswhichmakessoilerosiondifficulttounderstand—andsotopredictandcontrol—isthatitisaffectedbybothcommonandrareevents,andsomustbestudiedoverbothshortandlongtimespans.Erosionisalsoaffectedbyfactorsonverysmallandverylargespatialscales,andhasitsimpactsoverasimilarlywiderangeofspatialscales.

Thetimescalesoferosion

Soilerosionoccursbothincrementally,asaresultofmanysmallrainfallorwind-blowevents,andmoredramatically,asaresultoflargebutrelativelyrarestorms.Itisthelargestormswhichproducethebighard-to-misserosionalfeatuessuchasdeepgullies.Butwhileerosionduetosmallcommoneventsmayappearinsignificantonthefield,itscumulativeimpact(bothontheerodingfield,andelsewhere)may,overalongtimescale,besevere.

Thespatialscalesoferosion

Watererosion'scomplexhierarchyofprocessesmeanthaterosionbywateroperates(andisstudied)overawiderangeofspatialscales.Rainsplashredistributionandtheinitiationofmicrorillsandrillsoccuratascaleofmillimeters.Rillerosiononagriculturalhillslopesoperatesatascaleofmeterstotensofmeters,whilegullyerosioncanoccuronascaleofhundredsofmeters,orevenkilometers.Theoffsiteimpactsoferosioncanaffectverylargeareas,sometimeshundredsoreventhousandsofsquarekilometers.

Ateveryspatialscale,however,erosionishighlypatchy.Eveninareasofsevereerosion,ratesofsoillosscanvarygreatlyfrompointtopointonthelandscapeasthevagariesoftopographyandlanduseconcentrateerosiveflowsonawiderangeofspatialscales.Obviouserosioninonefieldcanbefoundside-by-sidewithvirtuallyuntouchedareas;andwithinanerodedfield,theseverityoferosioncanvarymarkedly.

On-siteeffectsofsoilerosion

Themainon-siteimpactisthereductioninsoilqualitywhichresultsfromthelossofthenutrient-richupperlayersofthesoil,andthereducedwater-holdingcapacityofmanyerodedsoils.Inaffluentareasoftheworld,acceleratedwatererosion'son-siteeffectsuponagriculturalsoilscanbemitigatedbyincreaseduseofartificialfertilizers;howeverthisisnotanoptionformuchoftheearth'spopulation.

Erosion'sremovaloftheupperhorizonsofthesoilresultsinareductioninsoilqualityi.e.adiminutionofthesoil'ssuitabilityforagricultureorothervegetation.Thisisbecausetheerodedupperhorizonstendtobethemostnutrient-rich.Also,becausethefinestconstituentsoferodedsoiltendstobetransportedfurthest,erodedsoilsbecomepreferentiallydepletedoftheirfinerfractionovertime;thisoftenreducestheirwater-holdingcapacity.Inotherwords,“Erosionremovesthecreamofthesoil”.Increaseduseofartificialfertilizersmaytoanextent,andforatime,compensateforerosion-inducedlossofsoilqualitywhereeconomiccircumstancesarefavorable.Thisisnotusuallyfeasibleindevelopingcountrieshowever.Lossofsoilqualityisalong-termproblem;globally,soilerosion'smostseriousimpactmaywellbeitsthreattothelong-termsustainabilityofagriculturalproductivity,whichresultsfromthethe'on-site'damagewhichitcauses.

Theon-siteimpactoferosion:severerillingonahillslopeat

RottingdeanontheUKSouthDownsin1987.Photo: John

Boardman

Cropsareparticularlyreliantontheupperhorizonsofthesoil,whicharethemostvulnerabletoerosionbywaterandwind.Inthissense,erosionremoves'thecreamofthesoil'.Agriculturaltillagealsoredistributessoil,resultinginthinnersoilsontopographicallyconvexareaswithinafield.

Thedamagingon-siteeffectsoferosion,intermsofdecreasedagriculturalyields,arewellknowninthedevelopingcountriesof

AfricaandAsia.Buteveninthedevelopedworldthereiscauseforconcern.WatererosionislocallysevereinAustralia,NewZealand,partsoftheUS,SouthernEurope,andEasternEurope(oftenasaresultoftheformerlargestate-controlledfarms).

Inerosion-proneareasofthemoreaffluentcountries,productivitymaybemaintainedintheshorttomediumtermbyincreasedfertiliserinput.Theeffectsoferosionarethusrarelyacknowledgedbyfarmersinrichercountries.Thisstrategyishoweverinfeasiblewithregardtoerosionindevelopingcountries.

Off-siteeffectsofsoilerosion

Inadditiontoiton-siteeffect,thesoilthatisdetachedbyacceleratedwaterorwinderosionmaybetransportedconsiderabledistances.Thisgivesriseto'off-siteproblems'.

Watererosionsmainoff-siteeffectisthemovementofsedimentandagriculturalpollutantsintowatercourses.Thiscanleadtothesilting-upofdams,disruptionoftheecosystemsoflakes,andcontaminationofdrinkingwater.Insomecases,increaseddownstreamfloodingmayalsooccurduetothereducedcapacityoferodedsoiltoabsorbwater.

China'sYangtzeRiverattheThreeGorges,inHubeiprovince.Note

thesediment-richwater.(Sourceofphoto:unknown)

AsatelliteviewofthedeltaoftheYangtzeRiverasit

Movementofsedimentandassociatedagriculturalpollutantsintowatercoursesisthemajoroff-siteimpactresultingfromerosion.Thisleadstosedimentationinwatercoursesanddams,disruptionoftheecosystemsoflakes,andcontaminationofdrinkingwater.Ratesoferosiondonothavetobehighforsignificantquantitiesofagriculturalpollutantstobetransportedoff-site.Thisisashorter-termimpactthanlossofsoilquality;inthemoreaffluentareasoftheworlditcanbethemaindriverforpresent-daysoilconservationpolicyinitiatives.Amoreminoroff-siteeffectcanoccurinsituationswhereerodedsoilhasadecreasedcapacitytoabsorbwater:increasedrunoffmayleadtodownstreamfloodingandlocaldamagetoproperty.

Anothermajoroff-sitedischargesintotheEastChinaSea.Thesedimentplumeisimpactresultsfromtheclearlyvisible.(Image:noaa) agriculturalchemicalsthat

oftenmovewitherodedsediment.Thesechemicalsmoveinto,andpollute,downstreamwatercoursesandwaterbodies.Whereinputsofagriculturalchemicalsarehigh-asinthemoreaffluentnations-costsofremovingsuchpollutantsfromdrinkingwatercanbeconsiderable.

Thereforetheon-siteimpactsofsoilerosionareapresent-dayproblemformanyofthedevelopingnations.Suchon-siteimpactswillbeaproblemonlyinthelongtermfuturefordevelopedareas:assuchtheyareoutsidetherelativelyshorttimehorizonwithinwhichtheirpolicymakerswork.

Intheshorttermhowever,erosion'soff-siteeffectscanbeanotableproblemfordevelopednations.Off-siteimpactsmaythereforebethemajordriverforpolicychangesinsuchcountries.

Soilerosion:whatwestilldon'tknow

Greaterunderstandingoftheoccurrence,processesandimpactsofsoilerosionbywater,windandtilla