Soil erosion in the West African Sahel: a review and an
Brief Description
erosionisnecessarilylinkedtohighlevelsofpopulation ortopoverty,claimswhichstillmakefrequentappearan- cesintheliterature(e.g.Baidu-ForsonandNapier,1998; theseerrorsareimpossibletoevaluate,especiallyinacli- matewhichisasvariableasthatoftheSahel,andonly someofthesedi cultiesareavoidedbytheuseofarti cialrainfalltosimulatedi erentrainfallintensities,or windtunnelstosimulatedi TheCaesium-137( Cs)techniquereliesonthefalloutfromatmo- sphericbombtestinginthemid-1960s.Thisproducedthearti isotopeCaesium-137,whichwasdistributedoveralltheearth ssurfa- ces.Inbrief,iflesscaesiumismeasuredinatopsoilthanis therehasbeenerosion,andthiscanbequanti ed.Therearemany availabledescriptionsofgreatercomplexities,forexampleinChappell (1998),andthepapersreferencedthere. notlistthelastofthese,butaddedfourinNigeriaand threeinGhana,givingamaximumof16forthewholeof WestAfrica.Ofthese,perhaps vecouldbesaidtobein theSahelandonemorecouldbeaddedfromHeusch workcitedabove.Thelongestperiodofcontinuous measurementinRoose sreviewwas14yr(Roose,1977a). Astowinderosion,theonlysourceofinformation untilthelastthreeorfouryearshasbeenfromvisibility data(i.e.,theextenttowhichairbornedustcarriedby windobscuresvision fromwhichdustinesscanbe frequencyofduststormsincreasedinthe1970sand 1980s,butneithertheynorthedustcollectionsallow ratesoferosiontobecalculated,becausethesourcesof wind-borneparticlesaredi culttopinpoint.Itispos- sibleto dustsources(Littmann,1991a; estimatesoferosionrateswilleverbepossiblefrom measuresofcollecteddust.Moreencouragingly,results arenowemergingfromresearchwhichismeasuring insituwinderosion,particularlyinsouthwesternNiger andwithitsrelevancetoSaheliansmallholders. EvenintheUnitedStates,wheretheywerecalibrated withsubstantialempiricaldata,somelargeassumptions hadtobemade.Foralltheveryrealvaluesoferosion plotsintheSahelprovidingwell-controlled,base-line dataforthelocalcalibrationofmodels,theyaremani- festlyandgrosslyunrepresentativeofthisvastareasin spaceorintime.Second,themodelsmustbeusedwith greatcare,ascautionedbytheirdesignersandtheir critics(e.g.Wischmeier,1976).TheUSLEandWEQand theirrevisionsarealmosttotallyempirical;theoutcome ofaverylargenumberofobservedstatisticalcorrela- tions.Manyofthekeyphysicalprocessesoferosionare evennownotwellunderstood(Rose,1998).Thisis particularlysofortheresistanceofthesoilsurface toerosion(theso-called factorintheUSLE),which canoftenbeinferredonlyasaresidualonceallthe otherfactorshavebeencontrolled;ithasprovedvery culttomeasuredirectly(Lal,1988b).Rooseand Sarrailh(1990),thoughgenerallysupportiveofthe USLE,found tovarywidelyinexperiments onindividualsoils. isstronglydependenton ThereisanotherwayinwhichmostSaheliansoilshave distinctivesoil-lifecharacteristics.Theyarebebeingre- placedinsigni cantmeasurebyinputsofdust(mostofit intheHarmattanwinds),whichmaysupplythemajor ectsofrapidremovalofsoil,whichisnecessaryto achieveexperimentalresults,comparedtothee ectsof slowerlossinaneroding eld,wheretopsoilfertilityhas timetorecoverfromerosion(Stocking,1984).Another 2.8.Thee aluationofimmediateimpact ItisherethatLavigneDelville sdictumhasitsstron- gestrelevance:immediateimpactscanonlybeevaluated beappliedtothesortsofagriculturepracticedbysmall- holders.Theapproachimpliesthaterosionshouldbe controlledbythetechnicaladjustmentofabstruse(and notalwayswellunderstood)qualitieslikesoilerodibility (theelusive factor),thecroppingpattern( ),andthe conservationpractice( )(bothmeasuredbyanindex relatedtoMid-Westernfarmingsystems)(Rooseand Sarrailh,1990;Roose,1993;Lal,1993).Thismightjustbe possibleinlarge-scalecommercialfarming,butsmall- holdersmanagetheirsystemsmuchmoreclosely,and, moreimportantly,theyadoptmorecomplexsystemsof riskmanagementandriskavoidance. Webelievethat,givenafreshapproach,theoutlook forresearchontheimmediateimpactsoferosionismuch brighterthanthisreviewmightsuggest,butonlyifitis placedinasmallholdercontext,asourdiscussionnow 3.Soilerosionandthesmallholder 3.1.Thesmallholderperspecti ThedebateaboutsoilerosionintheSahelisslowlybut surelybeingfocusedontheconcernsofthosewhomaybe itsvictims(Mortimore,1998).Thepurelyexperimental, technicalapproachhashadfewadherentsoverthelast decade,evenamongthescientistswhohadbeenmost deeplyinvolved(Pieri,1989;Roose,1993;Stocking,1986; Lal,1998).Manyforceshavedriventhismajorchangeof opinion.Thestrongestistheincontestablefailureto interestAfricanfarmersinsoilconservationprogrammes basedonthestrictlynatural capproach(seede BatterburyandKunzewerebothattachedasresearchstudentsto asoilandwaterconservationprojectinBurkinaFasointhe1990s.Our briefwastounderstandtheimpactsoflanddegradation,andthe impactsofprojectlandrehabilitationtechniques.Theneedforsoil conservationintheregionwasdeemedself-evident,andwasrarely farmers.Othertoolsweredevelopedtoaccountfordeci- sion-makingincontext,notablymulti-criteriaanalysisof long-terminvestmentinsoilconservation,andtech- niquestosensitisesurvey-deriveddatatogenderand Fig.1.LocationmapofFandouBe orientedmoretoindigenousconceptionsoftheenviron- ment,asintheworkofZimmerer(1994)inBolivia, stberg(1991)inEastAfrica,andLindskogandTen- gberg(1994)intheSahel.Athirdtype,ofmoreimmediate relevancetopracticalissuesofpolicyformulation,isthe actionresearch thataccompaniesdevelop- mentwork whereresearchismulti-authored, participatory,andlinkedtode ned,user-managed outcomessuchasnewextensionprogrammesorsoil Thevillageisinthe siteofHAPEX-Sahel(The AtmosphericPilotExperiment,aprogrammerunby post-colonialgovernancehaso eredandwithdrawn agriculturalassistanceandarangeoflocalservices. Fig.2.GIS-derivedmapshowingthelocationofsampled Table1 Variablescorrelatedtosoilerosionatthe95%con dencelevelusingmultipleregressionanalysis PredictorCoe cientStDev Constant40.8151.98620.550.000 Familylabour 1.98230.5524 3.590.004 Paidlabour 2.15480.84632.550.027 Distanceto eld(km) 4.5351.237 3.670.004 Equation:Erosion(tha 1.98Paidlabour 2.15Familylabour 4.54Distanceto eld(km), 68.0%for15 Note:Removedvariablesincludecorrelationsofwealthandtenure(tolabour)andsoilclassandtimeunderfallow(todistance). Averagenumberoffamilymembersworkingonthe eldoverthe1997season. Averagenumberofpaidlabourersworkingonthe eldoverthe1997season. Distancefromthehomesteadtothe .
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