Scientific registration n o : 2495 Symposium n o : 31 Presentation: poster. MAINAM Félix, ZINCK J. Alfred

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1 Sientifi registrtion n o : 2495 Symposium n o : Presenttion: poster Erodiility ssessment of seleted soils in northern Cmeroon using field rinfll simultor Estimtion de l'érodiilité de sols du nord Cmeroun à l'ide d'un simulteur de pluie MAINAM Félix, ZINCK J. Alfred ITC, PO Box 6, 75 AA Enshede, The Netherlnds Soil erosion y wter is the dethment nd trnsport of soil mteril used y the tion of wter. Soil erosion uses not only detrimentl on-site effets, suh s soil produtivity deline y removl of plnt nutrients nd orgni mtter, inresed omption nd inresed runoff, ut lso hrmful off-site effets, in prtiulr silting of reservoirs, lkes, irrigtion nls, strems nd ostl res (FAO, 99). Mny soil loss predition models hve een developed, whih differ from one nother ording to their ojetives nd sles. Soil erosion is studied t the wtershed level to ssess denudtion rtes of mjor river sins, mountin systems, eologil regions nd even ontinents. Studies re lso onduted t the plot level to evlute the stndrd erodiility. Finlly, soil erosion reserh is done t the miro-plot level ( to 2 squre meters) to investigte proesses suh s splsh nd rill formtion (Ll, 988). The present study intends to ontriute to the understnding of soil erosion t the miro-plot sle y investigting sptil nd temporl vritions of the soil surfe elevtion points s ffeted y soil properties, runoff nd trnsport of dethed prtiles. METHOD AND TECHNIQUES Nine sites, representing the regionl soil lsses (Alfisols, Vertisols nd Plnosols) with different erosion lsses, were sujeted to rtifiil rinfll in semi-rid re of northern Cmeroon lose to the ity of Mrou. Three erosion lsses were identified for eh soil lss, nmely slightly eroded (), modertely eroded (2) nd severely eroded () (tle ). A field rinfll simultor, developed y Asseline nd Vlentin (978), ws used for studying erosion nd hydrologil proesses t one-squre-meter plots. Three rin showers were simulted t different intensities nd durtions (tle 2). Plots were re nd ploughed with hnd hoe. The method llowed the expliit onsidertion of ftors determining oth runoff nd sediment onentrtion in detil. Smples of splshed-off mteril nd runoff were tken every ten minutes throughout eh

2 simulted rin. Soil surfe roughness ws ssessed y mesuring surfe elevtion points with ruler, from seline referene downwrds to the soil surfe, long trnsets 5m prt, on m y m plots. A first shower or pre-wetting rin (rin ) ws pplied to the re soil surfe nd, fter hnd-ploughing, the rle mirotopogrphy ws reorded. Mesurement of the elevtion points ws performed fter eh of two onseutive simulted rins (rin 2 nd rin ). Geosttistis ws pplied to produe mps displying temporl nd sptil vritions of the elevtion points using liner vriogrm modelling nd interpoltion through kriging. Tle : Surfe lyer fetures of different soil nd erosion lsses Soil lsses Surfe lyer Erosion lsses fetures Slightly eroded Mod. Eroded Sev. Eroded Alfisols Thikness Color Struture Texture Orgni mtter 6 m drk rown loky-grnulr sndy lom.5 % m light rown loky-mssive sndy ly.5 % 5 m red mssive ly % Plnosols Vertisols Thikness Color Struture Texture Orgni mtter Thikness Color Struture Texture Orgni mtter 5 m drk gry loky-grnulr sndy lom.5 % 5 m very drk gry orse prismti ly % 2 m light gry loky-mssive sndy ly.5 % 5 m drk gry fine prismti ly.5 5 m gryish rown mssive ly % m light gry loky-mssive sndy ly % Tle 2: Simulted rinfll hrteristis Rin Rinfll hrteristis No Imx(mm/h) Totl durtion(minute) Totl rin(mm) Remrks no mesurement Imx = mximl intensity in minutes durtion. CHANGES IN RUNOFF AND EROSION COMPONENTS Runoff nd erosion prmeter results for eh simulted rin on eh experimentl plot re presented in tle. Dt re ompred to highlight nd evlute the hnges in hydrology nd erosion ehvior ourring with time in eh erosion lss of the seleted soil lsses. Vritions etween rins, soil lsses nd erosion lsses re nlyzed. Vritions etween rins Runoff hrteristis vry onsiderly with time. Drsti vritions were found on severely eroded Vertisols. Pek runoff nd runoff oeffiient vlues re 7 mm/h nd 5.7% 2

3 fter the seond rin, ut inrese to 69 mm/h nd 72.8% during the third rin. No vritions with time were oserved on slightly eroded Vertisols nd modertely eroded Alfisols. While there is no runoff on the former, the lter exhiits reltively onstnt runoff oeffiient etween the seond rin (75.5%) nd the third rin (76.7%). Other soils show intermedite vritions. Regrding soil loss, three different situtions our with time: inrese, derese nd onstnt soil loss. Most of the experimentl plots on ll erosion lsses of Plnosols, modertely nd severely eroded Vertisols, nd severely eroded Alfisols, show n inrese in soil loss with time. Tle : Surfe runoff nd erosion prmeters Soil Erosion Rin 2 Rin lsses lsses qp Q Cr E qp Q Cr E Alfisols Plnosols Vertisols qp = pek runoff rte (mm/h), Q = runoff depth (mm), Cr = runoff oeffiient (%), E = soil loss (g/m2). As rinfll intensity nd durtion inrese from the seond to the third rin, there is positive orreltion etween erosion nd rinfll hrteristis. Sustntil inrese in soil loss ws found prtiulrly on severely eroded Vertisols nd severely eroded Plnosols. In ontrst, some soils show derese in soil loss despite sustntil inrese in rinfll hrteristis. This negtive orreltion my e ttriuted to seletive erosion nd onentrtion of orse mteril on the soil surfe or to diminishing roughness nd progressive rusting. Slightly eroded Vertisols did not produe soil loss. The differenes in response suggest tht two proesses loking infiltrtion hve tken ple simultneously t the soil surfe during the simulted rins. Firstly, n inrese of soil moisture in the topsoil lyer nd the filling of mirotopogrphi depressions t the soil surfe elerte the irultion of the overlnd flow. Seondly, surfe seling restrits infiltrtion nd enhnes runoff. The onsequene of this is tht lter runoff ehvior expresses minly rinfll hrteristis. Levy et l.(994) reported similr results. Vritions etween soil lsses The totl runoff mount produed on different soil lsses ws ompred. The order of inresing reltive suseptiility to runoff prodution of soil lsses hnges ording to erosion lsses. For instne, on slightly eroded soils the order is: Vertisols < Alfisols < Plnosols. But on severely eroded soils the order is: Alfisols < Vertisols < Plnosols. Soil loss results show similr hnges. The order of inresing reltive suseptiility to erosion

4 is: Vertisols < Alfisols < Plnosols on slightly eroded soils, nd Alfisols < Plnosols < Vertisols on severely eroded soils. Similr results were found y Seiny (99). Vritions etween erosion lsses As expeted, different erosion lsses show differenes in soil erosion. On Plnosols nd Vertisols, the order of inresing severity of runoff prodution in the lst rin is indited y: slightly eroded < modertely eroded < severely eroded. But on Alfisols, the order is: severely eroded < slightly eroded < modertely eroded. The generl tendeny is tht more eroded soils produe more erosion. This my vry over onsiderle rnges, suh s on Vertisols where no soil loss ws oserved on slightly eroded soils ut 6 g/m2 of soil loss ws mesured on severely eroded soils. Thee (987), Seiny (99) nd Mhop et l.(995) reported similr orders of mgnitude. CHANGES IN THE SOIL SURFACE MICROTOPOGRAPHY Vrition of the soil surfe elevtion points From the vrition of the soil surfe elevtion points with inresing rinfll, in terms of numer of events, durtion nd intensity, three outome situtions n e distinguished to highlight elevtion hnges ourring etween ploughing nd rin 2 nd etween rin 2 nd rin, respetively: () moderte derese followed y moderte derese, (2) sustntil derese followed y smll derese, nd () moderte derese followed y sustntil inrese (tle 4). Eh new mirotopogrphi onfigurtion is strongly relted to the reltive strength of the soil surfe ggregtes. Tle 4: Soil surfe elevtion point hrteristis Soil Erosion Tretment Men Stndrd Elevtion differene lsses lsses devition - - Alfisols 2 Plnosols

5 Vertisols = surfe elevtion fter ploughing, = surfe elevtion fter rin 2, = surfe elevtion fter rin, Men = men elevtion, vlues in mm. Moderte derese followed y moderte derese: eroding soil surfe ggregtes During rin 2 nd rin, moderte derese of the soil surfe elevtion points with time, ompnied y sustntil soil loss, ws oserved on severely eroded Alfisols. The positive orreltion etween the derese of the elevtion points nd the mount of rin reflets very eroding soil surfe ondition. In ddition, these soils exhiit n inresing nd higher stndrd devition, whih mens lrge vrition of the elevtion points due to seletive erosion res. Modertely nd severely eroded Vertisols nd Plnosols show similr ehvior. Sustntil derese followed y smll derese: modertely resisting soil ggregtes A sustntil derese of the elevtion points, ut ompnied y reltively low soil loss, ws oserved on slightly eroded Alfisols nd slightly eroded Plnosols fter pplition of rin 2. This denotes tht most sediment hs een redistriuted over the plot nd did not reh the plot outlet. Despite greter mount of wter pplied in rin, the elevtion of the points on slightly eroded Alfisols nd ll erosion lsses of Plnosols did not derese signifintly. This my indite tht flttening of the soil surfe, ompnied y rusting, hs tken ple during the previous rin. Stndrd devition dereses to reltively low vlues etween onseutive rins, refleting uniform removl of soil mteril t the terrin surfe. Lower soil loss is evidene of rust formtion, whih tends to inhiit soil dethment. Similr ehvior hs een reported y Levy et l. (994). Moderte derese followed y sustntil inrese: resisting soil surfe ggregtes On slightly eroded Vertisols, the pplition of rins uses n inrese of the elevtion points, refleting the presene of swelling lys (smetites). Dry lods sor wter, whih uses their expnsion. Swelling lods my promote resisting soil surfe. This trend n lso e tred on modertely nd severely eroded Vertisols, where only smll hnges ffet the elevtion points etween rins. The vrition of the soil surfe elevtion points with inresing rinfll is mesure of the reltive soil suseptiility to erosion. Suseptiility inreses ording to the following order of hnges in the elevtion points: inrese < smll derese < sustntil derese. Behvior differenes re ontrolled y differenes in soil properties, in prtiulr those properties tht ontrol permeility nd regulte infiltrtion nd peroltion. On Vertisols, 5

6 for instne, rks sor the rin wter nd it is only fter losing of the rks tht runoff strts. The horizon sequene in Alfisols nd Plnosols inludes orse-textured topsoil lyers (Ap nd A2) ove hevy, hrd nd strutureless susoil lyers (Btd nd Bt). Permeility nd runoff depend on the lest permele horizon in the lyer sequene. Bt horizons promote sturtion overlnd flow (de Ploey, 986; Kuipers, 986). Contrsting lyers result in higher runoff prodution, whih ontriutes to the deteriortion of the surfe struture nd the struturl stility. The differentil ehvior of the erosion lsses n e ttriuted to the modifitions tht ffet the topsoil hrteristis, suh s ly nd orgni mtter ontents, nd the thikness of the topsoil lyers ording to erosion degrees (Ll, 988). More eroded soils (Alfisols nd Plnosols) hve higher ly ontent (>2 %) in the surfe horizons thn less eroded ones. A higher ly ontent promotes stronger ohesion of the soil surfe ggregtes, whih resist to slking. Resistnt lods mintin the depression storge, enhne infiltrtion nd retrd erosion. Less eroded soils present thiker topsoil lyers (>5 m) nd higher orgni mtter ontent (.5-%) thn more eroded ones, providing lrger wter storge pity. But s the rin proeeds, sturtion overlnd flow ours nd the pprent resistne of eroded soils to runoff nd erosion dereses onsistently. On modertely nd severely eroded Vertisols, rks re less effiient in soring wter. The vrition of the elevtion points etween onseutive rins n e onsidered s n importnt prmeter for monitoring the soil erosion proess. Mwender nd Feyen (992) hve stressed the relevne of this prmeter. The rte of derese of the elevtion points my influene the distriution of the erosion fetures on the relief. Smll rtes use pronouned erosion fetures only downslope, wheres high rtes generte erosion fetures distriuted ll over the relief. Sptil distriution of sedimenttion nd erosion res Vriogrm prmeters were determined for severely eroded Alfisols, slightly eroded Alfisols nd slightly eroded Vertisols, representing eroding, modertely resisting nd very resisting soil surfe ggregtes, respetively (tle 5). The vrition of the nugget vlues reflets the differentil ehvior of the soil surfe elevtion points with inresing rinfll. Nugget vlues inrese on severely eroded Alfisols, inditing high vrition of the elevtion points within short distne due to erosion. Nugget vlues lso inrese on slightly eroded Vertisols, refleting sustntil vritions due to swelling lods. On slightly eroded Alfisols, derese of the nugget vlues with time denotes tht soil surfe mirotopogrphy eomes more uniform (figure ). Tle 5: Vriogrm prmeters for seleted soils Field Severely eroded Alfisols Slightly eroded Alfisols Slightly eroded Vertisols onditions Nugget Slope Nugget Slope Nugget Slope Ploughing After rin2 After rin

7 () () Figure : Soil surfe profile on slightly eroded Alfisols fter ploughing () nd fter rin (). CONCLUSION Correltion etween runoff, soil loss nd elevtion point dt llows to group the soils in four reltive erodiility lsses s follows: ()very low = slightly eroded Vertisols; (2) low = slightly nd modertely eroded Alfisols; () moderte = slightly nd modertely eroded Plnosols; nd (4) high = modertely nd severely eroded Vertisols, severely eroded Plnosols, nd severely eroded Alfisols. REFERENCES Asseline, J. et Vlentin C Constrution et mise u point d un infiltromètre à spersion. Ch. Pédol. ORSTOM, 5, 4: De Ploey, J. nd Imeson, A. C Proeedings XIII ISSS ongress, Hmurg, Vol 5:

8 FAO. 99. The onservtion nd rehilittion of Afrin lnd. FAO, Rome, ARC/9/4. Kuipers, H Proeedings XIII ISSS ongress, Hmurg, Vol 5: -9. Ll, R Soil erosion reserh methods. John Willey nd Sons Pulitions, New York, USA. Levy, G. J., Levin, J. nd Shinerg, I Sel formtion nd interrill soil erosion. Soil S. So. Am. Journ. 58: Mhop, F., Vn Rnst, E. et Seiny-Boukr, L Influene de l méngement des sols sur l effiité des pluies u Nord-Cmeroun. Etude et Gestion des Sols 2, 2: 5-7. Mwender, E. J. nd Feyen, J Estimtion of depression storge nd Mnning s oeffiient from rndom roughness mesurement. GEODERMA 52: Seiny, B. L. 99. Régime hydrique et dégrtion des sols du Nord-Cmeroun. Thèse de dotort e yle, Université de Youndé, Cmeroun, 226 p. Thee, B Hydrodynmique de quelques sols du Nord-Cmeroun. Bssin versnt de Moud. Thèse d Université, Université des Sienes et Tehniques du Lnguedo, Montpellier, 28 p. Key words: surfe elevtion points, vriogrm, interrill erodiility. Mots lés : modèle numérique d'ltitude, vriogrmme, érodiilité diffuse 8