Farming systems in Lake Tana basin, Ethiopia: Land surface management and its impact on runoff response

Elise Monsieurs Jan Nyssen Enyew Adgo Mekete Dessie
Afwatering van de akkers in Ethiopië, een vloek of zegen?Van jongs af aan staan kinderen in Ethiopië al mee op het veld met hun vader om de akkers te bewerken. De manier waarop hij dit doet, heeft hij geleerd van zijn vader die het op zijn beurt ook heeft geleerd van zijn vader enz. Zo worden akkers op grote schaal afgewaterd, waarbij de techniek gebaseerd is op duizenden jaren akkerbouw met de ploeg. Echter is deze techniek weinig omschreven, laat staan geanalyseerd.

Farming systems in Lake Tana basin, Ethiopia: Land surface management and its impact on runoff response

Afwatering van de akkers in Ethiopië, een vloek of zegen?

Van jongs af aan staan kinderen in Ethiopië al mee op het veld met hun vader om de akkers te bewerken. De manier waarop hij dit doet, heeft hij geleerd van zijn vader die het op zijn beurt ook heeft geleerd van zijn vader enz. Zo worden akkers op grote schaal afgewaterd, waarbij de techniek gebaseerd is op duizenden jaren akkerbouw met de ploeg. Echter is deze techniek weinig omschreven, laat staan geanalyseerd. We nemen jullie mee in onze zoektocht naar het gebruik van drainagevoren, en de gevolgen ervan.

Wanzaye, een plaats van contrastenVoor het onderzoek, uitgevoerd in samenwerking met de Bahir Dar University en een VLIR project, hadden we ons geen geschiktere omgeving dan Wanzaye kunnen inbeelden. Gelegen in Noord- Ethiopië, aan de voet van de hellingen rond het grootste meer van Ethiopië (Tanameer, ongeveer drieduizend km²) bevinden we ons in een tropisch bergklimaat met een lang droogseizoen en een regenseizoen van juni tot september. De vulkanische voorgeschiedenis van het gebied maakt dat de bodem hier enorm vruchtbaar is. Daarnaast omhelst het Tanabekken een gebied dat zich leent tot energieproductie door middel van waterkrachtcentrales, visvangst en toerisme, en dat een enorme biodiversiteit bezit.

Wanzaye telt ongeveer 500 uiterst gastvrije inwoners die leven van hun landbouwopbrengsten en kleinschalige handel. Dit maakt dat ze zeer kwetsbaar zijn voor de onvoorspelbaarheid van de neerslag, bevolkingsgroei en landdegradatie. Landbouwers zien hun bodems afspoelen en ravijnen groeien doorheen het regenseizoen. Drie verschillende landbeheersystemen worden in Wanzaye en omgeving door de landbouwers toegepast op hun akkertjes, afhankelijk van hun visie op de impact op bodemerosie: (1) een meter hoge stenen muurtjes die leiden tot terrasvorming, (2) een eeuwenoude techniek van drainagevoortjes (feses) en (3) een combinatie van beide.

Terrasmuurtjes en/of drainagevorenStenen muurtjes zorgen ervoor dat geërodeerde bodemdeeltjes niet met het afstromende regenwater worden afgevoerd maar worden vastgehouden op het akkerland. Het maken ervan is een zeer arbeidsintensieve karwij die wordt georganiseerd door de overheid (Ministerie van Landbouw) als gemeenschapswerk. De landbouwers zijn het er unaniem over eens dat de bouw van terrasmuurtjes de beste manier is om landdegradatie te beperken.

Tijdens het regenseizoen trekken de boeren met hun ossenspan schuin hellende drainagevoortjes (feses) om het teveel aan water weg te draineren (zie figuur). In Wanzaye zijn zulke voortjes gemiddeld 27 cm breed en 12 cm diep. Het gebruik van artificiële afwatering heeft vele voordelen voor een akkerland: het vermindert de bodemcompactie, het bevordert luchtcirculatie door de bodem en ook de wortelzone is dieper. Daarnaast  worden drainagevoortjes ook als bodemconserveringsmaatregel toegepast om ongecontroleerde waterstromen over het akkerland te vermijden. Desalniettemin wordt artificiële drainage vaak als een slechte praktijk aanzien, omdat de geconcentreerde stroom zowel op het akkerland als stroomafwaarts gaat leiden tot insnijding. Kortom, wetenschappers hebben tot nog toe weinig consensus bereikt over de finale balans van de positieve en negatieve effecten van drainagevoren.

Landdegradatie verder hellingafwaartsOm een idee te krijgen van de mate van stroomafwaartse erosie in relatie tot de drie verschillende landbouwtechnieken werd er gebruik gemaakt van topografische drempelwaardes voor ravijnontwikkeling. Die topografische drempelwaardes worden bepaald aan het ravijnhoofd, rekening houdende de hellingshoek en met het de oppervlakte van het bekken. Neerslag die afstroomt over het akkerland (runoff), zal stroomafwaarts niet tot ravijnerosie leiden als de topografische condities (hellingshoek en oppervlakte van het bekken) klein genoeg zijn, d.w.z. onder de drempelwaardes liggen voor een specifiek landbeheersysteem. We onderzochten hoe deze drempelwaardes voor de drie verschillende landbeheersystemen in relatie tot elkaar staan.

Bijgestaan door een lokale vertaler en het gebruik van een GPS werden 75 bekkens afgebakend, waarvan 26 met enkel gebruik van feses, 27 met enkel gebruik van stenen muurtjes (terrasbekkens) en 22 waar zowel terrasmuurtjes als feses werden aangewend. Aan de hand van een digitaal hoogtemodel werden de topografische factoren berekend en uitgezet op een grafiek. Hieruit bleek dat de drempelwaarde het hoogst ligt voor terrasbekkens en het laagst voor fesesbekkens wat betekent dat die laatsten het gevoeligst zijn voor de ontwikkeling van ravijnen. Tot nog toe werd deze analyse enkel toegepast om verschillende landgebruiken (bos, weiland, akkerland…) met elkaar te vergelijken, maar niet de beheertechnieken op de akkers zelf.

Erosie op het akkerlandOp terrein ondervonden we dat naast de stroomafwaartse erosie,  ook de landdegradatie op het akkerland zelf niet te verwaarlozen was. Echter staat internationaal onderzoek rond dit fenomeen nog in zijn kinderschoenen. Metingen in Wanzaye toonden dat on-site geulerosie in 41% van de gevallen veroorzaakt was door een slechte werking van feses, terwijl slechts 16% veroorzaakt was door een slechte constructie van terrasmuurtjes. Daarnaast werd ook een positieve correlatie gevonden tussen het totaal volume geulerosie en de densiteit van de voortjes, en een negatieve in relatie tot de densiteit van de stenen muurtjes. Een ander gegeven dat de bevindingen ondersteunt, vonden we terug in de debieten die op twaalf plaatsen werden opgemeten door medewerkers. We stelden een positieve relatie vast tussen de densiteit van de voortjes en de debieten  van het afstromende water waardoor er meer erosie kan plaatsvinden (ten gevolge van grotere kracht van het water) in de drainagevoortjes bij een hogere feses densiteit.

Als  nuance bij bovenstaande bevindingen komt nog dat feses in het studiegebied worden gemaakt met een hellingshoek die kleiner is dan de helling van het akkerland. Hierdoor zal het water minder snel stromen door de feses dan bij rechtstreeks afstromen over het land en dus ook minder bodemerosie veroorzaken.

Naar een finale balans ?We hebben bevestigd dat het gebruik van terrasmuurtjes een positief effect heeft bij het streven naar een verminderde landdegradatie. Voor het gebruik van traditionele drainagevoren is het resultaat echter minder eenduidig. We vonden zowel bewijzen die aantoonden dat dit erosie in de hand werkt als aanwijzingen van verminderde landdegradatie. Onze zoektocht eindigt hier dus niet, en het totale plaatje van het gebruik van drainagevoren moet verder aangevuld worden waarbij moderne wetenschap toelaat om verbeteringen te suggereren.

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Websites

 

CountrySTAT. http://213.55.92.108/countrystat/home.aspx?c=ETH. 24/03/2013.

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Universiteit of Hogeschool
Master of Science in de geografie
Publicatiejaar
2014
Kernwoorden