Terraces in Africa. Typology, efficiency, limits & improvements. By Eric Roose

Transcription

1 Terraces in Africa Typology, efficiency, limits & improvements By Eric Roose Soil scientist, Director of research emeritus of Institut de Recherche pour le développement (IRD), Montpellier, France

2 1. Introduction This meeting is concerned with terraces and their efficiency against natural risks What about in Africa? - Do we find the same types of terraces and the same functions? Do we have measurements of their efficiency to figth runoff, erosion, bush fires and to increase the crop production? What are the limitations to apply these expensive techniques? How can we improve the rentability of these systems?

3 2. Where terraces are observed? In mountains = refuges of animist populations When muslims riders/slave traders invaded Africa ex: Dogon in Mali, Kabiés in Togo/Bénin; Enugu in Nigéria, Mafa of Cameroon; Berbères in Rif and Atlas in Morocco + Algerian & tunisian mountains population: 20 to 100 hab./km² In mountains + fertile soils, climate + healthy, better irrigated Than in tropical plains ex: Rwanda, Burundi, Kenya, Tanzania, Ethiopia Comores, Morocco & Madagascar population: 150 to >800 Hab./km²

4 3. What type of erosion is the worse ennemy for farmers living in the mountains?

5 3.1 Sheet erosion is present every where * it is one cause of soil fertility degradation * but, not well known by the farmers * because there are very few spectacular signs

6 3.2.The main ennemi in mountains is linear erosion: *rills scours the humiferous horizon; *gullies reduce the field surface and the soil moisture storage *badland =dead of the hillside field

7 3.3. Mass movement: * farmers cannot fight against landslides & solifuxion * but slow creeping & tillage erosion may help to build embankments progressively

8 3.4. Conclusions In the mountains farmers are interested to water management on the hillslopes reduction of rill & gully erosion improvement of soil fertility

9 4. Terraces typology

10 4.1. Graded terraces Cropped surface quasi plane for irrigation; Various embankments : stone walls, grasses or bushes lines or even bare soil Terracing to capt runoff on hillsides, to irrigate by ditches= seguias - T. in valleys : stone walls in the river bed to capt floods

11 Graded Mediterranean terraces - Earth embankment covered by stones (Morocco) - Embankment = stone wall (Morocco)

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13 In the Rheraya valley (Atlas, Morocco) séguia+ narrow terraces + progresssive terrace

14 41. Overgrazed hilltop= impluvium Taneties collect runoff on progressive terraces Graded terraces in the riverbed = paddy fields in Madagascar terraces tanety

15 4.2. Progressive terraces Cropped fields are situated on sloping surface; The slope steepness decreases progressively by hydric and tillage erosion;but never 0% Embankments are permeable microdams growing progressively : stone bunds, living hedges, grassed lines

16 in Rif montains, northern Morocco Selected deforestation leaving olive trees

17 Half moon around olive tree capting the runoff from the stony hillside (Tunisia)

18 Meskat (Tunisia) hilltop overgrazed = impluvium for olive trees planted in basins

19 *Jessour: Little earth dam capting runoff + sediments from hillsides Olive, figue, palm-trees+rotation cereals - legumes

20 4.3. Earth banks Field cropped without changing the slope steepness, Chanel digged on the line for total infiltration, or With 0.2 to 0.4% slope for diversion Earth embankment : risk of overflow & gully never producing a terrace except if the earth is banked up Like Fanya juu

21 Earth bank for total infiltration (Tunisia) build manuelly local salaries Diversion du ruiss. vers chemin d eau

22 Overflow destructing the earthbank *Banquettes de diversion (Algérie, Maroc)

23 Reforestation on earth banks for total infiltration Algeria

24 Mechanized earth banks in Tunisia State funds to big foreign societies *Banquettes de diversion (Algérie, Maroc)

25 Channel + earth banks = lost areas 3.3. La diversion = 10 du to 32% ruiss. of vers the field chemin d eau *Banquettes de diversion (Algérie, Maroc)

26 4. Results: 4.1. Efficiency on runoff Plots of Rwanda/Burundi: slope >20% Bare soil 20-30% Cropped 10% Crop.+ graded terraces (year 1) 1% Cropped + hedges (year 2) <2% Watersheds a few ha: Nigeria (Lal) decrease of the flow discharge Tunisia (Albergel) increase the flow duration 50% reduction of max flow

27 4.2. Efficiency on erosion Plots Rwanda/Burundi: Slope>20% E t/ha/year Bare soil, standard 500 to 700 Cropped 150 to 300 Cropped + graded terraces (year 1) 1 to 5 Cropped + hedges (year 2d) <2 Watersheds a few ha: Nigeria (Lal) reduction the turbidity Tunisie (Albergel) reduction the sediments transfer reduction the Max diameter of particles

28 4.3. Efficiency on the production Plots Rwanda (poor ferrallitic soil) If only terraces no difference If Ter. + OM + NPK Yield = 2 to 4 times - On semi-arid plots (soil richer) : *Kenya, Ethiopia + 30 to 60% *Algeria: earth banks Surface loss=15% Yield = no change

29 4.4. Efficiency on bush fires Bush fires are frequent in tropical Africa No experimentation on terraces, But on Terr. the whole biomass is exported probability of fire is very low on terraces

30 5.1. Discussions : slope effects 1.Ditches & earth banks change only the slope length, not the slope gradient (except if earth rejected upstair) 2. Progressive terraces modifie the length and progressively the slope gradient (but never 0%) 3. Graded terraces modifie radically the length and gradient of the slope, but also the soil profile. Risk of landslide & need more OM + NPK + labour.

31 20- Runoff % Slope gradient effects Erosion Runoff % I 2 I I I % Gradient % Runoff increased very fast with the slope,then decreases when the sealing crust is eroded Erosion increased exponentially with slope gradient untill 20-30%, then the processes changed:sheet rills mass movements

32 5.2. Risks increase in relation to terrace type If graded terraces: Risk of runoff excess : sealing crusts,compacted soil Risk of landslides increase of drainage & leaching, need of trees to increase ETR If progressive terraces : Risk of rills necessity to cover the soil surface, Risk of embankment slide bushes + various plants If ditches or earth banks Erosion on banks & waterways + acceleration of the Max flows in rivers

33 5.3. Limitations of terraces Topography : slope gradient = 40 to 60% ; if earthquake less than 20% Lithology : high risks of landslide if marl, schist, argillite, gneiss and micas strips; Climatic: to much rain during a month/week drain. Economic limits : labour and fertilizers cost; Graded terraces: 600 to 1500 days/ha + maintenance Progressive terraces : 20 to 60 days X 2 years Ditches or earth banks: 100 to 300 days/ha + maintenance Necessity to intensify farming system to valorise this investment: Gt terracing is less rentable than fertilizing

34 6. Conclusions 6.1.Terraces are efficient against natural risks Allow the intensification of the production; Reforestation with fruits/forage trees Improvement of biodiversity in arid zones Improve the field security for individual farmers

35 6.2. Possibilities to improve these terrace systems in order to valorize this investment - Reduce the cost of building terraces: development of progressive terraces; Select intensive & rentable cropping systems; Rationalise irrigation & fertilisation in relation to crops; Organisation of the market : roads, near the farm direct saling, tourism, villagers groups.

36 Overgrazed primitive forest, but New forest of fruit trees + irrigated forages Asni (High Atlas, Morocco) END

37 Agroforestry in the irrigated terraces near the valley On the hillsides you will find progressive terraces with cereals More investment near the village and the irrigation channel Atlas, Morocco END

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