Terraces in Africa

Typology, efficiency, limits & improvements

ByEric Roose

Soil scientist, Director of research emeritus of Institut de Recherche pour le développement (IRD),

Montpellier, France

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?

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 mountainspopulation: 20 to 100 hab./km²

In mountains + fertile soils, climate + healthy, better irrigatedThan in tropical plains

ex: Rwanda, Burundi, Kenya, Tanzania, EthiopiaComores , Morocco & Madagascarpopulation: 150 to >800 Hab./km²

3. What type of erosion

isthe worse ennemy

forfarmers living in the mountains?

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

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

3.3. Mass movement: * farmers cannot fight against landslides & solifuxion

* but slow creeping & tillage erosionmay help to build embankments progressively

3.4. Conclusions

In the mountains farmers are interested to

water management on the hillslopesreduction of rill & gully erosionimprovement of soil fertility

4. Terraces typology

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

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

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

41. Overgrazed hilltop= impluviumTaneties collect runoff on progressive terraces

Graded terraces in the riverbed = paddy fields in Madagascar

tanetyterraces

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

in Rif montains, northern MoroccoSelected deforestation leaving olive trees

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

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

*Jessour:Little earth dam capting runoff + sediments from hillsides

Olive, figue, palm-trees+rotation cereals - legumes

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

Diversion du ruiss. vers chemin d’eau

Earth bank for total infiltration (Tunisia)build manuelly local salaries

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

Reforestation on earth banks for total infiltrationAlgeria

*Banquettes de diversion (Algérie, Maroc)

Mechanized earth banks in TunisiaState funds to big foreign societies

3.3. La diversion du ruiss. vers chemin d’eau

*Banquettes de diversion (Algérie, Maroc)

Channel + earth banks = lost areas= 10 to 32% of the field

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

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

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

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

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.

5.1. Slope gradient effects

I I I20 40 60% Gradient %

Runoff %

10-

20-

30-

I2

Erosion

Runoff %

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

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 banksErosion on banks & waterways + acceleration of the Max flows in rivers

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

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

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.

Overgrazed primitive forest, but…New forest of fruit trees + irrigated forages

Asni (High Atlas, Morocco)

END

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 channelAtlas, Morocco

END