These are bench terraces which alternate with the original slope and are used for the mixed cultivation of annuals and tree crops. Should a farmer wish to cultivate more food crops at a later stage, he can build more benches on the original slopes. Alternatively, he can convert the whole area into tree crops should age, labour or economic circumstances make this desirable.
Criteria for selection
- In humid tropical regions: use reverse-sloped benches.
- In semi-arid regions: use outward sloped benches.
- To fit in with the mixed agriculture farming systems practised by small farmers.
- To allow for future development and provide flexibility for future land use.
- To protect sloping land with a simple terracing system.
- On slopes of less than 20 degrees (36%).
- For mixed farming systems.
- For farmers who have not finalized long-term plans for the use of their land.
Length: 100 m
Width: 3.5 m
Gradients: Horizontal grade is 0.5 to 1% humid regions and level to 0.5% arid and semi-arid regions. Reverse grade is 5%; outward grade, 3%.
Slope limit: 7 to 20 degrees (12 to 36%)
Riser slope: 0.75:1 for hand-made and 1:1 for machine-built terraces
VI and inclined distance: The same as for hillside ditches.
Fig. 10 shows a cross sectional view of convertible terraces.
The specifications and volumes of convertible terraces can be computed in the same way as for hillside ditches and for bench terraces (3.5 m wide)
The same spacings and principles used for staking out hillside ditches are applicable. After setting a base-line, the staking can be done in two ways, depending on the spaces required for the fruit and for food trees.
On gentle slopes, use D (inclined distance) or HI (horizontal distance) as centre-to-centre distance to stake out the terraces. On distance to stake out the terraces. The Wt of 3.5 m wide benches on different slopes can be obtained from the specification tables for bench terraces.
Starting from the centre stakes of the terraces, add the top and bottom stakes, using the Wt (terrace width) figures from the specification tables. Trees, whether planted in one or two rows, should not overshadow the terraces where food crops are grown. Only one row of trees should be staked out if they are a tall species or on sites with narrow spacings. Two or more rows can be staked out on sites where there are wider spacings and small trees such as coffee plants.
Construction is the same as for bench terraces. Owing to their width, (3.5 m), the terraces can be built either by hand or by a medium-size machine. Individual basins need to be made by hand in the spacings planted with trees.
Output is 3.5 cubic m a day by hand and 40 cubic m an hour using a medium sized machine. The calculation for cutting or filling of one ha of convertible terraces can be done in the same way as for other terraces, i.e. dividing volume per ha by output per man-day or per hour.
The cost of convertible terraces is about one-third that for full bench terracing. They are slightly more expensive than hillside ditches because the benches are wider.
Benches
Same as for bench terraces.
Spacing
Individual basins are required for tree crops and grass or perennial cover crops should be planted. When interplanting other crops with trees, auxiliary measures such as contouring, mulching, or furrows are needed.
Access roads
For manual cultivation, only a rough road is required. However, for mechanized cultivation, a 3.5 m wide road is usually needed.
The maintenance of risers is much easier than for bench terraces because they are only half the height of the latter. Grass planting is not necessary on the risers if grass species are already present in the spacings.
These are bench terraces in which only one terrace out of three is constructed at the beginning of the operation. The other two terraces can actually be built over the next two years or even over a longer period of time depending upon the farmer's plans and resources.
Criteria for selection
- In humid tropical regions: use reverse sloped benches.
- In semi-arid regions: use level bench terraces (with dykes) for storing run-off from the slope above or contributing area.
- To protect cultivated slopes by initially terracing one-third of the area.
- To enable the farmer to spread out over a period of years the cost and labour involved in a terracing operation.
- To do a quick rehabilitation job over a large area.
- In semi-arid regions, to collect run-off from the slope for irrigation of crops grown on the benches.
- On slopes from 7 to 25 degrees (12 to 47%).
- For small farmers.
- In semi-arid areas (for collecting run-off).
If run-off collection is planned, the design should be the same as that for irrigation or level bench terraces. Otherwise, the design should be that for reverse-sloped bench terraces.
Fig. 11 shows cross-sections of two types of intermittent terraces. Specification tables for reverse-sloped bench terraces can be used for intermittent terraces in humid tropical regions (see Table 1) .
The same principles used for layout and staking of bench terraces are applicable. After setting a base-line, the calculated terrace width (Wt), or the width taken from the specification tables is used to stake put the terraces from the top of the slope downwards. Run contours (or graded contours) for the top and bottom lines of the first terrace on the upward slope. A space equivalent to the width of 2 terraces (2 Wt) should be left for future terracing work. The 4th terrace should be staked out in the same way as the first one, leaving space afterwards for the next two terraces. Stake out the 7th, 10th, 13th terraces and so on in the same way, until the entire area is completed.
A 15 to 20 cm dyke is needed for level benches where run-off collection is planned. otherwise, follow the same construction procedure as that for reverse-sloped bench terraces.
The same as for convertible terraces.
One-third that of full bench terraces.
Benches
Same as for bench terraces
Spacings
Auxiliary measures are needed for crop cultivation. These include contour planting, contour furrowing, mulching, residue/trash covering, grass barriers, cover cropping and interplanting.
Access roads and waterways
An access road and/or waterway is needed.
Protection
In humid regions, a line of tall thick grass should be planted above the cut bank of the terrace on cultivated slopes to restrain soils from coming downslope. Other protection measures are the same as those indicated for bench and convertible terraces.
Maintenance
The maintenance for both reverse sloped and run-off collection types is the same as for bench terraces. With the run-off collection type, it is especially important to make sure the dyke is maintained properly.
