Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa


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<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa>

1.1.6 Tied Contour Ridges

Technical Description

Contour ridges are small earthen ridges, 15 to 20 cm high, with an upslope furrow which accommodates runoff from a catchment strip between the ridges (Figure 8). The catchment strip is usually uncultivated, but, where contour ridging is used to control erosion rather than for water harvesting, the whole area may be cultivated.

Ridges may be from 1.5 to 10.0 m apart, but, as this is a micro-catchment system and the catchment is a function of the distance between ridges, the precise distance should be calculated for the expected rainfall of the region (Figure 9).

Figure 8. Contour ridges as used in Kenya
(Critchley et al., 1992).

Small earthen ties are made within the furrows at 4 to 5 m intervals to prevent lateral flow (Figure 10). The objective of the system is to collect local runoff and store it within the soil profile in the vicinity of the plant roots. Micro-catchment contour ridging is usually not designed to accommodate overflow, so the system should be protected with a cutoff drain.

The tied contour ridging system is used for tree planting (with a wider distance between ridges) and crop production. Crops are planted on the ridges as well as in the furrows.

Figure 9. Field layout for contour ridging which varies according
to the catchment to harvest area ratio (Critchley et al., 1992).

Extent of Use

It has been adopted in Kenya, Niger, Zimbabwe, amongst others. It does not seem to be taken up spontaneously, however, and is mainly promoted through projects and government policy. Nevertheless, tied ridges are widely used in commercial farming situations in southern Africa as a means of controlling soil erosion.

Operation and Maintenance

Minimal maintenance is required if the ridges are properly constructed initially. Maintenance involves reconstruction of any lines and ridges that might have collapsed.

Level of Involvement

While possible to prepare with hand implements, most projects have used mechanised equipment to construct the contour ridges. Farming practices thereafter are left in the hands of the community. The siting of contours can be done by the community after training.

Costs

With human labour, an estimated 32 person days/ha is required. Using machinery, the time requirement is reduced, but the costs are increased to an estimated $100/ha. This technology is considered low cost, although the rate of its adoption has not been high.

Figure 10. Tied furrows as used in Zimbabwe (Critchley et al., 1992).

Effectiveness of the Technology

Data from Kenya suggest that there are considerable yield advantages in using the contour system. The data also show that, when used in combination with appropriate crops, it has a demonstrated ability to reduce the risk of crop failure due to drought by concentrating the runoff. This technology has been used with millet, cowpeas and sorghum.

The application and effectiveness of the technology is believed to be greatest in those areas where soils have been degraded to the extent that the people cannot reverse the trend using their own resources. An external input of mechanical equipment can have a large impact in these situations.

Suitability

The technology is being used in a variety of climatic and soil conditions and can be adapted to rainfall by adjusting the distance between contours and also the area of cropping. Water harvesting potential is reduced or lost if the catchment area is planted. At Baringo, Kenya, where there is a mean annual rainfall of 655 mm, the project area has a catchment to cultivated area ratio of 2:1. Further, a range of slopes may be treated, though the dimensions need to be increased as slope increases. In the relatively higher rainfall area of Zimbabwe, there is only a 1.5 m spacing between the ridges, and the ridges themselves act as the catchment. Planting is carried out alongside the furrow to take advantage of the concentrated water but to avoid waterlogging.

The system is suitable for areas where cultivation is limited by water shortages and in areas where there is severe environmental degradation.

Environmental Benefits

Benefits of land rehabilitation and reduced soil erosion are normal results when this technology is used.

Advantages

This low cost technology has the potential to increase food security in below normal rainfall years. The system can be implemented using either a mechanised or manual labour approach. As with other water harvesting methods, it is more likely to be successful in areas which experience severe dry spells and/or highly variable rainfalls. The technology reduces soil erosion and increases soil moisture content.

Disadvantages

The unusual cropping system of planting on ridges and next to furrows, but leaving the catchment unplanted, is thought to be a disincentive for adopting this technology. Further, the labour-intensive approach is not thought to be attractive in the areas where the technology has been tried. In Baringo, Kenya, farmers were reportedly reluctant to repair bunds after they were washed away. The relatively low planting density discourages farmers, especially in a good year, and the technique does not work well on steep slopes.

Cultural Acceptability

No data have been reported on this aspect of the technology.

Further Development of the Technology

Globally, this is a well-documented and widely-practised technology which can be adapted to a variety of conditions. However, in Africa, it requires effective extension and promotion before it is widely adopted.

Information Sources

Contacts

Institute of Agricultural Engineering, Post Office Box BW 330, Harare, Zimbabwe.

Bibliography

Critchley, W., C. Reij, and A. Seznec 1992. Water Harvesting for Plant Production. Volume II: Case Studies and Conclusions for Sub-Saharan Africa. World Bank Technical Paper No. 157, 133 p.

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