Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for Freshwater Augumentation
1.1.10 External Catchments Using Contour Ridging
|A further variation of the contour ridging technique described in
Chapter 1.1.6, this technology uses an external catchment and incorporates
a stone spillway into the contour bund, providing for excess runoff to
flow around the structure (Figure 16). Bunds are made of earth or,
occasionally, stone, and, in Niger, they are usually covered with a layer
of stone on the top and back slope Figure 17).
The area impounded by the bund is planted. The usual catchment to
cultivated area ratio is 2:1 but reaches 5:1 in Kenya where off contour
bunds are used as collection systems to channel runoff to cultivated
Figure 16. Macro-catchment water harvesting system (Imbira,
|For full utilisation of the cropping area, the spillway height should be
level with the base of the spillway on the next contour uphill. Levelling
of the ground between contours assists in water spreading when runoff is
collected. The spillway height determines the depth of water retained and
is usually about 10 cm.
Figure 17. Macro-catchment water harvesting in Niger (Critchley
et al., 1992).
Extent of Use
Several thousand hectares are treated with a variety of bund systems,
although the largest number of projects are in Niger and Kenya.
Operation and Maintenance
Maintenance is required to control erosion around spillways and bund wing
walls. Achieving adequate compaction of bunds with manual construction methods
is difficult and may result in breaches during the first year of operation.
Grass planted on the bunds and spillways helps to protect these surfaces from
erosion and reduces maintenance requirements, particularly since some resistance
to the repair of breached bunds was reported in Kenya (Figure 18).
Level of Involvement
The Kenyan programme of bund construction is all done using manual
construction methods, whereas, in Niger, the bunds are constructed by machine
and only the stone is laid by hand. Construction of these storages has largely
been done through food-for-work programmes and there is some concern about the
level of true involvement of people. In at least one application, it was
observed that there was little voluntary participation in the use of this
technology by the community.
Figure 18. Spillway construction for bunds with external
catchments (Pacey and Cullis, 1991).
In Niger, the estimated construction cost is about $500/ha for bunds, land
preparation and fertiliser. In Kenya, 100 person days/ha are commonly devoted to
construction, resulting in an approximate cost of between $52 to $202/ha.
Effectiveness of the Technology
This technology is effective in controlling erosion. Yield information is not
available for Niger, but, in Kenya, the comparison with control plots has shown
an highly significant increase in yields of sorghum and cow peas.
This technology is suitable for low and unreliable rainfall areas, with an
annual precipitation of 350 to 650 mm. It is also well-suited for use in the
reclamation of degraded land.
The technology has beneficial effects in the reduction and control of soil
The advantages are the concentration of runoff to allow the cultivation of a
crop where otherwise none would be possible. This results in increased and more
reliable level of production.
A high demand for labour in the construction and maintenance of these systems
may partly be a reason for the low level of acceptance by the community.
Approaches have not yet been found to fully involve the community.
These have not been fully assessed; however, there are no known limitations.
Further Development of the Technology
This technique is clearly an appropriate technology for the conservation of
soils and the recovery of degraded lands. Nevertheless, the technology needs to
be demonstrated as useful to the community and presented in a way that the
community is willing to manage.
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.
Imbira, J. 1989. Runoff Harvesting for Crop Production in Semi-arid Areas of
Baringo. In: D.B. Thomas et al. (Editors), Soil and Water Conservation
in Kenya, Proceedings of the Third National Workshop, Nairobi, 1986.
University of Nairobi, SIDA., pp 407-431.