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ContextIn the Sahel, the advance of desertification is a key factor contri buting to the impoverishment of the population. Climate change and population growth are exacerbating desertification. Comp etition for the few available natural resources is increasingly straining the environment and pressurising its users. Less land is available for agriculture and grazing, increasing the risk of conflict between stockbreeders and crop farmers. For many, the only solution is rural exodus.
The erosion of fertile soil of drainage basins due to rapid surface runoff of rainwater leads to a reduction in cultivable area and a significant drop in the ground water table, and hence reduced yields. Rehabilitating these basins helps to counter the negative effects.
Water-spreading weirs (seuils d' épandage) in semi-arid regions aim to alleviate desertification by reversing the devastation of drainage basins. This technique can significantly increase the cultivable acreage, enabling a wider range of products to be grown over a longer period, which can increase food security for rural households.
Waterspreading weirs slow the water f low and increase the regularly f looded area. They enable rainwater to be stored by seeping into the ground and raising the level of the groundwater table close to the surface.
The area suitable for cultivation is enlarged along the banks of the wadis (dry valleys) by retaining fertile soil. The maintenance of the weirs requires minimal effort if there is no major damage.
Waterspreading weirs need specific natural landscapes. Large widespread valleys with a low slope angle provide ideal conditions. Managing drainage basins requires the construction of a series of weirs, where each weir retains some of the water and alluvial deposits and gradually raises the bed of the wadi. Over a short period of time, this process also can raise the ground water table by up to 20 meters.
This reduces the costs for energy (pumping costs) and the required labour of women often drawing the water. It also facilitates animals’ access to water.
Water-spreading Weirs
Initially, Swiss and German Development Cooperation introduced waterspreading weir technology to the Sahel region in the 1990s. Based on the experiences made with the first weirs, the approach was refined during the last 15 years. The weir design and construction planning are carried out by different projects of German and Swiss Cooperation in collaboration with international and local engineering societies and the local municipalities.
The management of the construction is under the responsibility of local municipalities, often delegating the organisation of construction activities to local construction committees. A shortterm labourintensive approach is preferable. Therefore, masons and unskilled labourers from local villages are hired and taught the basic construction and maintenance techniques of the weirs.
Preconditions Building weirs needs a systemic approach: Weirs have to be planned by local communities or municipalities within a watershed. Inter communal agreements are necessary if the watershed comprises several communities.
Designing and constructing weirs requires significant technical knowledge and their implementation calls for a well organised population. Increasing food security through the optimal use of the improved acreage will only be achieved in combination with access to agricultural extension and training programmes, inputs and markets.
Combined with erosion measures such as stone bunds and with better farming methods, they significantly increase agricultural production.
Construction costs per weir are between 600 and 1.500 Euro per ha improved land. The average cost of a weir is between 20.000 and EUR 50,000 depending on natural conditions and including the contribution made by the local population.
Weirs allow up to three production cycles per year: during the rainy season staple products for subsistence followed by vege tables for selling with supplementary irrigation. During the dry season, a third product can be produced under irrigation. Hereby the income of farmers can be more than doubled.Through retaining the soil deposit, over time the cultivable area is increasing. In addition, waterspreading weirs have a stabilising effect on the environment.
So far, in Niger, Tchad and Burkina Faso, more than 370 water spreading weirs have been implemented covering an improved cultivation area of more than 20.000 ha benefitting more than 40.000 households.
Building water spreading weirs is a systemic approach Building weirs calls for technical skills on both sides: executing agencies and local actors. Participation and responsible acting of the local population, manifested by development plans, local agreements and charters and organised on the basis of legitimate, recognised structures represents a very high added value for the quality and sustainability of weirs.
Decentralisation is a new element. The new local bodies are im portant actors responsible for local development. Weirs cannot be funded by local people or small rural municipalities alone. If supported by funding measures and technical assistance from the government, they could play a vital role in alleviating poverty. The effectiveness of waterspreading weirs also depends on changes that the farmers are prepared to make in their farming techniques. Assessments are needed here, too, regarding the quality of seeds, techniques and production methods. The management and maintenance committees are monitored to ensure that they know what they need to do.
Building weirs and improving agricultural production requires both funds and technical advice. For German Cooperation, the cooperation of financial and technical assistance is a success factor for water spreading weirs.
Scaling upWeir construction adequately solves the problems faced by 90% of already identified lowlying areas in the Sahel. This measure can regenerate deteriorated lowlands and is therefore a useful tool for alleviating desertification. It can be transferred to other semiarid regions with comparable natural landscape conditions.
New information technologies support the identification and planning dimension: Satellite photographs available on Internet offer an opportunity for preliminary feasibility studies of water spreading weirs.
In order to disseminate the acquired technical knowledge, German Development Cooperation plans to develop a cap acity development program for technicians of hydrogeological infrastructures in different regions.
The whole systemic approach has been described by GIZ and KfW Entwicklungsbank and will soon be published in English and French. It can be requested from the following address: [email protected]
For any further information, please contact: [email protected] and [email protected]
Imprint: November 2011
Published by:Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbHPost Box 518065276 EschbornGermanyT: +49 6196 79 0F: +49 6196 79 11 15
KfW BankengruppePalmengartenstraße 5-960325 Frankfurt am MainTelefon 069 7431-0Telefax 069 7431-2944 KfW Entwicklungsbankinfo@kfw-entwicklungsbank.dewww.kfw-entwicklungsbank.de
Design:creative republicThomas Maxeiner Visual Communications,Frankfurt a. M./GermanyT: +49 69 .91 50 85-60 E: [email protected]: www.creativerepublic.net
Photos: © GIZ/Klaus Wohlmann, Heinz Bender,
Marc Cleriot, Aboubacar Mounkaila
