1.2 Contact details of resource persons and institutions involved in the assessment and documentation of the Technology

Name of project which facilitated the documentation/ evaluation of the Technology (if relevant)

Book project: Water Harvesting – Guidelines to Good Practice (Water Harvesting)

1.3 Conditions regarding the use of data documented through WOCAT

The compiler and key resource person(s) accept the conditions regarding the use of data documented through WOCAT:

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2.1 Short description of the Technology

Definition of the Technology:

Water-spreading weirs are structures that span the entire width of a valley to spread floodwater over the adjacent land area.

2.2 Detailed description of the Technology

Description:

Over the last 12 years water-spreading weirs have been introduced and improved as a new rehabilitation technique for degraded dry valleys in Burkina Faso, Niger and Chad. In Chad 104 water-spreading weirs were constructed in the scope of the two development projects, initiated by the German Technical Cooperation (GIZ) and the Swiss Development Cooperation (SDC) in the 1990s. Water-spreading weirs are made of natural stones and cement, and consist of a spillway in the actual riverbed and lateral abutments and wings. Floodwaters are spread over the adjacent land area above the structure, where they eventually overflow the lateral wings and then slowly flow back towards the riverbed below the structure. As a result the land area below the weir is flooded. The lateral spreading of the water causes the land area above and below the structure to be flooded and supplies it with sediment. Water infiltrates, gullies in the valley are filled and the riverbed is raised. Thanks to the infiltration, the groundwater table also rises in a few years.

Purpose of the Technology: In dry valleys in which water flows in the rivers for only a few days a year, the weirs serve to distribute the incoming runoff over the valley floor and allow as much water as possible to infiltrate the soil. The aquifer is thus replenished and is then available for agricultural use. In contrast to the various types of dams, the goal of water-spreading weirs is not to create reservoirs for later use. What water-spreading weirs do is cause a temporary flooding of the adjacent land area above and below the weir. Depending on user preferences, the primary goal may be 1) agricultural use, 2) sylvo-pastoral use or 3) the replenishment and rising of the water table.

Establishment / maintenance activities and inputs: Water-spreading weirs require detailed technical planning and experienced engineering and construction firms. The bulk of the work is performed using local materials and by village craftsmen and helpers.

Natural / human environment: Compared to small impoundment dams, retention basins and microweirs, water-spreading weirs are especially well-suited for shallow, wide valleys that, due to severe gully erosion, are no longer inundated by small and medium
volume floodwaters. The flooding no longer takes place because the actual riverbed has been deeply eroded and enlarged. However, water-spreading weirs are also suitable for improving agricultural productivity in more or less intact valley floors. Water-spreading weirs are successful in regions where precipitation during the growing season is erratic and where the weirs ensure a more evenly distributed water supply for crops, as well as in zones in which water enrichment makes one or two additional growing seasons possible. At the present time they are in use in a broad area where annual rainfall ranges from 50 to 1,200 mm/year.

2.3 Photos of the Technology

2.5 Country/ region/ locations where the Technology has been applied and which are covered by this assessment

2.6 Date of implementation

If precise year is not known, indicate approximate date:

• 10-50 years ago

2.7 Introduction of the Technology

Specify how the Technology was introduced:

• through projects/ external interventions

Comments (type of project, etc.):

The first water-spreading weirs were introduced during the 1990s in Chad through Swiss cooperation

3.1 Main purpose(s) of the Technology

• improve production
• reduce, prevent, restore land degradation

3.2 Current land use type(s) where the Technology is applied

Comments:

Major land use problems (compiler’s opinion): Substantial degradation of drainage basins in the Sahel due to population growth and intense land-use pressure has been observed since the 1960s. Climate change has further amplified this trend. The expansion of agriculture and intensification of grazing and logging have caused the natural vegetation cover to decline. This process has been accelerated by severe droughts and has led to the degradation of the soils. Sparse vegetation cover and structurally damaged soils reduce rainfall infiltration into the soil, resulting in more runoff and soil erosion on plateaus and slopes. Runoff is concentrated in the valleys, in which heavy floodwaters
wash away fertile soils and lead to deep erosion of the riverbed. The annual, recurrent small and medium-size floods that normally cause temporary inundation of the valleys and deposition of fertile sediments no longer occur.

3.4 Water supply

other (e.g. post-flooding):

• Post-flooding

Comments:

Water supply: Also mixed rainfed - irrigated and rainfed

3.5 SLM group to which the Technology belongs

• cross-slope measure
• irrigation management (incl. water supply, drainage)
• water diversion and drainage

3.6 SLM measures comprising the Technology

3.7 Main types of land degradation addressed by the Technology

Comments:

Main type of degradation addressed: Wt: loss of topsoil / surface erosion, Wg: gully erosion / gullying, Hg: change in groundwater / aquifer level

Main causes of degradation: soil management (abandonment of fallow periods), deforestation / removal of natural vegetation (incl. forest fires) (for agricultural expansion), over-exploitation of vegetation for domestic use, droughts, population pressure

Secondary causes of degradation: overgrazing

3.8 Prevention, reduction, or restoration of land degradation

Specify the goal of the Technology with regard to land degradation:

• reduce land degradation
• restore/ rehabilitate severely degraded land

Comments:

Main goals: rehabilitation / reclamation of denuded land

Secondary goals: mitigation / reduction of land degradation

4.1 Technical drawing of the Technology

4.3 Establishment activities

	Activity	Timing (season)
1.	Excavating the steps
2.	Excavating the wall foundations
3.	Pouring the foundations
4.	Building the walls
5.	Finishing the walls and filling the stilling basin

4.4 Costs and inputs needed for establishment

	Specify input	Unit	Quantity	Costs per Unit	Total costs per input	% of costs borne by land users
Labour	Labour	ha	1.0	750.0	750.0	100.0
Equipment	Total costs for equipment	ha	1.0	750.0	750.0
Construction material	Stone	ha	1.0	750.0	750.0
Total costs for establishment of the Technology					2250.0

4.6 Costs and inputs needed for maintenance/ recurrent activities (per year)

	Specify input	Unit	Quantity	Costs per Unit	Total costs per input	% of costs borne by land users
Labour	Labour	ha	1.0	50.0	50.0	100.0
Equipment	Total costs for equipment	ha	1.0	50.0	50.0
Construction material	Stone	ha	1.0	50.0	50.0
Total costs for maintenance of the Technology					150.0

Comments:

The costs were calculated per structure (one diversion weir). The length of the weir varies depending on the width of the valley it is constructed in. The weir has to span the whole valley which is usually between 100 and 1000 m wide.

5.1 Climate

5.2 Topography

5.3 Soils

If available, attach full soil description or specify the available information, e.g. soil type, soil PH/ acidity, Cation Exchange Capacity, nitrogen, salinity etc.

Soil fertility: High
Soil drainage/infiltration: Medium (ranked 1) and good (ranked 2)
Soil water storage capacity: Medium

5.4 Water availability and quality

Comments and further specifications on water quality and quantity:

Availability of surface water: excess (during heavy rainfalls in the rainy season) or poor/ none (in the dry season)

5.5 Biodiversity

5.6 Characteristics of land users applying the Technology

Indicate other relevant characteristics of the land users:

Population density: < 10 persons/km2
70% of the land users are poor.
Market orientation: Mixed (rainfed crops are for subsistence, post-rainy season crops and irrigated crops (vegetables) mainly for market)

5.7 Average area of land used by land users applying the Technology

5.8 Land ownership, land use rights, and water use rights

Land ownership:

• communal/ village

Land use rights:

• leased

Comments:

In Chad, only the rainfed fields are in private family ownership and inheritable. Reclaimed irrigated fields and vegetable production fields go back to the community and can be redistributed.

6.1 On-site impacts the Technology has shown

Socio-economic impacts

Production

Income and costs

Other socio-economic impacts

Socio-cultural impacts

Ecological impacts

Water cycle/ runoff

Soil

Biodiversity: vegetation, animals

6.2 Off-site impacts the Technology has shown

6.3 Exposure and sensitivity of the Technology to gradual climate change and climate-related extremes/ disasters (as perceived by land users)

Gradual climate change

Gradual climate change

	Season	increase or decrease	How does the Technology cope with it?
annual temperature		increase	well

Climate-related extremes (disasters)

Meteorological disasters

	How does the Technology cope with it?
local rainstorm	well
local windstorm	well

Hydrological disasters

	How does the Technology cope with it?
general (river) flood	well

6.4 Cost-benefit analysis

How do the benefits compare with the establishment costs (from land users’ perspective)?

How do the benefits compare with the maintenance/ recurrent costs (from land users' perspective)?

Comments:

Depending upon the users' experience and the availability of labour, it may take anywhere from 2 to 10 years before the rehabilitated land area reaches its optimum use potential.

6.5 Adoption of the Technology

Of all those who have adopted the Technology, how many did so spontaneously, i.e. without receiving any material incentives/ payments?

• 0-10%

Comments:

100% of land user families have adopted the Technology with external material support

6000 land user families have adopted the Technology with external material support

Comments on acceptance with external material support: Between 4000 and 8000 households are direct beneficiaries of the construction of water-spreading weirs in Eastern Chad.

There is no trend towards spontaneous adoption of the Technology

Comments on adoption trend: It is unlikely that communities will be able to adopt this technology without external funding. Even for maintenance activities it remains to be seen if the communities will be capable of funding more extensive maintenance work with their low budgets.

6.7 Strengths/ advantages/ opportunities of the Technology

Strengths/ advantages/ opportunities in the compiler’s or other key resource person’s view
Through the construction of water-spreading weirs, soils are regularly flooded and supplied with water and sediment. Thus, the arable land area and the yields of the rainy season crops serving as staple food increase. How can they be sustained / enhanced? Ensure proper maintenance of the system.
The more frequent flooding of the soils results in increased infiltration, and the groundwater level rises substantially.
Prior to rehabilitation, in most of the sites, it was only possible to grow a rainfed crop and perhaps an irrigated crop on some small areas of land. After, in addition to the rainfed crop grown on larger areas of land, it became possible to grow a post-rainy season crop (culture de contresaison) and, once the water table had risen, an irrigated crop (culture de décrue) as well. How can they be sustained / enhanced? Upscale water-spreading weirs to increase the number of people benefitting.
Post-rainy season crops and irrigated crops diversify agricultural production. They are used as a means of earning cash income. How can they be sustained / enhanced? Improve access to markets.
With their capacity to regulate annual floodwaters and harness them to stabilise production, water-spreading weirs are an effective measure for adapting to climate change in regions experiencing increasing variability in rainfall.

6.8 Weaknesses/ disadvantages/ risks of the Technology and ways of overcoming them

Weaknesses/ disadvantages/ risks in the compiler’s or other key resource person’s view	How can they be overcome?
It can be assumed that one third of the weirs will require complete renovation every 20 years	The renovation of these weirs can be done for approximately 10% of the initial construction costs
Maintenance of the weirs by the management committees is still a weak area. Funds expected from user fees for the plots are often inadequately collected and too low to cover costs. Some management committees lose their drive and neglect their duties.	New fundings sources have to be found and tapped.
In spite of the great potential for the use of water-spreading weirs and the very promising results, implementation will continue to depend in the medium term on outside funding, as it is unlikely that the communal budgets will be able to fund investments of this size.	The existing knowledge hast to be spread.
Know-how and experience for the construction of water-spreading weirs are still concentrated among a few countries.

7.1 Methods/ sources of information

7.2 References to available publications

Title, author, year, ISBN:

Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) (2011). Water-spreading weirs for the development of degraded dry river valleys. Experience from the Sahel. Frankfurt and Eschborn, GIZ and KFW.

Available from where? Costs?

http://www.gtz.de/de/dokumente/giz2012-en-water-spreading-weirs-sahel.pdf

Title, author, year, ISBN:

Direction du développement et de la coopération DDC (2012). Gestion des eaux de ruissellement dans le Tchad sahélien. Bern, DDC.

Available from where? Costs?

http://www.gopa.de/uploads/tx_bdojobopps/PRODOC_Tchad.pdf