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TABLE 35
Effect of buffer strips and contour cropping in humid and dry tropical zones (trials on erosion plots) (cf. Roose and Bertrand 1971)

dH = difference in level

TABLE 36
Effect of barriers of grasses (Setaria) or shrubs (Leucaena or Calliandra) on erosion: Ruhande hill near Butaré in Rwanda (cf. König 1991)

° hedges = three staggered lines on a horizontal terrace 1 m wide, every 10 m
° alley cropping = two lines of shrubs every 5 m
° if the hedges are to be effective, a filtering layer - a mulch of brushwood, weeds or prunings from the hedges - must be placed on the ground

° Note:

- the ineffectiveness of free-standing trees
- the reasonable effect of alley cropping
- herbaceous hedges are more effective than shrub hedges, which may be supplemented by adding covered ridges every 5 m

Erosion reaches 300 to 500 t/ha/yr on the bare control plot, and 200 to 250 t/ha/yr under traditional cropping, but falls toward 1 t/ha/yr on plots protected by grass or shrubs, clearly indicating that treated plots are stable after two to three years. So SWC has been successful, but what of crop yields?

Tests show that the biomass produced by shrubs increases gradually from 1.5 to 3.2 kg per linear metre of hedge, that the presence of grass (Setaria) at the foot of the shrubs gives good results in the first year but that after this Calliandra alone produces more than Leucaena, and lastly that annual nutrient intake by the deep-rooted shrubs can be as high as 78 kg of nitrogen, 10 kg of phosphorus, and 17 kg of potassium, which are then redistributed in the form of mulch during the crop growth cycle.

It had been hoped that a combination of prunings from hedges and 10 t/ha/yr of manure (dried paddock dung) would gradually improve crop yields. In fact, however, levels remained only fair (500 to 250 kg/ha of maize, plus 500 to 800 kg/ha of beans in the first season, and 450 to 650 kg/ha of sorghum in the second season). In the fourth year it eventually became clear that soil fertility had to be restored (a supplement of 2.5 t/ha of lime, 10 t/ha of paddock dung, and 300 kg/ha of NPK - 17.17.17) if crop yields were to increase significantly (more than 2 t/ha of beans and 1.4 t/ha of sorghum).

Complex grassed risers. After some years the microdams (grass strips, hedges, etc.) have produced gradual terraces divided up by steep-sided banks protected by a grass sward.

Figure 47 shows the development of a grassed riser on a 20 to 60% slope over a five-to ten-year period, during which it reaches a maximum height of 1 to 1.5 metres, after which there is an increased risk of destruction from animal burrows, gullying, and landslides, and farming problems as well (Roose 1990).

Since any break in the line of the riser across the hill compromises these measures, it is recommended that the whole length be pegged out with the rural community, and the first line of grass indicating the basic contour line planted across the whole slope or hill, leaving the individual farmers to complete the work during the five successive years, at their own pace on their own land. Imposing treatment of the whole hillside on the rural community can undermine the farmers' sense of responsibility, leading to neglect of the necessary upkeep (Ngarambé 1991). If the grassed strip is not continuous, runoff water will rush into the breach, carving out a gully and undermining the whole system.

Lines of Stones, Stakes, Grass or Straw (Figure 48a and b). This heading covers single barrier lines established along contour lines and permeable to sheet runoff. Many examples can be seen in Sudano-Sahelian semi-arid areas of Mali, Burkina Faso and Niger. Stone lines slow down the runoff so that it spreads out in sheets several centimetres deep, thus causing sedimentation first of sand particles, and then of finer particles which tend to clog the surface. The lines filter the water, trapping straw, animal faeces dropped during the dry season on rangelands, and various types of floating organic residue. This creates a localized deposit of fertilizer in the sedimentation area and in the watered area. And if these lines are laid out in the right direction, in the dry season they also trap sand carried off by wind erosion. They enhance production potential by concentrating both water and nutrients over an area 2 to 6 metres broad above the barrier, and redistributing it below it when there is excess water.

FIGURE 47 Development of a semi-pervious erosion control structure to dissipate runoff energy (cf. Roose 1991)

FIGURE 48
Examples of semi-pervious microdams in a semi-arid area (cf. Roose 1989)

Stone line or line of grass, straw, stakes

• a single row of semi-pervious obstacles
• slows down and spreads runoff
• traps wind-blown sand and fine particles in runoff
• delicate: knocked over by livestock and rills, buried by runoff

Figure A

Honeycomb network

• tying (r) reduction in lateral flow
• used to restore soil at tier, foot of hills for water harvesting

Figure B

Low stone bund

• at least 2 to 3 layers of well-fitted stones
• held together by:

- grass > < sideways movements
- hedge + trees > < livestock (downstream)

• traps 5 to 15 cm of sand + OM + silt
• filters floating organic matter
• slows and spreads flow over more time and a larger area

Figure C

Stone walls

• carefully stacked flat stones
• wall + upstream sand draining filter beneath the wall
• develops into gradual terraces

Figure D

Semi-filtering dam

• large stone dam across the head of the valley
• reinforced horizontal crest
• slows flow
• core of finer packed material if a sheet of water is to be held back

Figure E

Hazards. When sheet runoff builds up before the barriers, it eventually finds a way out, and at this point it speeds up (the Venturi effect), carving out a channel below the line, then under the stones, thus burying them.

As it spreads out in front of the barrier, the sheet creates a lateral movement which can lead to a local concentration of runoff and the formation of a more aggressive drainage line capable of carving out channels and shifting gravel and pebbles.

These lines are fragile: a single kick from the hoof of a passing animal can shift a stone, causing a breach through which the water will pour. The resulting rills then develop into gullies.

The lines have a limited life-span. The stakes and straw rot and are attacked by termites. The clumps of grass are choked in the centre, so that they thin out and leave pernicious breaches. The stones are overturned by livestock or buried under sand.

However, the organic matter accumulated in front of the barriers will have drawn termites, which often improve macroporosity and absorption capacity at this point, so that abundant grass and shrubs, and sometimes even trees, will grow there. In order to counter the lateral movement of water that leads to soil erosion, lateral ties can be introduced either by making small ridges perpendicular to the barrier, or by tying the field laterally in a beehive pattern - as is often done by the Mossi in north-western Burkina Faso to restore soil at the foot of hills. Another option is to wait a year until the drainage lines become clear and then use large stones to reinforce the weaker points where water collects. The soil must also be kept rough through regular tillage in order to break up the slaking or sedimentation crust which seals the soil surface. Hoeing the earth up into tied ridges lowers the risk of water concentration. Lastly, a filter of straw or a hedge above the stone or grass lines will make them much more effective.

Low stone bunds (Figure 48c).

Two or three layers of stones are so arranged along the contour line as to reinforce one another, a technique widely used in Yatenga Province of Burkina Faso. Lining one hectare with these stone bunds (400 m) takes about 30 to 60 days' work, plus transport between quarry and field (1 day's use of a truck). The bunds slow down runoff, spreading it in sheets so that it is absorbed in less than one hour, thus causing successive sedimentation of sand, aggregates, and fine, humus-bearing particles, which will then form a sedimentation crust. Only the excess water flows over the first layer of stones. More water is stored than in the case of stone lines, and the sheet of water often covers 5 to 8 metres in front of the permeable barrier. The bund filters straw, animal faeces and various kinds of floating, organic matter, so that farmers see one of its most useful functions as that of maintaining soil fertility.

Theoretically, these stone bunds are laid perpendicular to the direction of the water flow, but not necessarily to winds. They thus do not always trap sand blown across the ground during the dry season.

The bottom of the first layer of stones is planted several centimetres deep in the ground, with soil between them, so that 5 to 15 cm of filtering, organic, sandy soil collects above the bund, improving soil storage capacity and forming new topsoil.

The second and third layers downhill (made of smaller stones or grass) divide the excess flow, absorbing the runoff energy and preventing channels from being dug out downstream of the barrier during major rainstorms. Tillage of the cropped strip and erosion will soon cause an embankment to form. It must be stabilized with grass, for example Andropogon and Pennisetum.

Hazards. If the top of the bund is not strictly horizontal (as in the case of smoothed contour lines), the sheet runoff flows toward the lower points and forms drainage lines, speeding up and carving out channels, which then develop into gullies and drain water from the whole slope. However, a strictly followed contour line will produce fields of widely varying sizes and shapes (a deviation of 10 metres for the slightest termite hill on 2% slopes), which causes problems for mechanized cropping. Even then, small drainage lines will form, but these can be treated in various ways:

• using large stones to reinforce the points where the water collects, and gradually levelling these areas;

• increasing the roughness of the soil through coarse tillage with toothed implements, repeated hoeing, and tied ridging;

• sowing grass (Andropogon) above and around the bund to brake sheet runoff;

• tying the field for 5 metres above the bund with earth ridges or stone bunds - although the latter can cause problems for mechanized cropping (Lamachère and Serpantié 1991).

Damage to the bund by passing livestock can be reduced by planting grass (to cover the stones), and a hedge below the bund, as well as trees which will eventually "tie" the landscape, creating an area dominated by hedges. In areas where stones are scarce, the same effect can be achieved by sowing a 50-cm strip of grass (Andropogon) or a hedge (at least three lines staggered on alternate rows) between two contour ridges (20 cm high) (Roose and Rodriguez 1990). In mountainous areas farmers often collect stones that come to the surface and heap them up on the edges of their fields (particularly on banks around the fields). If these heaps of stones are arranged along contour lines, they act as stone bunds.

A team of ORSTOM scientists (Serpantié, Lamachère, Martinelli and others 1986-1992) have studied the combined effects of stone bunds and tillage, comparing this with a control plot in a water-harvesting area in the Bidi region near Ouahigouya in Yatenga, in northwestern Burkina Faso (Table 37, Figure 49) (Serpantié and Martinelli 1987; Lamachère and Serpantié 1991).

Stone walls (Figure 48d).

These walls are carefully built by piling up flat stones, wedged with small rock chips. They are often found in sandstone hills, for example near Bamako in Mali. The first step is to dig a trench along the contour line, digging down to a coherent horizon, and build a drainage filter made of a layer of sand and gravel on the sides and bed of the trench.

On a medium to steep slope, gradual terraces are quickly formed by throwing the earth from the trench uphill and by water erosion, but above all by dry mechanical erosion during tillage.

Hazards. The direct pressure exercised on the wall by the soil cover and ground water can push it out of true so that it will bulge and, eventually collapsing unless there is good drainage above the wall and a bed of gravel below.

As time passes, the foot of the wall will be eaten away by water erosion or tillage in the field below it. This process can be slowed by planting grass or fruit trees on the bank that develops at the foot of the wall, which will prevent soil from creeping downhill.

Pervious and semi-pervious check dams (Figures 48e and 52).

This approach entails heaping up a line of large stones with the crest following the contour line, in order to dam the head of a valley, thus slowing flow and facilitating groundwater recharge. It can take 300 to 600 days' work to build these enormous stone bunds which are 200 to 300 metres long and 1 to 2 metres high. The length of the dam depends on its height and the depth of the gullying at each point. The crest of the dam must follow the contour line strictly and the dam itself must be set in a foundation trench 20 to 40 cm deep, which must also be covered by a pervious bed. This barrier slows down runoff to the valley bottom, although it passes quickly through the large stones - unless the dam has been built with a pervious core of finer gravel, which can hold back water for several days.

If a sheet of water is to be kept upstream of the dam (in other words, if it is to be a semi-pervious rather than a pervious dam), for example to grow rice, a clayey core behind the gravel filter will be necessary.

Sedimentation upstream of the stone dam is fine and slow (1 mm per year) in a fairly undegraded, undulating landscape, but can be fast (10 to 50 cm per year) in a gullied, hilly landscape (e.g. the area north of Ouagadougou - Dezilleau and Minoza 1988).

Hazards. If water filters too fast through the dam, erosion can burrow through it and below it. If there is too much pressure beneath, the water carves out a gully which will eventually breach the dam through headward erosion. A gravel and sand filter must therefore be built to slow down runoff: this material is poured into the foundation trench under the structure, as well as between the large stones that make up its core (Figure 52).

If the sheet of water flowing over the dam is over 20 cm, the structure may well be swept away by the speed of the current. This can be prevented by placing large stones, or better a small gabion, on the brow of the dam, and also on the slope of its upstream side (2/1 slope).

The water normally drains away fast after the rains stop, although the land immediately above is left waterlogged, so that the sorghum traditionally grown in flat valley bottoms rots and suffers from waterlogging, while water is not available long enough to grow rice - which is prized for feast-days. Only off-season gardens and fruit trees located around the valley in fact profit from the increased groundwater recharge from a check dam. It is therefore important to have a clear idea of objectives. Check dams recharge groundwater but do not hold back much water - in any case not enough to produce rice four years out of five in the Ouahigouya region. If the aim is to create a rice field, it is best to choose an impervious soil-saving earth dam, which will hold back a large enough sheet of water for this crop.

TABLE 37
Tests on the effect of stone bunds and tillage on runoff, erosion and millet yields at the Bidi Station, Samniweogo, northern Yatenga, Burkina Faso (cf. Lamachère and Serpantié 1991)

Tropical, sandy-clayey, well-drained, ferruginous veil, 25 to 220 cm deep over ironstone. Slope 2.5%. Plots of 3100 m³, receiving runoff from a 1250 m² catchment area until the end of 1987.

35 = Markedly different from control treatments

FIGURE 49 Diagram of the experimental layout at Bidi in Burkina Faso

FIGURE 50 Comparison of runoff: control plot/treated plot. Average rainfall

FIGURE 51 Comparison of runoff: control plots/treated plots as a function of rainfall aggressiveness

FIGURE 52 Pervious or semi-pervious check dam

Effects:

• sorghum yields
• security
• slowed runoff
• gully erosion

Problems:

• cost: c. 500.000 CFA francs + collective labour profiting 1 or 2 landowners
• zones

± humid
± waterlogged
± silted

Problems connected with land ownership should also be borne in mind, for building a check dam of this type requires considerable work on the part of the local community (15 people for 30 days) and the transport of a huge volume of stones (from 100 to 500 m³, at an average cost of 4 to 40000 FF). And although the structure takes all this effort on the part of the community, it in fact allows improvement of a mere 0.5 to 1 hectare, probably belonging to a single family. In order to avoid wrangling, advance plans should be made to redistribute the improved land to those taking part - not always a possible solution. Another solution is to negotiate a form of payment by the beneficiary to the neighbours who help in the work, by creating a bank which advances money and recovers it on improved harvests in the following years. Food for work, payment in kind, or an undertaking to work for the others for some days, are other possibilities.

The same amount of effort and the same volume of stones could go to improve 10 to 20 hectares of sloping land belonging to twenty families, who could easily build the necessary stone bunds on their own. It has not yet been ascertained whether this form of watershed management can offer the same production security as management of the valley bottoms, bearing in mind that although certain bottoms are completely flooded in the rainy season, in particularly dry years they are the only places where villagers can be sure of a certain level of productivity.

The importance of ensuring household food security makes it vital to improve both the land in the valley bottoms, which will produce even in dry years, and the land on the hillsides, which will produce best in wet years.

However, it must always be borne in mind that, like other methods, this is not the universal solution. It is valid for certain gullied valley bottoms, but much less so for flat bottoms, where there is very little sedimentation.

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