Similar names: cordons pierreux, stone bunds.
Stone lines are stones grouped in the shape of a line and placed along contours. The stones can be of different sizes. The goal of these lines is to conserve the soil and reduce runoff, as they are used to slow down water runoff and break its velocity. Hence, they increase infiltration and retain sediment and seeds to make water and nutrients available for crops. Stone lines are most suitable for water harvesting on slightly sloping plains (up to 5%) in semi-arid regions. For slopes starting from 5%, stone bunds can be used (see eyebrow terraces).
METHOD OF APPLICATION
- Pick-axe, shovel (to dig out stones)
- Wheelbarrow (to transport the stones), or something more adequate if stones are far away from the site.
- Water tube level or something to measure the contour lines; here are some examples of tools that could be used: a string line level or spirit level instrument, hose pipe half filled with water, bottle of water half filled with water, A-frame level.
A stone line is usually 25 cm high and 35-40 cm wide. Both small and big stones are used to build a stone line and they are placed along the contour lines. Smaller stones are placed upslope whereas the larger ones face downslope in order to slow down the runoff, hold nutrient-rich soil sediments and improve water infiltration. The distance between the lines can vary according to the slope and the amount of material available. However, it is recommended to keep the lines 20 m apart from each other for slope less than 1% and 15 m for slopes between 1-2%.
- Collect stones on site or transport them with the help of a donkey cart or lorries (which is much more expensive).
- Align stones along a contour line, with a maximum of 3 stones in width, measured with the help of a water tube level.
- To maintain the stone lines: check and repair the stone lines annually and following heavy rains.
Vegetation is sometimes planted to stabilize the lines, usually where stones are scarcer. The vegetation used usually consist of a perennial grass, as it develops long roots which resist soil erosion (see also the intervention Vegetative lines with vetiver grass).
Stone lines are an easy and cheap intervention if stones are available in the immediate surroundings. This intervention is widely used in Africa, both in dry and humid areas. Moreover, stone lines are often used in combination with planting pits intervention for the rehabilitation of degraded and crusted lands. It is applied in semi-arid areas, on sandy and loamy soils where the slope is lower than 5%. A great example occurs in Niger, where the combination of the two techniques is applied in order to capture runoff, making infiltration more efficient and improving nutrient availability. The pits have a diameter of 20-30 cm, a depth of 20-25 cm and are spaced about 1 m apart in each direction. Stone lines are spaced 20-25 m apart on slopes of 2-5%. With this layout stone lines are quite small, usually, three stones wide and only one stone high and they are placed, along the contour lines, by hand. Very often grass grows between the stones leading to a greater infiltration and helping the accumulation of fertile sediments. Maintenance wise, stone lines need to be repaired annually, in particular after heavy precipitation events.
- Simple and easy to implement (no special equipment or training needed)
- Reduced soil erosion
- Increased water harvesting and infiltration of water in soils
- Increase in organic matter
- Increase in yields
- Rehabilitation of degraded lands
- This intervention can be quite labor intensive if the stones are not available on the place and need to be transported.
- Maintenance: stone lines need to be maintained, especially after heavy rainfall.
- This technique cannot work properly if the slope is too high.
- The spacing of the stone lines can affect the levels of water runoff and crop performance. It is therefore fundamental to well plant their establishment and reinforce the lines if necessary.
COSTS & BENEFITS
Estimation of costs & benefits of intervention
The costs are based on 300m stone lines per hectare, with a slope of 3-4%.
The availability of stones and the slope determines the establishment cost.
Maintenance costs also vary according to the slope, the availability of stones, and on how well the stone lines were built.
|Establishment cost||US $85/ha|
|Labor time||25 person days|
|Maintenance cost||US $1.5|
|Maintenance labor time||1 person days|
Source: case study based in Niger.
CASE STUDIES & USEFUL LINKS
In some cases, stone lines are particularly effective in stabilizing the soil on very steep slopes. Here is a good example of stone lines built to decrease soil erosion and improve yields.
The spacing of stone lines can affect the levels of water runoff and crop performance. Here you can find an interesting study in a sorghum cultivation.
With the aim to reduce severe soil erosion and improve crop growth, large-scale stone lines were established. In this study you can read about the benefits of stone lines or stone bunds to fight desertification and benefit agriculture.
- A very useful video about the mechanism of stone lines and their benefits can be found here.
- Here you can find more information about the establishment of this intervention.
- Good instructions on how to lay out a contour or a graded contour and measure the slope of the land can be found here.
- Stone lines are commonly known in French as “cordons pierreux” and are widely used in Algeria, Burkina Faso, Morocco, and Tunisia. Here and here you can find more about it (available in French only).