Stone lines are stones grouped in the shape of a line and placed along the contours. The stones can be of different sizes. The goal of these lines is soil conservation and runoff reduction, as they are used to slow down runoff water 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).
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%.
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.