Soil loss caused by wind erosion is a widespread phenomenon in the Sahelian zone of WestAfrica. According to Sahelian farmers, scattered vegetation standing in amongst the crop has the potential for a wind erosion control strategy. This study was conducted to study the effect of single vegetation elements on the pattern of average wind speed and sediment transport. This was done by two experiments that were carried out during the rainy seasons of 2002 and 2003 in north Burkina Faso, West Africa. Wind speeds were measured using three sonic anemometers, at a sampling frequency of 16 Hz. Sediment transport was determined by calculating the mass fluxes from 17 MWAC catchers. In this study, a shrub was defined as a vegetation element with branches until ground and a tree as a vegetation element with a distinctive trunk below a canopy.Behind shrubs wind speed near the soil surface was reduced up to approximately seven times the height of the shrub. The observed reduction in wind speed in the area where wind speed was reduced was 15 per cent on average. At the sides of the shrub, wind speed was increased, by on average 6 per cent. As the area of increase in wind speed is one-third of the area of decrease in wind speed, the net effect of a shrub is a reduction in wind speed. A similar pattern was visible for the pattern of sediment transport around a shrub. Downwind of a shrub, sediment transport was diminished up to seven times the height of the shrub. Probably most of this material was trapped by the shrub. Trees showed a local increase of wind around the trunk, which is expected to relate to an increase in sediment transport around the trunk. Mass flux measurements of sediment transport were not made, but visual observations in the field substantiate this. Behind the canopy of a tree, a tree acts similarly to a shrub regarding its effects on average wind speed, but as a tree is generally a larger obstacle than a shrub the extent of this effect is larger than for shrubs. Thus, whereas shrubs are more effective than trees regarding their direct effect on soil loss by trapping sand particles near the soil surface, trees are more effective in affecting soil loss indirectly by reducing the wind speed downwind more effectively than shrubs. Therefore, to reduce soil loss in an area, the presence of both trees and shrubs is crucial.

Wind erosion is an important soil erosion and hence a soil degradation problem in the Sahelian zone of West Africa. Potentially, the characteristic dryland vegetation with scattered trees and shrubs can provide for soil erosion protection from wind erosion, but so far adequate quantification of vegetation impacts is lacking. The aim of this study was to develop a model of windblown soil erosion and sediment transport around a single shrub-type vegetation element. Starting with the selection of a suitable transport equation from four possible sediment transport equations, the effects of a single vegetation element on wind speed were parameterized. The modified wind speed was then applied to a sediment transport equation to model the change in sediment mass flux around a shrub. The model was tested with field data on wind speed and sediment transport measured around isolated shrubs in a farmer's field in the north of Burkina Faso. The simple empirical equation of Radok (Journal of Glaciology 19: 123-129, 1977) performed best in modelling soil erosion and sediment transport, both for the entire event duration and for each minute within an event. Universal values for the empirical constants in the sediment transport equation could not be obtained because of the large variability in soil and roughness characteristics. The pattern of wind speed, soil erosion and sediment transport behind a shrub and on either side of it was modelled. The wind speed changed in the lee of the vegetation element depending on its porosity, height and downwind position. Wind speed was recovered to the upstream speed at a downwind distance of 7·5 times the height of the shrub. The variability in wind direction created a 'rotating' area of influence around the shrub. Compared to field measurements the model predicted an 8% larger reduction in sediment transport in the lee of the vegetation element, and a 22% larger increase beside the vegetation element.

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Wind and water erosion are widespread phenomena throughout the Sahel, especially in the early rainy season, when highintensity rainstorms are often preceded by severe windstorms. This paper describes the results of a survey on the farmers' perceptions of wind and water erosion processes and control measures. In three villages in northern Burkina Faso, 60 farmers were interviewed using semi-structured questionnaires. According to most of the farmers, wind-blown particle transport has an influence on the fertility and the infiltration capacity of the soils. They considered wind-blown particle transport damaging for their crops. Seedlings are damaged by scouring grains of sand or lost when buried in sand deposits. Only 32 per cent of the farmers observed runoff and 15 per cent observed erosion and deposition during periods of high intense rainfall. According to 22 per cent of the farmers intense rainfall has a negative effect on crop production. Plants die because of the effects of stagnating water and plants are damaged by the forces of flowing water.All farmers are familiar with techniques to reduce soil erosion and 96 per cent applied one or more of these techniques. The indigenous techniques are application of manure and mulch. The main constraints to apply these techniques are lack of labour, manure and mulch. New techniques introduced by agricultural organizations are / zaï (a traditional agricultural practice whereby pits of diameter 10-15 cm are filled with compost for sowing) and half-lines, stone rows and sand ridges.Farmers have a good knowledge of wind erosion processes, but do not report the effects of water erosion processes. The farmers are willing to apply new techniques to control soil erosion, but the main constraints to apply these measures are insufficient knowledge and lack of labour.