Dynamics of wind erosion and impact of vegetation cover and land use in the Sahel : a case study on sandy dunes in southeastern Niger

In the Sahel, wind erosion increases the vulnerability of sandy soil through the removal of the finest and most fertile fraction of the soil, including nutrients, potentially limiting vegetation development. To provide experimental evidence of the role of different land uses on wind erosion, measurements of horizontal sand fluxes and associated meteorological parameters were taken for four years in Kilakina (southeast of Niger), where large rangeland surfaces coexist with cultivated surfaces and moving dunes in a typical Sahelian semiarid regime (average annual precipitation in the order of +/- 300 nun yr(-1)). The horizontal sand fluxes were monitored using BSNE sediment traps on the three sandy surfaces representative of the main land uses in the region: a cultivated field, a pastured land and a mobile dune. In addition, a meteorological station was installed to monitor the wind speed and direction and rainfall. Saltation flux on the bare mobile dune appeared to be an excellent proxy for wind erosivity in this region. The mobile dune produced saltation fluxes 2 to 40 times higher than the cultivated field and the rangeland. The annual fluxes on the dune varied from 2000 to 6000 kg m(-1) due to the variability of the winds from year to year. The sediment fluxes are higher in the millet field than those on the pastured land due to a larger fraction of bare surface compared to the relatively homogeneous grass cover that exists on pastured surfaces. The vegetation produced in a given year affects the wind erosion of the following year through the effect of dry vegetation and litter cover. These results highlight the sensitivity of wind erosion to dry vegetation, even for low cover rates. This outcome suggests that any agricultural practice that decreases dry vegetation cover, such as the uptake of vegetation residues from the fields and the pastured land, has a strong impact on wind erosion. Wind erosion in East Niger can be higher during the Harmattan season than during the monsoon season, while similar measurements in West Niger showed that saltation fluxes were higher during the monsoon season. This outcome suggests that the intensity of the Harmattan surface winds could decrease from east to west. In addition to land use changes, the regional pattern of the surface winds and its future evolution clearly deserve further investigation based on relevant long-term in situ measurements