Vegetative control of soil erosion by wind in the Sahel region of Mali, west Africa

This thesis is an investigation of soil erosion by wind within the region of Mopti located on the edge of the Inland Delta of the Niger River. Analysis of recent meteorological records showed an average of 164 dust events occur each year. The landscape types investigated were: Hardpan--a wash surface on the Inland Delta with crusted soils and no vegetation. Rice Cultivation--ploughed soils in flooded areas with no vegetation during the dry season. Millet Fields--on sandy soils, mounded by cultivation and with tall (9m) widely spaced trees (Faidherbia albida). Coppice Dune--vegetated (Leptadenia hastata) dunes on abandoned rice cultivation sites, mean vegetation height is 1m and cover is 19%. Steppe--on flat silty-clay soils with low (2.2m) shrub and tree species (Acacia spp.), mean vegetation cover is 13%.;Mean surface (0.6m) wind speeds relative to the wind speed at 10m were: Steppe, 46%; Coppice Dunes, 49%; Miller Fields, 56%; Rice Cultivation, 58%; Hardpan, 68%. The use of roughness concentration (Lc) proved to be an unsatisfactory indicator of surface erosion susceptibility because of inter-site differences in vegetation structure and canopy porosity. For example, a forest, vineyard and wheat field had similar Lc's to the Millet Field, but wind flow and wind erosion control would be different within each of these landscape types.;Shear stress ratios computed for Coppice Dune and Steppe showed shear stress is partitioned approximately 70% to the vegetation and 30% to the ground. The shear stress ratios were higher than predicted by Gillette and Stockton (1989) and this was attributed to vegetation porosity. The mean porosity of common deciduous Acacia species in the study region was 39%.;Shear stress partitioning predicted wind erosion of soil is prevented 80-85% of the time where the natural cover of vegetation is intact. In contrast, agricultural landscapes were predicted to be erosive and no erosion control strategies were observed within agricultural fields in the study region. This suggests there is the potential to reduce soil erosion by wind by using the existing body of knowledge on wind breaks, conservation tillage, cover crops and agroforestry. Finally, there is a need to develop land use policies for the implementation of strategies which reduce the problem of soil erosion by wind in the Mopti region.