Simulations of vegetation and wind erosion from two typical Sahelian millet fields and rangelands

Caroline Pierre 1 ,2, Laurent Kergoat2, Gilles Bergametti1, Eric Mougin2, Christian Baron3, Amadou Abourhamane Toure5, Jean-Louis Rajot6, Pierre Hiernaux2, Beatrice Marticorena1, Claire Delon4
1LISA, Creteil, France, 2GET, Toulouse, France, 3CIRAD, Montpellier, France, 4LA, Toulouse, France, 5Niamey University, Niamey, Niger, 6iEES-Paris/IRA, Medenine, Tunisia

Quantifying wind erosion and dust emissions in the semi-arid Sahel remains challenging because of the large seasonal and interannual dynamics of surface properties, like vegetation cover. In addition, there is an increasing conversion of Sahelian rangelands into croplands, which raises the question of the difference in wind erosion between these two surface types. Wind erosion models have been so far applied separately to croplands and grasslands. The novelty of our study consists in proposing a common modeling approach to simulate the horizontal flux from Sahelian rangelands and croplands. This is achieved through pair simulations of vegetation and wind erosion for these two typical Sahelian land surface types over a 3-year period for two instrumented sites in Mali and Niger. We used two different vegetation models specifically designed to represent the phenology and growth of a rangeland grass and a millet crop, respectively. These models account for the local cropping and pastoral practices. The simulated vegetation compares satisfyingly with field measurements, including the strong seasonal dynamics. We then established specific parameterizations of the aerodynamic surface roughness length (Z0) as a function of vegetation variables, using measurements from the two sites. As a result, the simulated horizontal flux turns out to be higher for a cropland than for a rangeland by approximately a factor 1.5. This suggests that increasing Sahelian cropped areas would increase dust emissions from this region. This difference is mainly due to a time shift between grass and millet growth: indeed, the latter starts growing about 2-3 weeks later than annual grass. The amount of dry vegetation remaining during late dry season, which depends on human activities, also appears to have a noticeable impact on the Sahelian wind erosion.