Geomorphologic approach for modelling the surface features of arid environments in a model of dust emissions: application to the Sahara desert

Mineral dust emissions from arid regions are influenced by the surface features encountered in the source regions. These surface features control both the erosion threshold and the intensity of the dust flux. Recently, a soil-derived dust emission scheme has been designed in order to provide an explicit representation of the mineral dust accounting for the influence of the surface features on the dust emissions. This physical scheme has been validated with micro-scale field measurements. Its large scale application has required the development of additional relations to estimate the input parameters from more accessible data: the mean height and the covering rate of the roughness elements and the mineralogical soil type. The determination of these surface data has been based on a geomorphologic approach which describes the surface features of arid areas in a 1 × 1° grid. Inside each square degree, up to five different areas characterised by different surface features have been distinguished. However, these areas have not been located inside the square degree. Each area can be constituted by several combined surface features, including roughness, vegetation, granulometry. Five main types of landscapes and eight main types of surface features have been distinguished. This approach is based on the combination of various data, mainly topographical, geological maps and climatological analysis. In addition to the problem of scale transfer, the main constraints to obtain a quantitative assessment are the confidence level of the existing data and the number of parameters to document. On the opposite, with this method, the fine scale required by the dust modelling can be separated from the scale accessible by the mapping approach, of the order of the square degree. This method can also be easily improved by aggregating new data and can be extended to other deserts. An example of application is given for the north-west of the Algerian Sahara where the method has been elaborated. The data provided by the modelling of the surface have been used to simulate dust emissions for 1990, 1991 and 1992 over the central and western Sahara. Over these three years, the mean annual dust emission is about 760 Mt·year–1 Although a significant interannual variability exists (mainly due to changes in the wind pattern), the most intensive emissions remain quite constant in terms of location. The percentage of agreement with satellite observations higher than 0.7 is 74 %, but only 32 % when using a model having a single threshold function for dust emission (i.e. the same surface feature for the whole Sahara) (cf. later Marticorena et al., 1997). © 2000 Éditions scientifiques et médicales Elsevier SAS modelling / geomorphology / arid environments / dust emissions / sahara Résumé – Les émissions d’aérosols désertiques à partir des régions arides sont fortement influencées par les caractéristiques de la surface des sols en zone source, aussi bien en ce qui concerne les seuils d’érosion que l’intensité des flux de poussières émis. Récemment, un modèle physique a été développé permettant de rendre compte de l’influence des caractéristiques de surface sur les émissions de poussières. Ce modèle a été validé à partir de mesures de terrain à micro-échelle. Son application à plus grande échelle a nécessité le développement de relations supplémentaires permettant d’estimer les paramètres d’entrée du modèle à partir de données plus accessibles : la hauteur moyenne et le taux de couverture des obstacles présents à la surface et le type minéralogique de la couche superficielle du sol. Ces données de surface ont été déterminées au travers d’une modélisation, à partir d’une approche géomorphologique, de la surface des zones arides en secteurs d’un degré carré. Cinq types principaux de paysages et huit types principaux d’états de surface ont été distingués. L’approche est fondée sur l’utilisation de données variées, essentiellement les cartes topographiques, géologiques et les données climatologiques. L’utilisation de « points de calage » (observations de terrain, photographies aérien * Correspondence and reprints. E-mail address: [email protected] (Y. Callot). Geodinamica Acta 13 (2000) 245–270 © 2000 Éditions scientifiques et médicales Elsevier SAS. All rights reserved S0985311100010445/FLA