干旱区生态环境知识资源中心

180 samples representative of surface soils erodible by wind were collected in South Tunisia to retrieve their grain size distribution. Dry sieving using 13 sieves was favour to preserve as much as possible the soil aggregates in their natural state. Two different methods were used to characterize the grain size distribution; the phi method of moments and the use of a combination of log-normal statistical functions. Both methods underline the ubiquitous and dominant presence of a very fine sand population (around 100 mu m in diameter), very well sorted (sigma=1,23) which corresponds to the grain size range for which the wind energy required for initiating the aeolian erosion is minimum. This clearly suggests that most of the soils in South Tunisia have the potential to be easily eroded by wind. However, in about 1/3 of the samples mainly those located in Dahars and in the vicinity of mountains, a secondary population of coarse grains (1500 mu m in diameter) is also observed. This coarse mode could significantly reduce the wind erosion of the soils where it is present by covering a part of the surface and by limiting the efficiency of the wind transfer to the surface. (C) 2011 Lavoisier SAS. All rights reserved