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In Sahelian ago-ecosystems, sandy microdunes often provide a privileged habitat for primary production and can also be potential initial points for the regeneration of eroded surfaces. The aim of this study is to understand the role of sandy aeolian deposits in the retention of rain and runoff water on overgrazed Sahelian hillslopes. It focuses on the interactions between wind and water processes. Water is beneficial to the development of vegetation if it (i) infiltrates through soil surfaces; (ii) reaches a sufficient depth so that it does not rapidly evaporate; and (iii) is accessible to plant roots. In this study, the wetting-front depth (WFD) proved to be a relevant indicator of these three criteria. Investigations were conducted within three plots with surface area of 14,000, 376 and 36 m(2) and grids of observations of 4, 0.5 and 0.2 m, respectively. The largest plot was a micro-catchment patched with sandy aeolian deposits. The two other plots were isolated microdunes with a typical asymmetric shape. A cone penetrometer was used to estimate the WFD and survey its spatial variability. Soil surface conditions (micro-relief, plant cover, crusted areas) were also estimated following the typology of Casenave and Valentin (Casenave, A., Valentine, C., 1992. A runoff capability classification system based on surface features criteria in semi-arid areas of West Africa. Journal of Hydrology 130, 231-249). At the catchment scale, WFD values ranged from 0-0.82 m. The deepest infiltration occurred within sandy deposits which supported an herbaceous cover of > 50% (drying surface type) and along rills filled with coarse sands (runoff surface type). Minimum WED values were observed on bare crusted surfaces with gentle slopes (erosion surface type). At the microdune scale, the important penetration depths coincided with bare windward surfaces with steep slopes. This unexpected result seems to be due to the combined effect of wind deflation and splash erosion, which would hamper the development of impervious crusts. These results could have significant impacts for better understanding of rangeland sandy soil in semi-arid areas. (c) 2006 Elsevier B.V. All rights reserved.