SURFACE SEALING AS AFFECTED BY VARIOUS ROCK FRAGMENT COVERS IN WEST-AFRICA

Field studies on the influence of rock fragments on surface sealing, hence infiltration, remain scarce and contradictory. To document this issue, rainfall simulation experiments were carried out on 37 undisturbed 1-m2 plots located along a pedo-climatic transect across West Africa. An important part of the variability of infiltration coefficient (R2 = 0.71) could be explained by a simple model based on the position of rock fragments in the top layer and accounting for the areal percentages of three types of soil surface: (1) bare surface, (2) soil surface covered with rock fragments embedded in the soil surface, and (3) surface with rock fragments resting on top of the soil surface. An even higher determination coefficient (R2 = 0.76) was gained when rock fragment size were accounted for in combination with vesicular porosity. Furthermore, a positive linear relationship was obtained between infiltration coefficient and mean annual rainfall suggesting that additional factors related to climate might be involved, including organic matter content and clay mineralogy. Two main regions could thus be differentiated. In the arid and semi-arid zones, coarse gravel and cobbles embedded in a seal are predominant and generate high runoff. Conversely, fine and medium gravel, mainly free at soil surface, are dominant in the wetter zone, favouring therefore higher infiltration rate.