Technical NoteLaboratory investigation on the role of slope on infiltration over grassy soils

•The role of slope on infiltration is investigated by laboratory experiments.•Deep flow decreases with increasing the surface slope in both bare and grassy soils.•In sloping bare soils there is runoff even for rainfall rate less than Ks.•In grassy soils the effect of slope gradient on infiltration is greatly reduced.

Even though natural surfaces are rarely horizontal, infiltration modeling has been primarily confined to horizontal surfaces, and there are not enough studies to clarify the effects of slope on the partition of rainfall into surface and subsurface water. Besides, previous experimental results on the effects of slope provide conflicting conclusions perhaps because of the existence of erosion and crust formation. In this study, new laboratory experiments, performed in the absence of the last two processes, highlight the effect of the slope angle, γ, on infiltration into a grassy soil. The results are compared with those from previous experiments performed on a bare soil and interpreted in terms of an effective soil saturated hydraulic conductivity, Ke (γ). The grassy soil dampens the variation of Ke with γ compared to bare soil. For example, for γ = 10°, the reduction of the gravitational infiltration with respect to the saturation condition was ∼80% for the bare soil, while we find it to be ∼20% for the grassy soil. Finally, we point out that the presence of grass does not affect the results through the development of a two layered soil, but through a substantial variation of roughness.