SURFACE-ROUGHNESS EVOLUTION OF SOILS CONTAINING ROCK FRAGMENTS

Soil surface roughness is a dynamic property which determines, to a la rge extent, erosion and infiltration rates. Although soils containing rock fragments are widespread in the Mediterranean region, the effect of the latter on surface roughness evolution is yet poorly understood. Therefore, laboratory experiments were conducted in order to investig ate the effect of rock fragment content, rock fragment size and initia l moisture content of the fine earth on the evolution of interrill sur face roughness during simulated rainfall. Surface elevations of simula ted plough layers along transects of 50 cm length were measured before and after simulated rainfall (totalling 192.5 mm, I = 70 mm h(-1)) wi th a laser microreliefmeter. The results were used to investigate whet her systematic variations in interrill surface roughness along stony h illslopes in southeastern Spain could be attributed to rock fragment c over and rock fragment size. Soil surface elevations were measured alo ng the contour lines (50 cm long transects) with a contact microrelief meter. Roughness was expressed by two parameters related to the height and frequency of roughness elements, respectively: standard deviation of de-trended surface elevations (random roughness: RR), and correlat ion length (L) derived from exponential fits of the autocorrelation fu nctions. The frequently used assumption that surface roughness (RR) of cultivated topsoils decreases exponentially with cumulative rain is n ot valid for soil surfaces covered by rock fragments. The RR of soils containing small rock fragments (1.7-2.7 cm) increased with cumulative rainfall after an initial decrease during the first 17.5 mm of rainfa ll. For soils containing large rock fragments (7.7 cm), RR increased w ith rainfall above a threshold rock fragment content by mass of 52 per cent. For a given rainfall application, RR increased non-linearly wit h rock fragment content. The correlation length for soils containing s mall rock fragments decreases with rock fragment content and is signif icantly lower than for soils with large rock fragments. Soils covered with small rock fragments (large RR and small L) are thus well protect ed against raindrop impact by a water film in the depressions between the rock fragments. On abandoned agricultural fields along hillslopes in southeastern Spain, rock fragments cover increases non-linearly wit h slope owing to selective erosion of finer particles on steep slopes. The increase of surface cover by large rock fragments (>25 mm) is eve n more pronounced. The simultaneous increase of rock fragment cover an d rock fragment size with slope explains the non-linear increase of RR with slope. These relationships differ for soils covered by platy mis aschists and those covered with cubic andesites. The variations in cor relation length along the hillslopes are not clear, probably owing to a simultaneous increase in rock fragment cover and rock fragment size. These findings may provide a better prediction of soil surface roughn ess of interrill areas covered by rock fragments using slope angle and lithology.