Influence of vegetal cover on sediment particle size distribution in natural rainfall conditions in a semiarid environment

Abstract

Sediment samples were collected during four years from two plots, one covered by natural vegetation and the other, a “replica” of the same plot, from which the vegetation was removed manually (disturbed plot), in order to evaluate the effects of vegetal cover on sediment particle size distribution. Particle-size was measured using a Coulter LS130 laser diffraction device. The effective size distribution of the sediment was compared with equivalent measurements of the same samples after chemical and mechanical dispersion (ultimate size distribution) to investigate the detachment and transport mechanisms involved in sediment mobilisation. The ratio between the ultimate particle size composition of the transported sediment and that of the parent soil provided a measure of the particle-size selectivity of the transported sediment. The median effective particle was coarser than the median ultimate particle for almost all the events recorded in both plots, confirming that much of the sediment in the runoff includes a substantial proportion of aggregates. Moreover, the texture of the sediment was finer than that of the matrix soil, with a depletion of clay and sand in exchange for an enrichment in silt, indicating the existence of transport selectivity in both plots. The effect of vegetal cover on the size distribution of the sediment was, particularly evident after high intensity rainfall events. An I 30 of 40 mm h −1 was found to be the threshold value for changes to be produced in the sediment texture of the disturbed plot, with aggregates being broken down and 10%–20% more sand particles being observed at this value than after events of lower intensity. No differences were found in the natural plot after rainfall of varying intensities. The vegetal cover in the natural plot reduced the energy available for erosion from the rainfall by 50%, and, the energy from the runoff by 75%, hindering the break down of aggregates and the transport of dense particles. Moreover, the presence of vegetation meant the raindrop was the predominant contributor to soil loss in contrast to the disturbed plot where both raindrop and runoff contributed to the erosion.