Erosion dynamics at the catchment level : spatial and temporal variations of sediment mobilization, storage and delivery.

Abstract

Soil material exported from river catchments by soil erosion is a key issue in environmental sustainability. Although soil erosion processes have been thoroughly investigated, their dynamics, specifically the continuity of erosion processes and sediment source locality, are less studied. The aim of this investigation was to evaluate the changes in the fluxes and characteristics of sediments during their downslope and downstream transport. The study was conducted in a 1000 ha catchment of the Drakensberg foothills, South Africa. Sediment fluxes were monitored at nested scales during the 2009-2011 rainy seasons using 1×1m and 2×5 m erosion plots and H-flumes coupled to automatic samplers from 23 ha, 100 ha catchments. In addition, soil texture, colour and total organic carbon and nitrogen contents in sediments exported from the nested scales and a 1000 ha catchment were compared to in-situ surface and sub-surface soil horizons in a 23 ha catchment river bank and hillslope soils and fluvial sediments. There was a sharp increase of sediment fluxes with increasing slope length (846±201 gm-1y-1 for 1 m2 vs 6820±1714 gm-1y-1 for 10 m2), revealing a limited contribution of splash erosion compared to rain-impacted flow erosion. Sediment fluxes decreased to 500±100 gm-1y-1 and 100±10 gm-1y-1 at the 23 ha and 100 ha catchments respectively, indicating the occurrence of sedimentation during sediment downslope and downstream transport. A principal component analysis (PCA) suggested that rain impacted flow erosion efficiency at the 10 m2 scale was significantly correlated with soil bulk density, clay content and antecedent rainfall (P<0.05). Moreover, strong correlations existed between runoff, sediment concentration and soil loss and selected soil surface and environmental variables at the plot scales. Correlations became weaker at the catchment scales due to increasing landscape heterogeneity and the complexity of soil erosion dynamics. An additional PCA suggested that stream bank erosion contributed to 63% of the soil loss from the 23 ha catchment. During their downstream transport, sediments were discriminated by the second PCA axis, which correlated with the clay and fine silt content, 100 ha sediments showed negative coordinates to this axis while 1000 ha catchment sediment had positive coordinates.