Spatial and temporal variability of the erosion resistance of loess-derived topsoils during concentrated flow

Knowledge of the spatial and temporal variability in soil erosion resistance during concentrated runoff (SER) is indispensable for accurate predictions of soil losses due to rill and gully erosion. Information on soil erodibility during concentrated flow is scarce compared with the well-studied rill and interrill erodibility. Nevertheless, soil erosion during concentrated flow is responsible for the main part of the total sediment budget in the European loess belt. This dissertation aims at explaining the dynamics of SER to allow the development of effective soil erosion control practices. To determine SER, undisturbed topsoil samples were taken regularly on cropland in the Belgian loess belt and subjected to simulated concentrated runoff in a laboratory flume. Measuring soil and environmental characteristics contemporary with sampling allowed to explain the observed dynamics in SER. Together, flow shear stress and soil characteristics (initial soil moisture content, bulk density, root density, soil shear strength, crop residue amount and microbiotic crust cover) explain 78% of the observed variability in soil detachment rates. Field mapping of rills and gullies in the Belgian loess belt demonstrated that 74% of the variations in measured channel cross-sectional areas can be predicted based on the estimated SER and flow intensity. In addition, a procedure to predict soil detachment rates in the presence of crop residues or vegetation was experimentally validated. This procedure allowed explaining the erosion reduction when reduced tillage is applied instead of conventional ploughing. Only a small part of the difference in soil erosion for both tillage practices can be explained by the crop residues at the soil surface that slow down runoff. The improved soil structure under reduced tillage is responsible for the main part of the reduction in soil losses, implying that the effect will even increase on the long-term. Finally, soil erosion control practices for rill and gully erosion, based on enhancing SER, were proposed and evaluated. Application of reduced tillage and the establishment of grassed waterways are very effective. Besides, small control practices such as the stimulation or preservation of microbiotic crusts and double drilling or compaction of the thalweg can cause a considerable decrease in soil losses.status: publishe