Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices

Soil erodibilty during concentrated flow (K_c) and critical flow shear stress (τ_cr), both reflecting the soil's resistance to erosion by concentrated runoff, are important input parameters in many physically-based soil erosion models. Field data on the spatial and temporal variability of these parameters is limited but crucial for accurate prediction of soil loss by rill or gully erosion. In this study, the temporal variations in K_c and τ_cr for a winter wheat field on a silt loam soil under three different tillage practices (conventional ploughing, CP; shallow non-inversion tillage, ST; deep non-inversion tillage, DT) in the Belgian Loess Belt were monitored during one growing season. Undisturbed topsoil samples (0.003 m³) were taken every three weeks and subjected to five different flow shear stresses (τ = 4–45 Pa) in a laboratory flume to simulate soil detachment by concentrated flow. To explain the observed variation, relevant soil and environmental parameters were measured at the time of sampling. Results indicated that after two years of conservation tillage, K_c(CP) > K_c(DT) > K_c(ST). K_c values can be up to 10 times smaller for ST compared to CP but differences strongly vary over time, with an increasing difference with decreasing soil moisture content. The beneficial effects of no-tillage are not reflected in τ_cr. K_c values vary from 0.006 to 0.05 sm⁻¹ for CP and from 0.0008 to 0.01 sm⁻¹ for ST over time. Temporal variations in K_c can be mainly explained by variations in soil moisture content but consolidation effects, root growth, residue decomposition and the presence of microbiotic soil crusts as well play a role. τ_cr values increase with increasing soil shear strength but K_c seems more appropriate to represent the temporal variability in soil erosion resistance during concentrated flow. The large intra-seasonal variations in K_c, which are shown to be at least equally important as differences between different soil types reported in literature, demonstrate the importance of incorporating temporal variability in soil erosion resistance when modelling soil erosion by concentrated flow.