Impact of tillage erosion on water erosion in a hilly landscape.

Little has been known of the interaction between tillage erosion and water erosion, while the two erosion processes was independently studied. Can tillage-induced soil redistribution lead to exaggerated (or retarded) runoff flow and sediment concentrations in steeply sloping fields? A series of simulated tillage and artificial rainfall events were applied to rectangular runoff plots (2m×8m) with a slope of 15° to examine the impacts of tillage erosion intensities on water erosion in the Yangtze Three Gorges Reservoir Area, China. Mean flow velocity, effective/critical shear stress, and soil erodibility factor K were calculated to analyze the differences in hydrodynamic characteristics induced by tillage. Our experimental results suggest that mean runoff rates were 2.26, 1.19, and 0.65Lmin(-1) and that mean soil detachment rates were 1.53, 1.01, and 0.61gm(-2)min(-1) during the 70-min simulated rainfall events for 52-, 31-, and 10-year tillage, respectively. A significant difference (P<0.05) in cumulative detachment amounts was found among different tillage intensities. Compared with the soil flux of 0kgm(-1), cumulative detachment amounts for the soil fluxes of 9.86 and 24.72kgm(-1) increased by 40.02% and 100.94%, respectively, during the 30-min rainfall event. The results imply that soil and water losses tended to increase with increasing tillage intensity. A significant difference in mean flow velocity occurred near the upper and lower slope boundaries of the field, while significant differences (P<0.05) in runoff depth and effective shear stress were observed among different slope positions. Soil erodibility factor K for the soil fluxes of 9.86 and 24.72kgm(-1) were 2.40 and 5.11 times higher, respectively, than that for the soil flux of 0kgm(-1). As tillage intensity increased, critical shear stress trended to gradually decrease for all soil fluxes. Our results indicate that tillage erosion increases soil erodibility and delivers the soil for water erosion in sloping fields, accelerating water erosion.