Critical thresholds for stage division of water erosion process in different ridge systems in mollisol region of Northeast China

Extreme rainfall events on a slope under ridge tillage systems cause concentrated stream soil loss. To analyse the critical thresholds for different stages of water erosion process of ridge systems, simulated rainfall-erosion experiments for the contour wide ridge (CWR), contour narrow ridge (CNR), longitudinal wide ridge (LWR), and longitudinal narrow ridge (LNR) were conducted under four rainfall intensities, with slope gradients of 3° and 5°. For the runoff event, the runoff depth order was LNR > LWR > CWR > CNR; the soil loss order was CNR > LNR > CWR > LWR. The product of slope factor (S) and rainfall erosivity (R) or runoff depth (D), can be adopted as critical thresholds for different stages of runoff and soil erosion process. For the longitudinal ridge systems, R values were provided for LWR and LNR and were the beginning of sheet flow, whereas the product of rainfall erosivity and slope factor (RS) values were provided for LWR and LNR as the beginning of the accelerated concentrated flow. For the contour ridge systems, R values were provided for CWR and CNR as critical thresholds for the beginning of overflow. The product of runoff depth and slope factor (DS) values were 9.98 and 7.73 mm for CWR and CNR, respectively, and were critical thresholds for the beginning of ridge failure; the DS values were 18.45 and 12.75 mm for CWR and CNR, respectively, and were critical thresholds for the beginning of the formation of ephemeral gully erosion. The critical thresholds can distinguish different stages of soil erosion process modelling.