Effect of tillage layer depth on erosion driven by surface-subsurface runoff coupling under rainfall simulation conditions

The surface tillage layer structure of sloping farmland has a significant impact on rainfall-runoff distribution; however, the relationships between the Tillage Layer Depth (TLD) and surface-subsurface runoff, and the coupling effects of surface-subsurface runoff on soil erosion are still unclear. Thus, a set of laboratory experiments were conducted to reveal impacts of tillage layer depth (10, 20 and 30 cm) on surface-subsurface runoff relationships, eroded sediment processes, and soil erosion pattern evolution under the long-duration (180 min) rainfall simulation tests. A deeper TLD mitigated soil erosion. When the TLD increased from 10 to 30 cm, the average surface runoff decreased by 13 %, subsurface runoff increased by 5 %, and soil loss rate decreased by 19 g m −2 min −1 . The interaction between surface runoff and subsurface runoff, influenced by the tillage layer depth, significantly impacts soil erosion. Both surface runoff and subsurface runoff promoted soil erosion at shallow tillage layer depths (10 and 20 cm). Conversely, at TLD 30, the diversion effect of subsurface runoff on surface runoff was enhanced, which played a role in alleviating soil erosion. With the increase of TLD, the soil erosion pattern changed from rill erosion to sheet or splash erosion. During the interill erosion stage, soil loss primarily occurred in the early stage, wherein the Variation Ratio (VR) of soil loss rate and surface runoff coefficient ranged from 2.16 to 4.99. At the rill erosion stage, the VR was approximately 1.0, and the soil loss rate was 2.7- to 6.3- fold greater than that in the interrill erosion stage. These results increase understanding of the effects of TLD on the coupling relationship of surface-subsurface runoff, which is of great significance for alleviating slope farmland erosion.