Effects of rainfall, surface inflow and seepage on ephemeral gully erosion process under contour tillage in the Chinese Mollisol region

Seepage flow greatly enhances ephemeral gully (EG) erosion process, which exacerbates soil degradation, resulting in reduction of land productivity. However, the combined effects of rainfall, inflow and seepage on the EG erosion process remain unclear. Thus, a set of laboratory experiments was conducted to quantify the contribution of seepage to EG erosion process and elucidate its driving mechanisms. The experiments were performed under different rainfall intensities (50 and 100 mm h −1 ), lateral inflow rates (6–33L min −1 ), upslope inflow rates (15–90 L min −1 ), and seepage flow rates (0, 9 and 15 L min −1 ). The results showed that an increase in rainfall intensity, inflow rate, and seepage led to increases in EG erosion rates by 28.55%–304.93%, 35.47%–448.13%, and 14.60%–223.46%, respectively, highlighting the critical role of seepage in intensifying erosion. With increasing seepage flow rate, erosion rate of sidewall collapse varied by −40.00% to 300.00%, bed incision by −66.67% to 233.33%, and headcut migration by −53.49% to 166.67%. The contribution of EG channel erosion to total catchment erosion increased with seepage flow and inflow rate under both rainfall intensities, ranging from 64.82% to 97.03% with a mean of 84.13%. Furthermore, the contributions of rainfall, seepage, and surface inflow to EG erosion ranged from 1.49% to 57.14% (mean 16.39%), 11.16% to 71.43% (mean 37.95%), 4.76% to 86.46% (mean 45.67%), respectively. Seepage made a substantial contribution to EG erosion, primarily through increasing hydrodynamic parameters by 2.12%–244.75%. Dimensionless EG erosion rates exhibited power function relationships with dimensionless hydrodynamic parameters, with stream power outperforming flow velocity, shear stress, and unit stream power. This study emphasizes the necessity of considering combined effects of rainfall, surface inflow and seepage on EG erosion under contour tillage in the Chinese Mollisol region when evaluating gully evolution and establishing an EG erosion dynamics model.