Influence of rock fragments on preferential flow in stony soils of karst graben basin, southwest China

•Alternative profile imaging methods can be conveniently applied to identify the rock fragment characteristics.•Rock fragments can both promote and depress preferential flow, with a threshold of 30%.•The coarse gravels (2–7.6 cm) dominate with a threshold of 17 % for promoting preferential flow.•Preferential flow indices (i.e., the preferential flow ratio and length index) are linearly related to the volume fraction. In karst areas, the distribution of stony soils results from bedrock weathering over time, influencing the development of preferential flow to some extent. In this study, field dye tracing experiments and stereological profile imaging methods were performed to identify the effect of different-sized rock fragments in regolith layer on preferential flow. Results show that 95 % rock fragments (5–250 mm) in karst graben basin are scattered below the 15 cm soil layer. The coarse gravels (20–76 mm) dominated the stony soils of karst graben basin, with high number density and volume fraction, 1.4–3.3 times and 2.1–3.7 times higher than those of the medium gravels (5–20 mm) and cobbles (76–250 mm), respectively. The local effective rock fragments can either promote preferential flow with flow paths along the soil–rock interface or depress it, with the total volume fraction threshold of 30 % (17 % of the coarse gravel). Indices describing the degree of preferential flow development, i.e., the preferential flow ratio and length index, are linearly increasing in relation to the volume fraction. The influence of rock fragments in regolith layer on the preferential flow cannot be ignored and provide directions for the underground hydrological processes in karst stony soils.