Quantification study of rainfall intensity and slope gradient impacts on ephemeral gully morphological characteristic on steep loessial hillslope

Abstract: Ephemeral gully is formed in the cycle of concentrated flow and tillage practices, and contributed a lot to the sediment yield on the loessial hillslope as well as in the watershed. Ephemera gully morphology is the base of establishing ephemeral gully erosion prediction model on the steep loessial hillslope. Thus, to quantify the ephemeral gully morphological characteristics on steep loessial hillslope in different rainfall intensities and slope gradients, an 8 m long, 2 m wide and 0.6 m deep slope adjustable soil pan was used to make an initial ephemeral gully channel on the soil bed according to the topographic characteristics of natural ephemeral gully after tillage and before rainy season. The initial ephemeral gully was placed at the middle of the soil bed with a depth of 12 cm. The soil used in this study was loess soil (fine-silty and mixed, with 28.3% sand, 58.1% silt, 13.6% clay), classified as Calcic Cambisols (USDA Soil Taxonomy). Soil materials were collected from 0 to 20 cm in the Ap horizon of a well-drained site in Ansai County, Shaanxi Province, and packed according to natural soil structure of the farmland on Loess Plateau. Two rainfall intensities (50 and 100 mm/h) and three typical slopes on which ephemeral gully occurred and developed (15°, 20°, and 25°) were designed in this experiment. Simulated rainfall and runoff scouring experiments were carried out at rainfall simulation laboratory of the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau. Needle board method was used to measure the topography after 70 min experiment and the DEMs (digital elevation models) were generated in Surfer 10 with a resolution of 3 cm ×10 cm. The results showed that, the increasing of rainfall intensity and slope gradient accelerated the ephemeral gully erosion processes on the steep loessial hillslope. Average ephemeral gully width and depth in 25o and 100 mm/h condition were 1.40 and 0.61 times larger than those in 15° and 50 mm/h condition. Based on DEMs after rain and flow path figures, it could be concluded that the increasing of slope gradient increased the slope length required for the converging of rills and ephemeral gully channel, and the angle of rill and ephemeral gully channel at converging point was decreased; while the increasing of rainfall intensity decreased the slope length required for the converging of rills and ephemeral gully channel. Gully density, surface dissected degree and tortuosity complexity of ephemeral gully increased with the increase of rainfall intensity and slope gradient, varying from 0.74 to 1.48 m/m2, from 0.13 to 0.29, and from 1.64 to 2.84, respectively, while ephemeral gully channel width to depth ratio ranged from 0.65 to 1.27 and was the smallest when slope gradient was 20o. Directional derivative distribution was generated from original DEMs after rain according to the relationship between the neighbor grids, and it could be concluded that contour map of directional derivative grids reflected the length, surface area and gully bottom position of ephemeral gully and rills. More studies should be done on the ephemeral gully morphology and hillslope ephemeral gully erosion prediction model.