其他摘要 | 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. |
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