雨强和坡度对黄土陡坡地浅沟形态特征影响的定量研究

浅沟形态特征是建立陡坡地坡面浅沟侵蚀预报模型的基础。为了定量研究黄土陡坡地浅沟形态特征,在长8 m、宽2 m、深0.6 cm的试验土槽上制作了雏形浅沟,设计了2个降雨强度(50、100 mm/h)和3个浅沟发生的典型坡度(15°、20°、25°),利用模拟降雨和径流冲刷(10 L/min)相结合的试验方法定量分析了黄土陡坡地的浅沟形态特征。结果表明:降雨强度和坡度的增加均加快了坡面浅沟侵蚀过程并使浅沟沟槽宽度和深度不断增加,25°和100 mm/h降雨强度下的浅沟沟槽平均宽度和深度比15°和50 mm/h降雨强度下的分别增加1.40和0.61倍。根据测针板法得到的3 cm×10 cm精度的地表高程值数据,在Surfer软件中生成不同试验处理下的地面数字高程模型(DEM,digital elevation model)及水流流路图等,发现坡度的增加使两侧坡面细沟汇入浅沟沟槽的坡长增大,而降雨强度的增加则导致浅沟沟槽两侧坡面细沟汇入浅沟沟槽的坡长缩短,同时,沟道密度、地面割裂度和浅沟复杂度均随着降雨强度和坡度的增加而呈现增大的趋势,三者分别变化于0.74~1.48 m/m2、0.13~0.29和1.64~2.84之间,而不同降雨强度和坡度条件下浅沟沟槽宽深比变化于0.65~1.27之间。基于不同试验处理下的DEM,根据相邻格网关系在水平方向上计算方向导数后发现,方向导数格网等值线图可以有效地反映坡面浅沟和细沟的长度、表面积及侵蚀最严重的浅沟沟底位置。

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.