基于模拟降雨的北京褐土坡地土壤团粒流失特征试验

Particle Characteristics of Eroded Cinnamon Soil from Beijing Based on Simulated Rainfall

选取20°北京典型褐土坡面径流小区为试验对象,基于野外人工模拟降雨试验和有无雨滴打击作用对坡面侵蚀的影响,研究了坡面土壤团粒组成及其变化特征,揭示了坡面侵蚀过程中泥沙团粒的分离和输移规律。试验处理包括3种代表性降雨强度（35、65、100 mm/h）和3种植被盖度（0%、30%、80%）。结果表明,消除雨滴打击作用后,坡面侵蚀特征变化明显,坡面含沙量和土壤分离率分别减少25.91%~31.15%和35.10%~41.20%,坡面侵蚀泥沙团粒中值粒径均小于雨滴击溅坡面。通过侵蚀泥沙有效粒径分布和最终粒径分布的比值（E/U）分析泥沙团粒的粒径分选特征,发现产流初始阶段粗砂、细砂、细粉粒和粘粒多以团聚体形式存在,而粗粉砂以初级粒子形式存在;随着降雨历时延长,侵蚀泥沙各粒级的分离程度增加,泥沙颗粒逐步分解为初级粒子。坡面侵蚀泥沙分离规律表明,泥沙团粒结构变化与坡面水动力学特征密切相关,土壤团聚体分形维数（D）与时间（T）呈幂函数关系。坡面产流前雨滴击溅对土壤分离有重要作用,其对土壤分离贡献率为28.09%,而无雨滴打击坡面土壤团聚体分形维数增量是有雨滴打击增量的48.43%。在该区坡地泥沙颗粒输移过程中,稳定性较差的砂粒被分解为细小颗粒,粗粉砂多以初级粒子形式存在,对坡面侵蚀泥沙颗粒分离过程具有重要影响,而粘粒在侵蚀坡面则逐渐富集。

In order to study and illuminate the effects of different raindrops and runoff characteristics on size-selectivity of sediments crumb structure detachment and transportation in soil erosion process,a rainfall simulation study was conducted to examine the sediment particle dynamic distribution characteristics in Beijing, China. Erosion plots（ 5 m × 1 m） were installed in a 20° hillslope.Experiment treatments included three rainfall intensities（ 35 mm / h,65 mm / h and 100 mm / h） of representative rainfalls and three average vegetation coverages（ 0%,30% and 80%）. The changes of sediment crumb structure at erosion plots with and without raindrop impact were observed through placing the nylon net over the soil surface. The results showed that sediment concentration and soil loss were reduced by 25. 91% ~ 31. 15% and 35. 10% ~ 41. 20%, respectively, after eliminating raindrop impact. The sediment mean particle size（ D50） of the slope without raindrops was less than that of the slope with raindrops,which was decreased by 23. 47%,27. 17% and 31. 63% under the three kinds of rainfall intensities. At the initial stage of runoff,coarse sand-size,fine sand-size,fine silt-size and claysize existed in the form of aggregation,and coarse silt-size was in a state of primary particles. With theextension of rainfall,the E / U（ effective-ultimate size） ratio of each graded size was gradually tended to be the value of 1,which indicated sediment particles existed in the form of primary particles. The correlation analysis of the sediment particle size and water flow power under different vegetation coverages showed that the slope erosion process was closely related to the hydrodynamic characteristics. The effect of splash on soil surface accounted for 28. 09% of the total detachment degree before the runoff generation,and the soil aggregate fractal dimension increment with no raindrops on the slope was48. 43% of that with raindrops on the slope. In the sediment transport process,sand particles was decomposed into small particles,coarse silt particles were transported as primary particles,and clay particles was gradually enriched in the erosion process. The sediment size-selectivity characteristics by erosion dynamics in the process of erosion were revealed,which deepened the research on erosion regularity of different erosion processes.