森林植被对坡面土壤水蚀作用的动力学机理

Dynamics mechanism of the effect of forest vegetation on hill-slop sater erosion

水力侵蚀是目前世界上分布最广、危害也是最为普遍的一种土壤侵蚀类型。坡面土壤侵蚀主要是由雨滴击溅、坡面径流引起,而森林植被作为陆地上最重要的生态系统以其林冠层、林木茎杆、林地上富集的枯枝落叶层、根系层以及发育疏松而深厚的土壤层截持和蓄储大气降水,发挥着其特有的水文生态功能,从多个角度影响降雨和坡面流的水力特性,在防治土壤侵蚀方面有其不可缺少的意义,然而目前对森林植被防治坡面土壤水蚀机理系统的研究还较少。系统的总结了森林植被各个垂直层次对坡面水蚀作用的动力学机理以及不同学者在此领域所做出的研究成果及此项研究的研究现状,并从以下几个方面指出了林地坡面水蚀作用动力学机理研究中尚存在的问题及发展方向:林冠对降雨重新分配出现林冠截持和干流等现象,降雨雨滴的大小、分布、降落速度和动能等性质发生变化,林冠层通过改变雨滴特性来影响坡面流水力特性,进而改变坡面流对坡面的侵蚀机理;森林植被茎干对径流的分散阻止作用,增大地表径流的阻力系数,茎干绕流现象对坡面土壤侵蚀的作用有正反两方面,林木在一定种植密度内,会使得泥沙起动流速减小,增加坡面侵蚀,因此应合理选择林木的种植密度才能起到减少坡面水蚀的作用;坡面流在枯落物层中流动并穿过枯落物层后下渗进入土壤的过程,类似于水流在多孔介质中的流动,枯落物的物理性质如分解程度、空隙度等的变化,引起水流流动的状态变化复杂,有必要应用渗流理论来深入研究以搞清其流动机理;根系层的存在能逐步改善土壤的内在特性,稳定表土层结构、提高土壤入渗性能使其抗侵蚀能力加强,植物根系层对坡面水蚀作用的研究是一个崭新的领域,需从土力学和植物根系影响土壤力学性质的角度研究土壤的抗侵蚀能力。

Water erosion is the most universally-distributed soil erosion type at present and does a lot of harm. Soil erosion is a chief cause of land degradation, which is also an important factor of eco-environment deterioration. The process of hill- slope water erosion depends on the interaction of rainfall, the erosion power of runoff and the ability of the soil to resist erosion. Forest vegetation participates in the soil erosion process, regulating and controlling water erosion with its peculiar hydro-ecological characteristics, changing the erosivity of rainfall and runoff to hill-slopes. Hydraulic characteristics of overland flow, such as flow velocity, flow depth and friction coefficients, etc. , and their relationships have been studied widely on overland flow. However, few studies have been done examining inter-rill flow in vegetation-covered plots under rainfall conditions. This paper illuminates dynamic mechanics of each vertical part of forest vegetation influencing hill-slope water erosion, summarizes the research achievements and research progress that different scholars have made, and points out the problems and developing directions in this field that currently exist： Forest vegetation canopy redistributes rainfall and changes the characteristics of rainfall such as the raindrop size distribution, rainfall velocity and raindrop kinetic energy. These have an influence on hydraulic characteristics of overland flow and can change the mechanism of soil erosion on hill- slope. Trunks prevent and disperse runoff and increase the surface runoff resistance coefficient. Runoff flowing around trunks has both positive and negative functions on hill-slope water erosion. Within a certain planting density, forest vegetation can decrease initial velocity of sediment and increase hill-slope water erosion, so reasonable planting density should be chosen to decrease water erosion. Runoff flows in and through the litter layer and infiltrates into the soil, similar to the flow in porous media. Litter has a different flow state for different degrees of decomposition, so it is necessary to do further research to get the flow mechanism by percolation theory. The forest vegetation root system gradually improves the inherent characteristics, structure stability and infiltration of the soil, and strengthens the anti-erosion ability. Research on the role of plant root system to the slope soil erosion is a new field. Anti-erosion ability of soil should be studied from the aspects of soil mechanics and plant roots influencing soil mechanic characteristics.