Impact Of Herbaceous Plant On Size Selectivity Of Erosion Process In Northern Earth-rock Mountainous Area

Northern earth-rock mountainous area suffers serious water and soil erosion problem, which severely restrains the agricultural and forestry productions and leads to continuous degradation of ecological environments, because rainfall in this area is concentrated within several months and soil contains many gravels and is very thin in depth. Forming grass strips by planting herbaceous plant can effectively improve the physical and chemical properties of soil and ecological environments, hence reduces significantly the water and soil loss in this region. The study of impact of herbaceous plant on the size selectivity of the splash erosion, sheet erosion and rill erosion process could give us a better understanding of soil erosion process. It is theoretically and practically significant for building soil erosion predicting models and understanding the influence mechanism of herbaceous plant on soil erosion process and arranging plant measures. This paper conducted laboratory experiments to explore the impact of herbaceous plant on the size selectivity of splash erosion process on cylindrical plexiglas container with different vegetation coverage, which consisted of Pennisetum alopecuroides L. and Lolium perenne L. and Festuca ovina L., at Jiufeng Beijing forestry artificial rainfall simulation hall. Field experiments were conducted to investigate the influence of herbaceous plant on size selectivity of sheet erosion and rill erosion process on runoff plots with different plant densities of Medicago sativa L. at Yanqing soil and water conservation demonstration garden, Beijing. Conclusions obtained in this article were shown as below: (1) The mass percentages of clay, fine silt, coarse silt, fine sand and coarse sand in the splashed particles were significantly correlated with rainfall intensity, vegetation coverage and total splashed amount. The impacts of herbaceous plant exerted on size selectivity of splash erosion process achieved through absorbing rainfall kinetic energy and restraining the development of soil crust.(2) We found that fine size particles including clay and silt were the major composition of splashed particles and increased with vegetation coverage. Meanwhile, coarse (sand) size particles decreased with vegetation coverage but increased with rainfall intensity and total splashed amount.(3) The proportions of clay and fine silt in the splashed particles decreased with the increase of splash distance at shorter distance (0-12.5cm), then increased with splash distance. While the proportions of coarse silt and fine sand increased with the increase of splash distance at shorter distance (coarse silt:0-17.5cm; fine sand:0-12.5cm), then decreased with splash distance. The proportion of coarse sand decreased consistently with the increase in splash distance.(4) The proportion of fine silt was the highest in eroded sediment, followed by the proportions of coarse silt and fine sand, and the proportions of coarse sand and clay were the lowest on alfalfa slope. The temporal fluctuation proportions of different particle size fractions and mean weight diameter (MWD) increased with the increase of vegetation density under the same flow discharge. Meanwhile, the enrichment ratio (ER) of fine particles (clay, fine silt, coarse silt,<50μm) increased and the ER of coarse particles (fine sand and coarse sand,>50μm) decreased. Moreover, under the same vegetation density, the temporal fluctuation of proportions of different particle size fractions and MWD decreased with the increase of flow discharge. At the same time, the enrichment ratio (ER) of fine particles decreased with the increase of flow discharge, which generally exceeded 1, while the ER of coarser particles showed an opposite tendency, which was smaller than 1. There was a negative correlation between MWD and fine particles content, while positive correlation between MWD and coarser particles content.(5) MWD of eroded sediments in rill erosion process was significantly larger than that in sheet erosion process, while the temporal fluctuation of MWD of eroded sediments showed an opposite tendency.(6) The influences of herbaceous plant exerted on size selectivity of sheet erosion and rill erosion process achieved through reducing flow drag forces and increasing the consumption of flow energy. The increases of flow shear stress τ and stream power ω shortened the sheet erosion process and prolonged the erosion process (except under the vegetation density of 60 individual plant/m2 and led to the increases of flow erosion power and coarse particles transported by flow.