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Impacts of Young Trees on Hydraulic Characteristics of Overland Flow and Sediment Particle Size in Loess Plateau

ZHANG Ying1,2,ZHENG Xi-lai1,2,ZHANG Xiao-hui3,ZHANG Jian2(1.Key Lab of Marine Environmental Science and Ecology,Ministry of Education,Qingdao,Shandong 266100,China;2.Environmental Science and Technology Institute,OceanUniversity of China,Qingdao,Shandong 266100,China;3.Beijing Water Bureau,Beijing 100038,China)  
The typical trees(one-year-old Platycladus orientalis,Pinus tabulaeformis Carr,Acer truncatum Bunge and Robinia pseudoacacia) on the Loess Plateau were planted in self-designed soil box with adjustable gradient three years ago.The experiments were carried out by a simulated rainfall in laboratory.The effects of forest vegetation canopy and root on the characteristics of overland flow and sediment particle composition were analyzed.The results showed that:(1) Runoff yield was large for all plots.The average overland flow depth of each slope was less than 1 mm.Fr of bare land slope was greater than 1.0,as supercritical flow,for others it was smaller than 1.0,as subcritical flow.After the canopy layer was removed and the topsoil was scrapped off,overland flow of Robinia pseudoacacia slope and Acer truncatum Bunge slope was close to the critical flow,the function of reducing sediment yield and runoff by Robinia pseudoacacia and Acer truncatum Bunge was significantly weaker,while the function of Platycladus orientalis and Pinus tabulaeformis Carr slope became stronger.(2) Clay loss ratio of the slope covered with vegetation canopy was 1.40%~2.00% higher than that of bare land slope.The proportion of sediment particles with diameter bigger than 0.05 mm was 1.04%~1.26% lower than the bare land.But there was no apparent coarsening soil layer on soil surface.The obvious rough bedding texture layer appeared on bare slope.(3) After removing the forest canopy layer and the disturbed topsoil,the average particle size of sediment particles was larger than that covered by canopy but smaller than the bare land slope.And the percentage of the lost clay content with diameter smaller than 0.001 mm fluctuated with the time past.It is suggested that forest root systems could increase anti-erosion capability of forest soil.
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