Research Of Runoff,Erosion And Biogenic Element Processes On Covered Slopes

Soil erosion is the dominant environmental problems all over the world,it also is the main driver of natural deterioration,including land degradation,vegetation destruction,sediment deposition,non-point source pollution.Slope is the unit of erosion.Slope erosion process consisted of soil particles' detachment,transportation,deposition caused by rainfall splashing,runoff and rill erosion.Cover implement on slopes was an effective measure for soil and water conservation.Against the erosion problem in mountainous areas in North China,simulated rainfall method was used to conduct erosion experiments in cinnamon soil slopes.Three rainfall intensities(30,60,90mm/h)combined with three slope gradients(10,15,20°)were applied to various cover types.The objectives of our study were 1)observe the runoff and sediment yield process under diffement experimental conditions,explore the effects of different cover types on erosion process;2)character hydraulic properties in different cover slopes,coupe the relationship of runoff hydraulic properties with erosion characteristics.Based on the analysis results of runoff and sediment loss process,the influence mechanism runoff hydrodynamics on soil erosion was studied,and coupling mechanism between soil particle initiation force and sediment production intensity was studied.Finally,the slope erosion process was modeled.Combined with the verification,the main slope soil erosion prediction model was coupled to provide theoretical basis for the soil erosion prevent factors.The results are:(1)The relationship among runoff generate time,slope,rainfall intensity,and coverage was exponential(T=a ?b Rc Cd;T is the runoff generate time,a is slope gradient,R is rainfall intensity,and C is slope coverage).Litter delayed runoff the most,followed by ryegrass,and finally was gravel cover.Rainfall intensity played dominate role in runoff generation.Among six kinds of litter,litter from Ziziphus jujuba var.spinose had the strongest effect on delaying runoff,followed by litter from Platycladus orientalis and Pinus tabuliformis,and then were litter from Vitex negundo var.heterophylla and Broussonetia papyrifera,the worst was litter from Quercus variabilis.Cover increased soil infiltration and reduced runoff effectively.Compared with bare slope,litter,gravel,and ryegrass cover reduced slope yields by 34.13%,43.45%,and 36.91%,respectively.The decrease rates of runoff on Pinus tabuliformis,Platycladus orientalis,Quercus variabilis,Broussonetia papyrifera,Vitex negundo var.heterophylla,Ziziphus jujuba var.spinosa covered slopes were 9.13%,9.54%,14.20%,13.56%,17.27%and 9.47%,respectively.(2)Cover reduced runoff generation and sediment transportation,so that the amount of soil erosion was reduced and cover exerted water quality and conserved soil.Litter,gravel,and ryegrass reduced mean sediment concentration in runoff by 53.74%,18.50%,and 30.14%,reduced total sediment amount by 79.66%,68.18%,and 58.00%.Sediment concentration in runoff on Pinus tabuliformis,Platycladus orientalis,Quercus variabilis,Broussonetia papyrifera,Vitex negundo var.heterophylla,Ziziphus jujuba var.spinosa covered slopes were 10.62%,8.07%,4.75%,2.23%,2.69%and 7.48%,respectively.Total sediment amounts on these slopes were reduced by 45.16%,49.23%,24.00%,76.76%,76.27%and 48.97%.Sediment particle size transported out the slope mainly concentrates in the range of 2-50 ?m.Cover conditions on slope surface could significantly change the characteristics of sediment particles,reduced large particle sediment yield effectively.Cover spatial distribution also changed the function,lower part cover could protect soil surface better.(3)Cover had no significant effect on available nitrogen,available phosphorus,and organic matter in sediment.The concentration of available potassium increased 144%to 325%in comparison to bare slope.TOC concentration of runoff on bare slopes gradually increased during rainfall event,while TOC concentration on covered runoff gradually decreased during rainfall event.TOC concentration in runoff was increased by cover,with the covereage increase,TOC concentration gradually increased.There was no significant effect on IC concentration.TOC concentration in sediment increased with the increase of coverage,cover increased TOC loss rate with sediment.TOC concentration was higher when cover distributed on the lower part of slope.(4)Runoff velocity was slow down on cover condition slopes,cover also changed runoff dynamic characteristics,so that affected sediment transport capacity of runoff.Litter,gravel,and ryegrass cover reduced flow rates by 17.7%,67.91%,and 27.17%.Pinus tabuliformis,Platycladus orientalis,Quercus variabilis,Broussonetia papyrifera,Vitex negundo var.heterophylla,Ziziphus jujuba var.spinosa decreased flow velocity by 32.03%,40.11%,19.79%,47.76%,41.46%and 57.44%,respectively.Single-width traffic was the dominate factor affecting the runoff on the slope.With the increase of single-width traffic,slope runoff increased significantly.Runoff velocity was the key factor determining Reynolds number and Froude number.Cover reduced Reynolds number and Froude number significantly,cover also changed runoff regime,and weakened the disturbance of runoff on the soil surface.The increase of coverage reduced runoff Reynolds number and Froude number effectively,while the increase of rainfall intensity and slope increased Reynolds number and Froude number.Slope gradient had less influence on runoff regime in comparison to rainfall intensity.Cover also increased surface roughness degree and runoff resistance coefficient.With the cover of litter,gravel,and ryegrass,Darcy-Weisbach resistance coefficient was 2.85,1.67,6.04 times of bare land.Runoff resistance coefficient on Pinus tabuliformis,Platycladus orientalis,Quercus variabilis,Broussonetiapapyrifera,Vitex negundo var.heterophylla,Ziziphus jujuba var.spinose covered slopes were increased 1.34,2.38,0.96,2.80,2.90,5.38 times.There were logarithmic relationships between sediment yield rate and resiatance coefficient(Dr=-a 1n f+b).(5)Runoff shear stress and runoff power decreased with the increase of coverage,while they increased with the increase of rainfall intensity and slope gradient.Mean critical shear stress on litter covered slope was 0.8-1.3 N/m2.Critical shear stress on bare slope was 0.5 N/m2 in the same condition.Litter cover increased critical shear stress by 62.5%-70.8%.Mean critical shear stress on the gravel covered slope was 0.7-0.9 N/m2,which was 35.4%-93.8%of that on bare slope.Compared to bare slope,shear stress on Pinus tabuliformis,Platycladus orientalis,Quercus variabilis,Broussonetia papyrifera,Vitex negundo var.heterophylla,Ziziphus jujuba var.spinose covered slopes were increased by 79.2%,1.4%,0.4%,1.0%,0.1%,and 2.2%.Shear stress was an important sensitive factor in WEPP model.The critical shear stress in this study will be used as an important parameter for WEPP model operation.The critical runoff power of bare slope was 0.0003 N/(m s),runoff power on litter covered slope was 0.0115-0.0190 N/(m s),and critical runoff power on gravel covered slope was 0.012-0.015.N/(m s),on ryegrass covered slope,runoff power as 0.0148-0.0312 N/(m s).The critical runoff power on Pinus tabuliformis,Platycladus orientalis,Quercus variabilis,Broussonetia papyrifera,Vitex negundo var.heterophylla,Ziziphus jujuba var.spinose covered slopes were 0.021,0.024,0.020,0.127,0.377,and 0.027 N/(m s),respectively.(6)RUSLE and USLE-M are improved models from USLE,whereas WEPP model is a physical model based on soil erosion process.In this research,soil erosion processes on covered slopes were simulated with the models,RUSLE,USLE-M,and WEPP model.The results showed that on vegetated slopes,Ens of USLE-M ranked in the beginning.On Pinus tabuliformis,Ziziphus jujuba var.spinose covered slopes,USLE-M performed the best in soil erosion simulation.On other covered slopes,USLE-M ranked the second in soil erosion simulation.This was for that in USLE-M,runoff factor was added based on EI30,make the simulation ability better.Then was WEPP model,WEPP model worked the best on Platycladus orientalis,Broussonetia papyrifera covered slopes.WEPP model was more accurate for runoff simulation on forest cover slopes.However,runoff of WEPP model to run USLE-M weakened its simulation ability,and the Nash-sutcliffe efficiency coefficient will decreasee.USLE-M had better simulation results when there was corresponding runoff data.Finally,the forest cover factor and WEPP model interception parameters of the RUSLE model were revised.The C values for the cover of Pinus tabuliformis,Platycladus orientalis,Quercus variabilis,Broussonetia papyrifera,Vitex negundo var.heterophylla,Ziziphus jujuba var.spinose were 0.38,0.25,0.52,0.10,0.15,0,26,respectively.In this study,the interception model of WEPP model was improved based on the actual measured interception data.The improved WEPP model verified that the improved WEPP model simulated the slope erosion process of forest vegetation.The efficiency coefficient of Ens were improved significantly.