| The Loess Plateau is one of the main sloping cropland area in China.The soil and water loss on sloping cropland will not only lead to the destruction of the arable layer,the loss of soil nutrients,the decline of land productivity,but also the increasing river sediment concentration and the spread of pollutants across regions.Therefore,it is of great significance to study the law of soil erosion on sloping cropland and how surface conditions affect it.The surface condition of slope cropland mainly includes the soil surface roughness and crop coverage.The soil surface roughness refers to the micro terrain with high and low undulation caused by tillage practices and soil erosion.It is one of the important factors that affect the flow velocity,flow pattern and resistance characteristics,and has an important impact on runoff and sediment yield.Crop is also an important factor affecting soil erosion on sloping cropland.In this study,the soil surface roughness formed by different tillage practices and the crops were taken as the research objects.Three common tillage practices,which included manual dibbling(MD),manual hoeing(MH),coutour drilling CD)and three common crops,which included millet,maize,soybean were selected as the tested surface conditions,and the smooth surface without any tillage practices and crops was taken as the control group.Based on the research method of indoor runoff scouring experiment,the roughness characteristics of tilled surface,the characteristics of crop root system,runoff initiation time,flow velocity,flow pattern,resistance characteristics and erosion law were systematically studied.The main conclusions are as follows:(1)The flow velocity on the artificial simulated rough surface was less than that on the smooth surface,and there was a negative correlation between the flow velocity and the surface roughness,which showed that the artificial simulated roughness reduced the flow velocity.Slope gradient and unit discharge had the effect of increasing flow velocity,while the effect of unit discharge on flow velocity was more significant than slope.With the increase of roughness,the change of Reynolds number showed that the flow pattern was changing from turbulent flow to laminar flow.However,as the depth and roughness of overland flow tended to be close,the influence of roughness on the boundary conditions of overland flow was enhanced,resulting in significant turbulence characteristics of the flow,which showed that the Reynolds number of open channel could not be the only criterion for judging the flow pattern of overland flow.The increase of surface roughness would also reduce the Froude number of overland flow and increase the Darcy-Weisbach friction factor,which indicated that the artificial simulated roughness had a significant resistance effect.(2)The relationship between the soil surface roughness of the tilled surface and the depression storage capacity was not simply positive correlation so that soil surface roughness could not precisely represent the morphological characteristics of tillage practices.The results also showed that depression storage capacity decreased with the slope gradient.the The runoff initiation time had a positive correlation with soil surface roughness when compared with the smmoth surface,which meant soil surface roughness delayed the runoff initiation.This delay effect of soil surface roughness on runoff initiation was more significant under the condition of low slope gradient and unit discharge,and gradually weakened with the increase of slope gradient and unit discharge.The flow velocity of tilled slope was lower than that of smmoth surface and showed a trend of CK(0.331 m/s)>MD(0.190 m/s)>CD(0.167 m/s)>MH(0.137m/s).The results showed that,due to the change of the surface morphology in the process of erosion,the soil surface roughness was also reduced,and resistance effect decreased with time.In the view of erodibility,the trend of CD>MH>MD was presented.The flow pattern of tilled surface was turbulent,and with the increase of slope gradient and unit discharge,the flow turbulence was gradually increasing.The average Reynolds number of the tilled surface was lower than that of the flat surface,which indicated that the tilled surface reduced the flow turbulence.The runoff on tilled surface was generally slow flow,while that of flat surface was mainly rapid flow,which showed that soil surface roughness had significant resistance effect.The average Darcy-Weisbach friction facor of rough surface was larger than that of the flat surface,and the decreasing rate of the surface roughness and resistance coefficient showed a positive correlation with initial surface roughness.Both f-Re and f-fr were positively correlated and showed a relationship of power function.The coefficient of determination of f-fr regression equation was in high level,which indicated that the Darcy-Weisbach friction factor and Froude number were highly correlated under the condition of tillage practices.(3)The soil surface roughness of the tilled surface increased the soil detachment rate as rough surface was found promoting runoff concentration.The soil detachment rates of different tillage measures were CD(6.762 g/m~2 s)>MH(4.180 g/m~2 s)>MD(3.334 g/m~2 s)>CK(3.214 g/m~2 s).Slope gradient and unit discharge were positively correlated with soil detachment rate,and the increasing rate was CD>MH>MD>CK.Slope gradient,unit discharge and soil surface roughness showed power function relationship with soil detachment rate,and the influence of slope and unit discharge on soil detachment rate was greater than that on soil surface roughness.Unit length shear force,shear stress,unit stream power and stream power were all positively correlated with soil detachment rate under tillage practices,and the correlation was stream power>unit length shear force>unit stream power>shear stress.(4)The delay effect of different crops on the runoff generation time was different and the runoff initiation time was millet(3.99 min)>maize(2.98 min)>soybean slope(2.36min)>flat surface(1.67 min).The result showed that the runoff initiation time was mainly affected by the aboveground part(ground diameter and planting density)and the resistance effect of crops was posive correlated with ground diameter and planting density.The runoff initiation time on cropland first increased and then decreased with slope gradient and decreasd with unit discharge,which was caused by the combination of resistance effect of root system and promoting effect of slope and unit discharge.The increasing of unit discharge would quickly lead to gully erosion,and directly increased flow velocity and thus delayed runoff initiation time.The flow pattern on cropland was turbulent and the Reynolds number was smaller than that of flat surface,which indicated that crops were helpful to reduce the flow turbulence.There showed a positive correlation between Reynolds number and slope gradient and unit discharge,and the effect of unit discharge on flow turbulence was more significant than slope gradient.The flow pattern on cropland included both rapid flow and slow flow,while mainly showed as slow flow generally.Crops effectively reduced the Froude number,which meant that crop root system reduced the inertia force of runoff by reducing runoff velocity.The Darcy-Weisbach friction factor of cropland is higher than that of flat surface and showed as millet>maize>soybean>smmoth surface.The Darcy-Weisbach friction factor of cropland basically decreased with unit discharge but increased with slope gradient while the friction factor of flat surface decreased with both slope gradient and unit discharge.It showed that as slope increased,the increasing rate of flow velocity was lower than that of hydraulic gradient and the hydraulic radius due to the resistance effect of root system.(5)The average root diameter,root weight density,root length density,root surface area density and root volume density of crops increased with growth period,and the growth rate was slow in 0-30 days after seedling emergence and showed a rapid increase trend after 30days.The root diameter of different crop slope was soybean>maize>millet;the root weight density,root length density and root surface area density were millet>soybean>maize;the root volume density was soybean>millet>maize.The root diameter showed a logarithmic function with seedling time,and the root weight density,root length density,root surface area density and root volume density showed power function with seedling time.The rill initiation time of cropland was longer than that of flat surface,and showed as millet>soybean>maize>flat surface,which indicated that crop root system had a significant effect in soil consolidation.However,with the increase of slope gradient and unit discharge,the ability of crops to delay the generation of rills was also weakened.Crops significantly slowed down the increasing rate of soil erosion with slope gradient and unit discharge.The planting density was millet>soybean>maize;the erosion reduction capacity was millet>soybean>maize;the erosion reduction capacity of single plant was maize>soybean>millet.This indicated that the erosion reduction of crops depends not only on crop species,but also on its planting density.With the increase of slope and unit discharge,the increase of soil detachment rate on cropland was smaller than that on flat surface.The average root diameter,root weight density,root length density,root surface area density and root volume density of crops all had negative correlation with soil detachment rate.The correlation between root length density and root surface area density and soil detachment rate was more significant,and result showed that the root system with root diameter in the range of 0-1 mm was the main factor affecting soil detachment rate.(6)The differences of flow hydraulic characteristics and erosion process between rough surface with tillage practices and crop surface were compared.The results showed that both tillage practices and crops could effectively evolve runoff initiation time and reduce flow velocity,and the effect of tillage practices was more significant.The internal mechanism of this effect was that the friction factor on tilled surface was larger than that on crop surface,which indicated that resistance effect of tillage practice was greater than the effect of runoff separation on crop surface,and the process is affected by time.This evolution law of rough slope over time may lead to the inconsistent results of previous studies on the research that how soil surface roughness affect erosion yield. |
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