| Infiltrating water accumulates in the plough pan because of the low permeability of this layer,which results in saturation or partial saturation?soil water contents between the field water capacity and saturated water content?of the cultivated layer during continuous rainfall.Even in sloping farmlands,soil infiltration via large pores created by roots,wormholes and straw rot continuously increases,and the infiltrating water forms subsurface flow above the plough pan.More serious problems of erosion and sediment transport caused by partial soil saturation urgently required attention.As one of the main forms of soil erosion,rill erosion is the key channel to transport eroded sediment.Scholars at home and abroad have conducted in-depth research and discussion on rill erosion,but less attention has been paid to the research on the erosion process after it forms a partially saturated slope,and there is no systematic theoretical system and research method.In view of this,the method of indoor water flow simulation scouring test was adopted in the present study.By constructing a soil tank water supply device to simulate the continuous partially saturated soil,the characteristics of rill detachment,sediment transport and erosion process of partially saturated soil were systematically studied.The relevant process models were further established and the differences between them and non-saturated soil?The soil water content is below the field water content,and the soil particles are in a holding state under capillary force or hydration?were compared.On this basis,through the mathematical model theory of soil erosion and sediment yield in WEPP model,the mechanical mechanism of interaction between runoff and soil in the process of partially saturated and non-saturated soil erosion was revealed,and the mechanical mechanism of partially saturated soil particles and the mechanism of sediment distribution,sorting and transportation in the process of erosion were defined.The research results will be helpful to understand the hydrodynamic mechanism of soil erosion under partial saturation and provide theoretical basis for improving the physical model of soil erosion.The main conclusions of the present study are as follows:?1?The detachment rate of rills in partially soil decreased linearly and exponentially with the sediment concentration and rill length,respectively.And the detachment process was more intensive than that of non-saturated soil.The detachment rate of rills in partially soil showed a decreasing trend with the increase of sediment concentration,and the relationship between them could be well expressed by linear equation.Similarly,the detachment rate of rills in partially saturated soil gradually decreased with the increase of rill length,and the relationship between them could be well expressed by exponential equation.The detachment rate of rills in partially saturated soil increased with the increase of slope and flow discharge as the sediment concentration and length of rills decreased.The detachment process of partially saturated soil was more intensive than that of non-saturated soil.The detachment rates of rills in partially saturated soil were 1.77,1.77,2.36 and 2.38 times higher than those in non-saturated soil when the slope gradients were 5°,10°,15°and 20°,respectively.The maximum detachment capacity of partially saturated soil was nearly twice that of non-saturated soil under the same hydraulic conditions.The detachment rate of partially saturated soil decreased 13%faster than that of non-saturated soil with the increase of sediment concentration in water flow,and partially saturated soil could reach the maximum detachment amount of water flow in a shorter distance than non-saturated soil.?2?There was a positive correlation between the sediment transport capacity and flow discharge in the partially saturated soil,and the presence of subsurface water flow significantly increased the sediment transport capacity of partially saturated soil.Under different conditions,the analytical sediment transport capacity for rill erosion of partially saturated soil ranged from 0.04-1.56 kg s-1 m-1.The sediment transport capacity increased continuously with the increase of slope gradient and flow discharge,and the relationship with slope gradient and flow discharge could be well expressed by nonlinear multivariable equations.The logarithmic equation model could more accurately describe the relationship between the analytical sediment transport capacity of partially saturated soil and the stream power.The presence of subsurface water flow significantly increased the sediment transport capacity of partially saturated soil.When the slope was 5°,10°,15°and 20°,and the flow discharge was 2,4 and 8 L min-1,the analytical sediment transport capacity of partially saturated soil was 1.08,1.32 and 1.50 times 1.20,1.29 and 1.59 times;1.86,2.06 and 1.74 times;2.35,2.28 and 1.85 times,than that of non-saturated soil,respectively.With the increase of slope and discharge,the difference between the sediment transport capacity of partially saturated and non-saturated soils gradually increased.?3?The distribution of sediment concentration partially saturated soil could be described by an exponential function,and it interacts with the sediment transport capacity and detachment rate.The distribution of sediment concentration partially saturated soil could be described by an exponential function,and it all increased gradually with the increase of rill length.The sediment concentration of rill erosion in partially saturated soil was positively correlated with slope gradient and flow discharge.According to the variation rule of sediment concentration along the course,the estimation equation of critical slope length for rill erosion and sediment transport was derived,and the critical slope length ranges for rill erosion and sediment transport in partially saturated and non-saturated soil were 6.62-12.69 m and 8.95-16.91 m respectively.In general,the critical rill length of partially saturated soil was 72%of that of non-saturated soil under the same hydraulic condition.After the soil tended to be saturated,the sediment concentration of the current would reach the maximum value corresponding to the sediment transport capacity in a shorter trench length.There was a certain correlation between detachment rate,sediment transport capacity and sediment concentration in rill erosion process of partially saturated soil.Under most hydraulic conditions,the sum of unit sediment transport capacity and detachment rate of partially saturated and non-saturated soil was basically equal to the value of sediment transport capacity.The osmotic pressure of soil particles on the downhill surface under the action of water flow in the partially saturated state leaded to the difference between the stress of soil particles with that of unsaturated soil.Under the action of rill water flow,the movement of soil particles was mainly caused by shear force generated by runoff,which broke the cohesive force between soil particles and separated them,and then overcame the friction force between soil particles and soil particles or between soil particles and the surface to start on the slope.The two processes were closely related to the shear characteristics of runoff,the characteristics of soil shear stress resistance and the distribution of sediment particle size.?4?The degree of runoff shear force greater than critical shear stress of soil in partially saturated state was higher than that in non-saturated state,which was the reason why the erosion of partially saturated soil was more intense.The shear force of partially saturated soil increased with the increase of slope gradient,and the relationship between them could be well expressed by logarithmic function model.The relationship between the maximum detachment rate and shear force could be well expressed by linear regression equation.The maximum detachment rate of soil in two different states all showed an increasing trend with the increase of shear force,but the increasing amplitude was different.Under the same shear stress condition,the maximum detachment rate in partially saturated soil was larger than that in non-saturated soil on the whole,and this trend was more obvious under the condition of large slope gradient.Under various hydraulic conditions,the erodibility parameters of the partially saturated soil were 1.33-2.33 times that of the non-saturated soil,while the critical shear stress of the soil was less than that of the non-saturated soil,which indicated that the erosion sensitivity of the partially saturated soil was significantly higher than that of the non-saturated soil,and the ability of the soil to resist runoff shear damage was also significantly reduced in the process of becoming saturated.The comparison between shear force and critical shear stress of soil showed that the degree of shear force greater than critical shear stress of soil in partially saturated state was higher than that in non-saturated state,which was also the reason why the degree of soil erosion in partially saturated state was more intense.The rising sediment concentration of the water flow reacted on the runoff,causing the viscosity of the runoff to increase continuously,the energy loss to increase continuously,and the turbulence intensity of the water flow to weaken,thus reducing the effective force of the runoff on the sediment particles on the bed surface and the flow velocity.This was also an important reason why the detachment rate of partially saturated and non-saturated soils decrease continuously with the increase of the sediment concentration of the water flow,and the sediment concentration decreased continuously with the increase of the rill length.?5?The distribution and sorting characteristics of the sediment as controlled by the aggregation state of the particles during the erosion process.Change in the stress on soil particles in partial saturation is an important factor affecting the erosion and sediment transport process.Fragmentation of aggregates during erosion easily led to enrichment of fine particles,especially clay-sized particles.Clay-sized particles were bound into aggregates and were transported in the form of aggregates during erosion process,but the degree of aggregation was not high.Most of the silt-sized particles were transported in the form of single particles during erosion,and enrichment occurred in eroded sediment,but the degree of enrichment was lower than clay-sized particles.The enrichment characteristics of clay-sized and silt-sized particles indicated that water flow preferentially transports fine particles during erosion.In the process of erosion,most fine sand-sized particles and coarse sand particles were formed by aggregation of other finer particles and were transported in the form of aggregates.Compared with fine sand-sized particles,the aggregation of fine sand-sized particles was relatively poor,and a certain amount of fine sand-sized particles were transported in the form of single particles.Compared with the tested soil,the content of sand-sized particles in eroded sediment was low and certain sedimentation occurred.Under different hydraulic conditions,the mass fraction of<0.021 mm particles was greater than 10%.With the increase of particle size,the mass fraction of erosion sediment particles of other particle sizes decreased gradually.Therefore,sediment particles were preferentially transported in the form of suspension/saltation of particles of less than 0.021 mm in the process of partially saturated soil rill erosion.Changes in the stress of soil particles under partial saturation led to differences between non-saturated soil and its erosion and sediment transport processes.In the process of soil approaching from non-saturated state to partially saturated state,the force dominated by van der Waals attraction among soil particles gradually changes into electrostatic repulsion among particles.The hydration repulsion and decomposition of ions increase,which weakened the aggregation force among particles.The binding substance in soil that played a binding role was dissolved by water and loses the binding force,resulting in the reduction of the binding strength.The soil was in a partially saturated state,with more soil particles in a dispersed state and more incipient sediment,which is an important reason why the erodibility of partially saturated soil was stronger than that of non-saturated soil.In summary,the erosion and sediment transport processes of partially saturated soil are obviously enhanced.The degree of runoff shear force greater than the critical shear stress of partially saturated soil is higher than that of non-saturated soil,which is an important reason for more intense erosion.In further,change in the stress on soil particles in partial saturation is an important factor affecting the erosion and sediment transport process.And the distribution and sorting characteristics of the sediment as controlled by the aggregation state of the particles during the erosion process.The results reported in this study have important implications for an in-depth understanding of hydrodynamic mechanism of soil erosion in partially saturated soil and provide theoretical basis for improving the physical model of erosion. |
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