Discussion On Migration Path Of Concentrated Water Flow Aggregates On Slope Surface Based On REE Method

Soil erosion has become one of the major environmental problems in the world,and it can cause the reduction of arable land and soil quality degradation,which threatens the sustainable development of agriculture.Soil aggregates are the main transport material in rill erosion on sloping land,and which closely related to the occurrence and development process of slope erosion.In order to clarify the grade by grade changes and migration process of aggregates,this paper selects red soil aggregates in three different land use types(garden land,grassland,and woodland)as the research object,and uses the tracking method of rare earth elements to analyze the migration pathway of aggregates from hydrodynamic properties and different characteristic aggregates perspective.Combined with the data,this paper can systematically identify the similarities and differences of aggregate migration pathways at different particle levels,characterize the quantitative relationship between hydraulics parameters and aggregate migration pathways at each particle level,and compare the effects of different land use types on aggregate migration pathways.The main conclusions obtained are as follows:(1)Reynolds number,Froude number,mean flow velocity,and water shear stress were positively correlated with the migration process of large macroaggregates,small macroaggregates,and microaggregates(p<0.001),and the positive correlation between Froude number,mean flow velocity and shear force on large macroaggregates fragmentation was particularly obvious(p<0.05).At the same time,the mobility was used to reflect the migration speed of each particle size aggregate in the flow,it was found that the migration process of large macroaggregates and microaggregates was the fastest in the flow,and they were positively correlated with the slope and flow.The migration of aggregates was dominated by downward fragmentation,and the migration pathways were mainly divided into nine.The six pathways in the direction of fragmentation,the migration pathways from large macroaggregates to small macroaggregates and microaggregates to silt and clay sized aggregates accounted for a relatively high proportion,all around 50%.The ratio of three migration pathways for large aggregate fragmentation is:large macroaggregates to small macroaggregates>large macroaggregates to silt and clay sized aggregates>large macroaggregates to microaggregates.The ratio of two migration pathways for small aggregate fragmentation is:small aggregate to silt and clay sized aggregates>small aggregate to microaggregate.Among the six migration pathways in the formation direction,the proportional relationship of the main three pathways is:silt and clay sized aggregates to microaggregates>silt and clay sized aggregates to small macroaggregates>microaggregates to small macroaggregates,while the proportions of the three migration pathways associated with large macroaggregates are all between 0%and 1%,which are not sufficient to reflect the migration changes of aggregates.(2)The differences in the migration pathways of each aggregates under the three land use types are mainly manifested in the migration of large macroaggregates to small macroaggregates(garden land<grassland<woodland)and the migration of microaggregates to silt and clay sized aggregates(garden land>grassland>woodland).These two pathways may be related to the change of mean weight diameter.The MWD value of woodland decreases the most,with a difference between 0.31-0.38 and the initial value.The grassland was followed by 0.49-0.55,and the garden land was the least reduced,0.55-0.63.At the same time,the degree of damage of the three aggregates measured by the W_r/W_i was mainly as follows:garden land>grassland>woodland.Among the three types of aggregates,there were two pathways with little difference in migration pathways:migration of microaggregates to small macroaggregates(mostly concentrated in 8%),and migration of large macroaggregates to silt and clay sized aggregates(mostly concentrated in 23%).The large differences in the proportion of migration paths of aggregates are mainly manifested in five pathways,namely,the migration of large macroaggregates to microaggregates,the migration of small macroaggregates to microaggregates,small macroaggregates to silt and clay sized aggregates,silt and clay sized aggregates to microaggregates,and silt and clay sized aggregates to small macroaggregates.Similarly,in the results of mobility,the speed of migration of the three particle levels of large macroaggregates,small macroaggregates and microaggregates in the water flow is also different from the different types of aggregates.