Study On The Mechanism Of Breakdown And Transportation Of Soil Aggregates In The Process Of Water Erosion On The Loess Plateau

The Loess Plateau is the main soil erosion area in China.The soil erosion caused by rainfall increases the amount of soil erosion of cultivated land by more than 30%.Dispersion and fragmentation of soil aggregates caused by splash erosion and slope erosion is a crucial step in the occurrence of soil erosion.Five different soil types（Lou soil,Linnamon soil,Dark loessial soil,Loessial soil and Aeolian sandy soil）are used as research objects in the Loess Plateau.The method of artificial rainfall and inflow simulation are used to explore the splash erosion and the characteristics of aggregate selected for five soil types under different rainfall conditions,analyze the characteristics of sediment transport for slope erosion and its impact on aggregate breakdown and transportation for five soil types,and discuss the differences of hydraulic characteristics parameters and aggregate characteristic parameters for different erosion conditions,in order to serve as the study of splash erosion and slope erosion on the Loess Plateau provides theoretical basis.The results are as follows:（1）The characteristics of splash erosion for different soil aggregates were revealed.Soil types with smaller clay contents were more likely to be transported under high rainfall intensity.For the same soil type,the distribution of splash aggregate fractions was similar at different splash distances,and the splash mass decreased exponentially as the splash distance increased.A model was developed to predict the splash erosion mass using the soil erodibility factor,splash distance,raindrop diameter and raindrop intensity.Compared with the rainfall conditions,soil properties had a greater influence on the splash aggregate distribution.（2）The erosion characteristics of different soil types under the condition of slope erosion were clarified.Under the condition of rainfall alone and inflow alone,the soil loss rate of the five soils increased with the increase of raindrop diameter or flow discharge,which was higher at the initial stage,and decreased sharply with time and gradually stabilized.The higher the sand content is,the higher the initial soil loss rate is,and the shorter the time that the soil loss rate stabilizes with time.Under the condition of rainfall+flow,when the water discharge flow is the same,the soil loss rate of the same soil type increases exponentially with the increase of raindrop diameter.When the raindrops diameter is 3.39 and 3.79 mm,the soil loss rates of the five soil types all increased first and then decreased with the increase of flow discharge,and the maximum value is at flow discharge 220 L/h,with the range of 476.45-808.49 and 767.04-1390.31 g m^-2min^-1,respectively.The results show that under the two rainfall+flow conditions,the impact of raindrops and the scouring effect of runoff reach the maximum.（3）The response of slope erosion to the hydraulic characteristics parameters of different soil types was discussed.Under the condition of rainfall alone and inflow alone,the flow velocity,flow depth,flow shear stress and stream power for the same soil type increase with the increase of raindrop diameter and flow discharge;the soil loss rate increases with the increase of flow velocity,flow depth,flow shear stress and stream power.Under the condition of rainfall+flow,for the same soil type,under the same flow discharge,the flow velocity decreases with the increase of raindrop diameter,and the flow depth,flow shear stress and stream power increase with the increase of raindrop diameter;the relationship between soil loss rate and flow velocity,the flow depth,flow shear stress and stream power is extremely significant linear function.For the same soil type,under the same raindrop diameter,the flow velocity,flow depth,flow shear stress and stream power increase with the increase of flow discharge.Under the condition of rainfall alone and rainfall+flow,the soil loss rate of different soil types can be well described by multiple regression equation for raindrop diameter,soil erodibility and hydraulic characteristics parameters.（4）The characteristics of aggregate fractions distribution of different soil types for the slope erosion were discussed.Under the condition of rainfall alone and inflow alone,the mass percentage of macro-aggregates（>0.25 mm）for the five soil types increased with the increase of raindrop diameter or flow discharge.Under the condition of rainfall+flow,the mass percentage of macro-aggregates gradually decreased with the zonal distribution of soil latitude for the same flow discharge,under five rainfall conditions.The mass percentage of macro-aggregates for Lou soil,Linnamon soil and Dark loessial soil fluctuated with the increase of raindrop diameter,and which were higher than those in Loessial soil and Aeolian sandy soil.（5）The influence of slope erosion on the characteristic parameters of different soil aggregates was analyzed.Under the condition of rainfall alone and inflow alone,the fractal dimension（D）of aggregates in the five soils decreased with the increase of raindrop diameter and flow discharge.The aggregate enrichment rate（ER）of Lou soil,Linnamon soil and Dark loessial soil increased with the decrease of aggregate fractions distribution,and the ER value of aggregate with aggregate fractions<0.053 mm was more than 1;the ER value of aggregate in Loessial soil and Aeolian sandy soil fluctuated with the decrease of aggregate fractions distribution.The mean weight diameter（MWD）of the five soil aggregates increased with increasing raindrop diameter and flow discharge.Under the condition of rainfall+flow,for the same raindrop diameter and soil type,the fractal dimension（D）of the aggregates of the five soils gradually decreases with the increase of the flow discharge.The aggregate enrichment rate（ER）of Lou soil,Linnamon soil and Dark loessial soil were enriched in small aggregate fractions（ER>1）,while Loessial soil and Aeolian sandy soil were opposite.The mean weight diameter（MWD）of five soil aggregates increased with the increase of flow discharge for the raindrop diameter 2.67 and3.05 mm,and fluctuated for the raindrop diameter 3.39,3.79 and 4.05 mm.