| The study of compound soil erosion under the superposition and interaction of multiple erosive forces is the focus and difficulty of current soil erosion research.Based on the deficiencies of current studies on alternation and superposition of compound soil erosion under multiple erosive forces,this paper employed field observation,rare earth element tracers and stereoscopic photogrammetry technology,simulated rainfall and upslope inflow,as well as wind tunnel experiment,indoor freeze-thaw simulation and soil physics experiments to study the seasonal variation of soil erosion and the characteristics of soil erosion-deposition spatial distribution on long and gentle hillslopes in Chinese Mollisol region;to expound the interaction between rainfall intensity and rainfall kinetic energy and the synergistic effect of rainfall and upslope inflow on hillslope water erosion;to reveal the mechanism of the effect of freeze-thaw cycle on soil erosion;and to separate the contributions of the interaction and superposition of multiple forces such as water,wind,and freeze-thaw to hillslope soil erosion.The results not only enrich the theoretical basis of compound soil erosion,but also provide important scientific bases for the construction of soil erosion prediction models with interactions of multiple erosive forces and the prevention and control of compound soil erosion.The main research conclusions are as follows:（1）The seasonal variation of soil erosion on long gentle hillslope and the characteristics of soil erosion-deposition spatial distribution of Mollisol under the interaction of multiple erosive forces were clarified.The erosion rates of snow melt,wind and rainfall on the long gentle hillslope of agricultural land were 537.3 t km-2 a-1,363.1 t km-2 a-1 and 2350.6 t km-2 a-1,respectively.The hydraulic driving force is the main external force of hillslope soil erosion.The water erosion caused by snow melting and rainfall accounted for 88.7%of the total annual soil loss.The slope erosion rate and its spatial distribution are obviously different in different seasons.In the snowmelt season,soil erosion from the top to the foot of the hillslope presents strong and weak alternating changes,and the erosion-deposition rate is between-3.3-4.4 kg m-2;In the wind erosion season that after snowmelt and before rainy season,the distribution of soil erosion and deposition on the hillslope has no obvious regularity,the distribution of soil erosion and deposition from top to the foot of the hillslope is alternating patchy,and the erosion-deposition rate is between-3.0-3.6 kg m-2;In the rainy season,soil erosion gradually increases from the top to the bottom of the hillslope,and the erosion-deposition rate is between-9.3-5.1 kg m-2.Soil erosion was most serious in the middle part of the hillslope,then it gradually weakens and the soil deposition mainly occurred at the foot of the hillslope.With the increase of slope length,the sediment transport ratios of snowmelt erosion and water erosion in rainy season in different segments from the top to the foot of the hillslope changed from 23.5%to 100%and 34.7%to 100%,respectively.（2）The effects of the interaction of rainfall intensity and rainfall kinetic energy on hillslope water erosion was clarified.Under the natural rainfall conditions,the most severe hillslope soil erosion is caused by the rainfall pattern with rainfall intensity concentrated at11.4-68.4 mm h-1 and the rainfall energy ranged between 16.4-42.2 J m-2 mm-1.The frequency of the rainfall pattern accounts for 34.2%of the annual erosive rainfall,and the soil erosion caused by it accounts for 47.2%-93.9%of the annual soil loss.Under simulated rainfall conditions,when the rainfall intensity increased from 50 mm h-1 to 100 mm h-1,the rainfall kinetic energy increased by 1.4-1.9 times,and the soil loss increased by 1.6-2.7times.With the increase of rainfall energy under the same rainfall intensity,soil erosion on the hillslope increased in steps,and the soil erosion under the two rainfall intensities increased by 1.0-1.8 times and 1.7-2.3 times respectively.The kinetic energy of natural rainfall is generally greater than that of artificial simulated rainfall.The soil erosion rate and rainfall kinetic energy of natural rainfall and artificial simulated rainfall both show an exponential function relationship.Based on the classification of rainfall patterns under natural rainfall conditions,the exponential function relationships between water erosion and rainfall kinetic energy were fitted,and a multiple nonlinear regression equation of slope water erosion and rainfall intensity and rainfall kinetic energy was constructed.The contribution of rainfall intensity and rainfall kinetic energy to hillslope water erosion was quantified.It was found that at 3°and 5°slope gradients,the contribution rates of rainfall intensity to soil losses reached 62.0%and 58.5%,respectively;the contribution rates of rainfall kinetic energy to soil losses reached 30.1%and 36.6%,respectively;and the contribution rates of the interaction of rainfall intensity and rainfall kinetic energy to soil losses were 4.5%and 4.8%,respectively.（3）The synergistic effect of rainfall and upslope inflow on hillslope soil erosion was studied.The contributions of ridge and furrow to longitudinal ridge-tillage hillslope erosion was distinguished.Under the experimental treatment of two rainfall intensities(50 and 100mm h-1)and five upslope inflow rates(10、20、30、40 and 50 L min-1)combinations,soil erosion increased by 1.4-5.2 times and 2.5-14.0 times,respectively,compared with inflow-only treatment.The synergetic effect of rainfall and inflow increased the soil loss by21.1%～56.7%compared with the sum of their individual effects.When the inflow rate increased from 10 L min-1 to 50 L min-1,soil erosion under the synergistic effect of rainfall and inflow increased by 6.2 and 4.1 times,respectively.Under the treatment of five upslope inflow treatments,soil erosion mainly occurred at the furrow bottom and the foot of ridge sideslope.However,under the synergistic effect of rainfall and inflow,soil erosion mainly comes from the ridge area,which accounted for 51.3%-60.9%and 53.0%-61.8%of the total soil loss on the hillslope under the rainfall intensity of 50 mm and 100 mm h-1,respectively.（4）The influence mechanism of freeze-thaw effect on soil erosion was revealed.The shear strength and cohesion of soil decreased with the increase of freeze-thaw cycles.When the number of freeze-thaw cycles increased from 0 to 13,the shear strength of the two Mollisols（KS thick layer black soil and BX thin layer black soil）decreased by 8.1%-32.8%and 4.9%-24.7%,respectively.Soil cohesion decreased by 5.7%-91.1%and 8.3%-85.0%,respectively.With the increase of freeze-thaw cycles,the disintegration rates of the two tested soils increased by 1.0-11.4 times and 1.1-8.2 times,and the soil dispersion rate of>1mm soil aggregates increased by 12.6-44.8%and 1.4-3.8%,respectively.After one freeze-thaw cycle,the content of micro-aggregates（<0.25 mm）in the two test soils increased by 6.9%-12.2%and 1.3%-13.4%,respectively.It is precisely because of freeze-thaw that the soil erosion resistance indices such as soil shear strength,soil cohesion,and water stability of soil aggregates are weakened,and the rate of soil erodibility index such as soil disintegration increased,resulting in an increase in the intensity of wind erosion and water erosion in agricultural land in Chinese Mollisol region.（5）The contribution of water,wind and freeze-thaw interaction to hillslope soil erosion was separated.Previous soil freeze-thaw significantly increased soil water erosion and wind erosion（P<0.05）,and the superposition of previous soil freeze-thaw and wind erosion in early stage significantly increased hillslope water erosion in later stage（P<0.05）.Compared with the experimental treatment without previous soil freeze-thaw,the soil water erosion amount increased by 52.5%-89.6%,the soil wind erosion amount increased by39.2%-141.9%,and the soil erosion amount increased by 13.4%-156.9%under the superposition test of wind erosion and water erosion.Previous wind erosion had a significant promoting effect on water erosion in later stage（P<0.05）.When the previous wind speed increased from 9 m s-1 to 15 m s-1,the soil loss increased by 61.6%and 30.8%,respectively,under the rainfall intensity of 50 and 100 mm h-1.Under the same wind erosion at the earlier stage,the soil loss increased by 8.7 and 7.0 times with the increase of rainfall intensity from 50 mm h-1 to 100 mm h-1.The results showed that the soil anti-erodibility was weakened by freeze-thaw action.The wind erosion dent formed by early wind erosion on the soil surface led to runoff convergence,increased runoff velocity,Reynolds number and Froude number,and decreases Darcy-weisbach coefficient,which increased runoff erosion and transport capacity,and then increased soil erosion amount.The contribution rates of the rainfall intensity,wind velocity and freeze-thaw action on soil erosion were50.9%,21.7%and 1.6%,respectively.The interaction contribution rate of rainfall intensity and wind velocity was 10.2%. |
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