The Effect Of Seasonal Freeze-thaw On Soil Erodibility In Wind-Water Erosion Criss-Cross Region Of Loess Plateau

Soil erodibility is an important indicator to evaluate the sensitivity of soil to erosion and a key parameter of soil erosion prediction model.The quantitative expression and its influencing factors of soil erodibility has always been a frontier science problem in soil erosion research.The current research of soil erodibility variation in seasonal freeze-thaw zones of loess plateau is inadequate.Three kinds of loam in the interleaving zones of water wind erosion in loess plateau was study in this paper.The process of freeze-thaw was simulated in laboratory based on field survey and meteorological data.The variation of Soil physical and chemical properties under the condition of freeze-thaw were study by the testing soil properties.The effect of freeze-thaw on soil erodibility was researched by the simulated test of scouring,artificial rainfall and wind tunnel.The results of this study can provide scientific reference for soil erosion control and prediction in the period of freeze-thaw in the zones of seasonal freeze-thaw of loess plateau.Main results of this study were as follows:?1?The variation of soil physical and chemical properties has been cleared under the condition of freeze-thaw.In the process of freeze-thaw,different kinds of soil structure were destroyed,the bulk density decreased,the porosity increased,the texture was coarse,the content of water stability aggregates was reduced,shear strength and firmness decreased.The changes of soil physical and chemical properties were significantly correlated with soil texture,freeze-thaw cycles and initial water content of soil.The change trend of soil physical and chemical properties after outdoor freeze-thaw was the same as that of indoor simulation under the same conditions,but the variation amplitude were less than the indoor simulation freeze-thaw.The information of the same category factors were overlapped,the degree of explanation was high,and different categories of factors were strong correlation as a result of the interaction influence.The correlation between soil texture and other factors was strongest.The factors of soil texture,structure,aggregates and intensity were highly sensitive to freeze-thaw,while the organic matter was a weak sensitive factor for freeze-thaw.?2?The effect of freeze-thaw on soil water erodibility was quantified.There was a significant positive correlation between the K value and the number of freeze-thaw cycles based on the model of EPIC and Dg?p<0.05?.The increase trend of logarithmic or linear function with freeze-thaw cycles was related to the initial moisture content.When the initial moisture content of the aeolian sand soil was low,the K_EPIC values had no significant correlation with the number of freeze-thaw cycles?p>0.05?,while the K_Dg values of the aeolian sand soil decreased with the increase of freeze-thaw cycles number.Bulk density is the most main factors that affect the K value.The direct and indirect effects of the content of clay particles?CLA?and geometric mean diameter?Dg?of soil texture on soil erosion were greater,but there were double effect on soil K values.Soil detachment capacity had significant positive correlation with slope,flow,initial water content and the number of freeze-thaw cycles under the condition of freeze-thaw?p<0.05?.After freezing and thawing 10times,the detachment capacity of aeolian sand soil,irrigation soil and loessal soil were significantly increased by 0.59,0.58 and 0.96 kg m^-2s^-1?p<0.05?.The soil rill erodibility?K_r?increased with the increase of freeze-thaw cycles number and then tend to stabilized.Before freeze-thaw,the mean K_r of aeolian sand soil(0.28 s m^-1)>loessal soil(0.26 s m^-1)>irrigation soil(0.25 s m^-1).After 10 times of freeze-thaw,the mean K_r of loessal soil(0.65 s m^-1)>irrigation soil(0.54 s m^-1)>aeolian sand soil(0.52 s m^-1).The critical shear force??_c?of three kinds of soil showed a significant decreased trend after freeze-thaw.Under the condition of different initial water content,the average values of?_c were:irrigation soil?0.54 Pa?>loessal soil?0.52 Pa?>aeolian sand soil?0.20 Pa?.Under the condition of freeze-thaw,the key indicators of influenced soil erodibility include the number of freez-thaw cycles?FT?,soil clay content?CLA?,geometric mean diameter?Dg?of soil particles,content of water stability aggregates of greater than 0.25 mm(WSA_>0.25),bulk density??_B?and soil shear strength???.The above indicators were also the key factors affecting the rill erodibility and critical shear force of the soil in the freeze-thaw process,which can be used to estimate soil erodibility:K_r=0.35e^0.57FTCLA^-0.016WSA^-0.067?^-0.03?_B^-0.189?R²=0.88,NSE=0.85?;?_c=0.0083e^-0.128FT0.128FT CLA^-0.174D_g⁰.378.378 WSA^0.099?^1.132?R²=0.84,NSE=0.81?.?3?The effect of freeze-thaw on soil wind erodibility was clarified.There was no significant difference in starting wind speed between before and after freeze-thaw when the initial moisture content was 2%and 8%.The starting wind speed of different soil after freeze-thaw was significantly less than that of unfrozen soil when the initial moisture content was 4%and 6%.The starting wind speed of loessal soil was the least.The rate of sediment transport before and after freeze-thaw were decreased exponentially with the increase of height.The contribution rate of wind speed to sediment transport rate?29.60%?>soil initial moisture content?14.69%?>soil types?14.29%?>number of freeze-thaw cycles?7.10%?.The mean sediment transport rate of irrigation soil?4.23 g/?m² min??>loessal soil?3.06 g/?m²min??>aeolian sand soil?2.23 g/?m² min??under the condition of different wind speed and moisture content before and after freeze-thaw.Initial soil water content is the most main factors that influence the soil wind erosion intensity,accounted for 41.05.Wind speed,soil types and number of freeze-thaw cycles on the contribution rate of wind erosion intensity is relatively small,at around 14%.Under the same conditions,the wind erosion intensity of 3kinds of soil after thawing was significantly greater than that of the unfrozen and frozen soil?p<0.05?.When the initial moisture content increased from 4%to 8%,the average soils wind erosion modulus of loessal soil(8.42 g?m² min?^-1)>irrigation soil(6.02 g?m² min?^-1)>aeolian sand soil(2.91 g?m² min?^-1)under the condition of same state of freezing and thawing and wind speed.There was a good nonlinear relationship between soil wind erodibility and soil moisture content after freeze-thaw,the number of freez-thaw cycles,the median size of soil particle size,average geometric diameter of aggregates,particle concentration of 0.05-0.5mm diameter,soil shear strength and bulk density.K_?=e⁰.015FT+14.97.015FT+14.97 DW^-3.2193.219 d₅₀⁰.456.456 MWD^-0.103WT0.05-0.5^0.401?-1.389?_B-8.641?R²=0.89,NSE=0.87?.?4?The erosion process and soil erodibility of slope under different forces were revealed.There was no significant difference in sediment flow structure between before and after freez-thaw?p>0.05?.After natural freeze-thaw,soil erosion modulus was significantly less than that of indoor freeze-thaw?p<0.05?.Compared with the unfrozen slope,the content of erosive particles decreased significantly after the wind erosion,while the content of the particles greater than 0.7 mm increased significantly?p<0.05?.After wind erosion,the content of greater than 0.7 mm particles of loessal soil and irrigation soil experienced freeze-thaw were significantly larger than that of the unfrozen soil,and the freeze-thaw effect increased the roughness of the slope in the wind erosion process.Under different treatment methods,the average flow rate of the slope of aeolian sand soil?9.97 min?was greater than that of the loessal soil?9.12 min?,which was greater than the irrigation soil?5.02 min?.After the combined effect of freeze-thaw and wind erosion on the slope surface,the increase amplitude of the start time of production flow was irrigation soil?84.75%?>loessal soil?72.13%?>aeolian sand soil?44.75%?.After the stability of the production flow,the flow rate of the irrigation slope in different treatment conditions was greater than that of the loessal soil and aeolian sand soil,and the infiltration rate was the opposite.The cumulative sediment yield of aeolian sand soil of contrast,wind erosion and composite effect of freeze-thaw and wind erosion slope(4.03,4.16,and 5.82 kg m^-22 h^-1)>loessal soil(3.50,4.11,and 5.54 kg m^-22 h^-1)>irrigation soil(0.54,0.62,and 0.79 kg m^-22 h^-1,p<0.05).The cumulative sediment yield of loessal soil slope(5.56 kg m^-22 h^-1)>aeolian sand soil(5.24 kg m^-22 h^-1)>irrigation soil(0.65kg m^-22 h^-1,p<0.05)after freeze-thaw.The soil erodibility after the combination of freeze-thaw and wind erosion in different textures slope was greatest,followed by freeze-thaw and wind-erosion alone slope,and the erodibility of contrast slope was minimal.The average soil erodibility of the same treatment method of aeolian sand soil and loessal soil was 7.10 and6.74 times of irrigation soil respectively.The soil erodibility parameter of compound erosion could be simulated by soil erodibility parameter of wind and water erosion better.K?=1.31K₀⁰.078.078 K_w^-0.16K_FT^1.044?R²=0.96,NSE=0.92?.