Experimental Study On The Effect Of Freeze-thaw On Water Erosion Processes Of Three Soil Types In The Loess Plateau

The seasonal freeze-thaw region of Chinese Loess Plateau is one of the region suffering the most severe soil erosion in the world largely attributed to the alternation of freeze-thaw in the late autumn and early spring seasons,which directly leading to soil degradation,land productivity deterioration.In order to explore the effects of seasonal freeze-thaw on the soil physical-chemical properties and the water erosion process,three kinds of loam in the seasonal freezing freeze-and thawing area region of the Loess Plateau was studied in this paper.The process of freeze-thaw simulated in the laboratory based on field survey and meteorological data.The variation of soil physical-chemical properties under the condition of freeze-thaw were studied by testing soil properties.The effect of freeze-thaw on soil erodibility was estimated by using the EPIC model.Exploring the effect of freeze-thaw on the process of runoff and sediment erosion by the simulated test of artificial rainfall.The result of this study provides a scientific basis for the prediction and prevention of soil erosion in the seasonal freeze-thaw region of Chinese Loess Plateau.Main results of this study were as following:(1)During the alternate freeze-thaw process,the soil temperature of each layer showed a fluctuating downward trend.The surface soil temperature changed significantly.As well as,the soil water content of each layer showed a fluctuating decrease trend,and the change process was greatly affected by temperature.During the alternate freeze-thaw process,the temperature,moisture content and salt content of each soil layer changed periodically.During the freeze-thaw process,the soil temperature of each soil layer showed a fluctuating downward trend.The surface soil temperature changed significantly.After 15 freeze-thaw cycles,only the 0 ～ 10 cm soil layer was the thawed layer.Meanwhile,the soil water content of each soil layer showed a fluctuating decrease trend which affected by temperature.After repeated freeze-thaw process,the soil water content of the non-thawing layer decreased due to the forms of water change,and the water content in the 0 ～ 5 cm layer was reduced by evapotranspiration.The soil salt content of each soil layer showed a fluctuating decrease trend,and its changing trend was roughly the same as that of the soil water content.After repeated freezing and thawing cycles,the soil salt content of the nonthawing layer decreased rapidly,while the soil salt content in 5 cm and 5 ～ 10 cm layer decreases less.(2)With the increase in the number of freeze-thaw process,the bulk density,soil shear strength,and soil hardness of lou soil,loess soil,and aeolian sand soil showed a significant decrease trend,while soil porosity,saturated hydraulic conductivity,and disintegration rate showen a significant increase trend.During the freeze-thaw process,the soil physical-chemical properties changed regularly with the number of freeze-thaw cycles and the initial water content.The bulk density of the three loam showed a significant decrease with the increase in the number of freeze-thaw cycles.The bulk density showed a gradual stabilization trend after 10freeze-thaw cycles,while showed a lowest level at 20 freeze-thaw cycles.The soil porosity of the three loam had a significant impact on soil porosity(p<0.05)with a significant increase trend.The soil porosity of the three loam showed a trend of gradual stability after 10 ～ 15 freeze-thaw cycles,and then reached a highest level at 15 ～ 20 times of freeze-thaw cycles.The soil shear strength of the three loam showed a significant decreasing trend with the increase of the number of freeze-thaw alternations(p <0.05),which reaching the lowest level at 20 freeze-thaw cycles.The soil hardness showed a significant decrease with the increase in the number of freeze-thaw cycles(p<0.05),while showed a stable trend after 10 ～ 15 freeze-thaw cycles.The saturated hydraulic conductivity of lou soil and loess soil showed a significant increase with the number of freeze-thaw cycles(p<0.05),and the saturated hydraulic conductivity of sandy soil changed with the number of freeze-thaw cycles which generally increases first,then decreases and then increases.The disintegration rate of the three tested soils showed a significant increase trend with the change of the number of freeze-thaw alternations(p<0.05),and the disintegration rate after 10 freeze-thaw alternations showed a stable trend.The organic matter content of loess soil,loess soil and aeolian sand soil genrally increases first,then decreases and then increases with the change of the number of freeze-thaw alternations.(3)Soil erodibility increased with the increase of the number of freeze-thaw cycles,and gradually stabilized after 10 ～ 15 cycles of freeze-thaw cycles.During the freeze-thaw alternation process,soil erodibility showed an increases trend with the increase in the number of freeze-thaw alternations,and gradually stabilized after 10 ～ 15 freeze-thaw alternations.Meanwhile,soil erodibility was significantly correlated with soil physical-chemical properties(P<0.05).The soil physical-chemical properties measured before and after the freeze-thaw cycle have significant correlations.Three principal components were selected,and the cumulative variance reached 91.02%.(4)The runoff of slopes with three tested soil generally increases first then decreases,and then increases with the increase in the number of freeze-thaw cycles,while sediment yield generally increased.Runoff rate increased sharply at the beginning and then tended to stable.Sediment yield rate increased sharply at the beginning and then showed a downward trend.During the freeze-thaw process,the slope runoff,total sediment yield,runoff rate and sediment yield rate changed regularly with the number of freeze-thaw cycles.The runoff of slopes with three loam generally increased first,then decreased,and then increased with the change of the number of freeze-thaw alternations.The total sediment yield showed an increases trend,and the maximum values were reached at 15 freezethaw cycles.Runoff rate increased sharply at the beginning,and then it fluctuated steadily.The fluctuation range becomes larger with the increase of the frequency of freeze-thaw alternation.Sediment yield rate increased sharply at the beginning,and then dropped sharply.With the increase in the number of freeze-thaw alternations,the sediment yield rate and runoff rate generally increase.Through correlation analysis,the total amount of sediment yield and total runoff,soil type,number of freeze-thaw cycles,soil erodibility K value,soil bulk density,porosity,shear strength,soil hardness,saturated hydraulic conductivity,disintegration rate,Clay content,powder content,sand content and organic matter content were all significantly correlated(P<0.01).The principal component analysis showed that these variables can explain 90.54% of the change.