Formation Of Soil Crust And Its Effects Of Different Spatial Distributions Of Physical Soil Crusts On Runoff And Erosion

Physical crust is a surface phenomena that generally exist in arid and semiarid region,which is a low porosity layer in surface produced by rainfall or irrigation.Soil crust increase runoff rates as a result of soil infiltration reduction,and their formation may also increase the soil strength compared to uncrusted soils,therefore it has an important influence on the erosion process.Soil crust is often accompanied by micro topography?surface roughness?in agricultural and their?structural crust and depositional crust?spatial distribution is affected by roughness,thus different erosion phenomenon occurs in these treatments.Therefore,under the simulated rainfall condition,we should refer to the actual situation of the slope land surface,we analyzed the strength characteristics of soil crust,the effect of crust strength on infiltration and soil erosion,Effects of different spatial distributions of soil crusts on soil erosion and the effect on soil erosion from soil crust and soil surface roughness.The main conclusions were as follows:?1?The soil physical and chemical properties of rainfall intensity were the important factors that affected the formation of soil crust,the greater the rainfall intensity was,the higher the crust strength was.The lowest the soil crust strength?11.68¹2.80 kPa?was formed under the 30mm h-1 rainfall,the soil crust strength formed under the 90 mm h-1rainfall was 15.02¹8.40 kpa,the 120 mm h-1 was the highest intensity?19.59²0.62 kPa?formed under rainfall.Under the same rainfall condition,the order of crust strength in different types is:Heilu soil>Huangshan soil>Huangmian soil>red soil>Lou soil>?black soil,Lou soil+30 t/ha biological carbon,Lou soil+7.5 t/ha biological carbon?,black soil and Biocharcoal soil with the smallest crust strength,and the Heilu soil with the highest.The greater content of the soil aggregate?>0.25 mm?,clay and organic matter,the smaller strength of the crust,while the correlation between the exchangeable Na⁺and the soil crust strength was positive.The degree of soil crust was positively related to rainfall intensity,but negatively correlated with micro aggregates and clay content,and the correlation between rainfall and crust formation was the strongest.?2?The size distribution of the soil porosity is the main factor affecting the infiltration,soil crust can effectively reduce water infiltration,and with the crust strength increases,the larger effect of reducing infiltration,the reduction effect of soil crust on infiltration is8.8%⁶0.3%.For the same type of soil,the infiltration rate decrease with the increase of crust strength,compared with noncrust soil,the reduction effect of lou soil with high strength crust,moderate strength crust,low strength crust is 37.4%,32.1%and 10.9%less than it's for the black soil?20.6%,17.2%and 8.8%?,which has a close relationship with the structure characteristics of the soil itself,the reason for the difference is the structure characteristics of the soil itself.Fractal dimension of soil particles can effectively characterize the structure characteristic,The fractal dimension of soil aggregate?D?have a negatively correlated with the content of large particle?>0.05 mm?,while there is a positive relationship with fine particles?<0.05 mm?.The fractal dimension and the infiltration parameters?F_i,F_s,F_a and F_t?were negatively correlated?P<0.05?,there was a quadratic regression equation between the fractal dimension and the characteristic parameters of infiltration?R2=0.554⁰.734?.?3?Soil crust can effectively reduce soil erosion under rainfall,but under high intensity rainfall,there is no significant effect on runoff.The greater soil crust strength,the greater reduction on sediment.With the increase of soil crust strength,the reduction effect is high intensity crust?61.8%⁸0.7%?>moderate intensity crust?34.9%⁶5.5%?>low intensity crust?19.2%³6.9%?.The flow velocity was greater on cursted soil than uncrusted,while the flow depth,shear stress and resistance coefficientwas much lower on crust soil surface than uncrusted soil surface.the correlation between soil erosion and hydrodynamic characteristics of overland flow was significant,soil erosion had a positive linear relation with shear stress,while had a negative linear correlation with resistance coefficient.The soil erodibility factor were 2.673,2.404,2.099,1.293 and critical runoff shear stress erosion were 0.127,0.145,0.168,0.176 for uncrust slope,low intensity crust slope,moderate intensity crust and high intensity crust respectively.With the increase of crust strength,critical runoff shear stress`increases but soil erodibility factor decrease,the degree of correlation between shear stress and erosion gradually weakened,and the correlation degree of resistance coefficient increases.?4?The sediment yield had significant difference under the different spatial distribution of structural and depositional crust.The effect structural crust on sediment reduction is42.5⁶6.5%,depositional crust on sediment reduction is 16.7³4.3%,the anti-corrosion effectwas of structural crust stronger than the depositional crust,When the more depositional crust was located on the lower of the slope,there is more sediment yield on the slope.When surface depression and crusts exist at the same time,the maximum sediment yield is when the depression is located in the middle part of the slope?M?.Compared with the uncrust soil surface,the sediment reduction of depression is 91.5%?L?,8.5%?LM?,5%?M?,39%?MU?and19%?U?,compared with the crust soil surface,the reduction rate was 89.5%,-16%,-28%,21%and 6.5%,respectively.As a whole,the sediment reduction effect is the largest when the depression located in the lower part of the slope,Compared with the uncrust slope,the depression and the crust can effectively reduce the slope erosion,but relative to the crust slope,the depression at the middle?M?of the slope of and the middle lower?LM?will increase the slope erosion.?5?Slope,tillage and soil crust have an important impact on the erosion process,the slope also affects the thickness of soil crust and surface roughness produced by tillage.The order of surface roughness on different tillage measures is contour?0.119⁰.152?>artificial digging?0.093⁰.115?>?0.067⁰.082?>hoeing plough?0.037⁰.058?.For the same kind of tillage measures,the surface roughness decrease with the increases of the slope.The crust thinckness is smaller in the greater slope,the average thickness of crust on 3 degrees slope were 1.06,1.13,1.19 and 1.32 times compared with 5,10,15 and 20 degrees slope,at the same slope,the thickness of crust increased with rainfall intensity enhancement,the thickness of crust under 120 mm h^-1 is more than 1.06 and 1.17 times of that under 90mm h^-11 and 60mm h^-1.The runoff and sediment yield increased with the increase of slope,in the range of small slope?3-15 degrees?,the increase rate of runoff and sediment was 16.7²1.0 g/°?m²?min,4.7³.01 16.7²1.0 g/°?m²?min respectively,in the large slope range?15-20 degrees?were3.01⁴.7 g/°?m²?min and 1.29³.196 g/°?m²?min,which the rate of increase on large slope is less than the increase rate of small slope.The smaller runoff and sediment yield on the slope with the bigger roughness,soil crst can effectively reduce soil erosion,but there is no significant effect on amount of runoff.Regression analysis showed that the slope and surface roughness have significantly correlated with runoff,the degree of influencing factors to soil erosion is slope>roughness>soil crust.