| In the hilly areas of Southwest China,slope farmland is not only an important kind of land and resource,but also an important way of production for human survival.It is one of the most serious types of soil erosion.The erosion process of slope farmland is more complex,as result of the influence of human activities.Reconstruction of farming plots is built through engineering measures to reduce the original slope farmland slope,so as to control the soil erosion effectively.The slope terrace is the most common form in the hill area with purple soils to build a farm field.So it is very important to study the new reconstruction soil which just after the reconstruction of farming plots and soil was in an unstable state.Recently,the characteristics of soil erosion on slope land in purple hilly area has done a lot of research,mainly concentrated in the surface effect of erosion process on slope,rainfall intensity,gravel content and the way of cultivation.Through a field scouring experimental study on slope farmland erosion characteristics focused on Slope land in Loess Area and industrial and mining accumulation body.Most the research focused on stability of soil by indoor simulation study,lack of research on unstable new soil erosion characteristic after the transformation of land consolidation project.Based on that,field scouring experiment,designed to have three flow discharges(30 L/min,15 L/min and 5 L/min)and different slope lengths(5 m,10 m,20 m,30 m,40 m,50 m)under farming plots reconstruction in hilly-mountainous region of Chongqing,was conducted on characteristics of water and sediment transport with runoff and hydrodynamic parameters.At the same time,the physical characteristics of the new reconstructed soil under different flow discharges and slope lengths are analyzed by the method of laboratory experiment.The main results obtained as follows: 1.Rules of water and sediment transport with runoff on the new reconstructed soilDuring the scouring process,changing trend of runoff yield rate was the increasing period-stable development period and the fluctuated period-stable development period.The changing trend of sediment yield rate was the fluctuated period-stable development period.The flow discharge and slope length are both the influence factors of water and sediment transport with runoff on the new reconstructed soil in in the hill area with purple soils.The runoff and sediment yield increased with the increase of flow discharge,and the influence of slope length was complex.The slope lengths of 20 m and 30 m were the critical value of runoff yield and sediment yield of 10°.Runoff and sediment yield were fluctuated with the increasing slope length,in which large and middle flow discharges(30 and 15 L/min)in large fluctuation range,5 L/min in low fluctuation range.There was a significant linear positive correlation between cumulative sediment yield and cumulative runoff yield of the new reconstructed soil.2.Hydrodynamic parameters characteristics of the new reconstructed soilThe flow discharge and slope length were both the influence factors of hydrodynamic parameters characteristics of the new reconstructed soil in the hill area with purple soils.Hydrodynamic parameters characteristics were fluctuated with the increasing slope length,in which large and middle flow discharges(30 and 15 L/min)in large fluctuation range,5 L/min in low fluctuation range.Under the experiment condition,new reconstructed soil surface runoff patterns were laminar flow in the final state.Compared with slope aspect,the influence of flow discharge on the Re,runoff shear stress and runoff power was more significant.The relationship between runoff yield and runoff shear stress was two times curve function.Runoff yield and runoff power were linear equation function.The influence rule of slope length was complex,and the slope length of 20 m~30 m was an important critical range which affects the average velocity,Reynolds number and Froude number of the new reconstructed soil on the slope of 10°.The influence of slope length on runoff velocity was greater than flow discharge.With the increase of slope length,the average flow velocity was three curve function change relation.However,the influence of flow discharge on the Reynolds number was more significant,and there was a two curve function relationship between the two factors.There was a certain relationship between the hydrodynamic parameters and the sediment yield of the new reconstructed soil.Runoff power had the greatest influence on the slope erosion and sediment yield.The relationship between sediment yield and runoff power was two curve function.3.Erosion response of soil physical characteristics on the new reconstructed soilThe erosion effects of different flow discharges on soil physical properties of the 10° in the hilly area of purple soil were different.After the discharge of water,the gravel content mainly increased in plots of slope length 10~30 m,and the others decreased.The spatial distribution characteristics of soil mechanical composition were mainly as follows: the content of clay and silt decreased,and the content of sand increased in the upper slope.In the middle slope,clay particles and silt increase and sand grains decrease.In the lower slope,clay particles and sand grains increase,and silt particles decrease.Compared with the stable cultivated land,the soil particles of the new reconstructed soil were not uniform,but the effect on the fractal dimension of soil particles was small.After drainage,and the majority of the macro-aggregate in each plot was damaged,and the change range was-0.92%(20 m uphill)~2.88%(10 m downhill).The change rate of the < 0.02 mm soil particle aggregation degree of the new reconstructed soil before and after the different flow discharges was increased,and the change rate was-0.9(20m middle slope position)~45.31(5m downhill).With the increase of the flow discharge,the percentage of aggregate disruption and the degree of < 0.02 mm soil particle aggregation respectively increased and decreased.With the change of the slope length,the change of the slope position was not obvious.The new reconstructed soil bulk density in slope length of 5~30 m plot showed an increasing trend,the increase range was 0.01~0.20 g/cm3.The 40 m and 50 m showed a decreasing trend,the change range was-0.01~-0.18 g/cm3.The soil total porosity and non-capillary porosity showed the following descending,the decline was 0.14%~10.67% and 0.12%~6.27% respectively.The variation range of total porosity and non-capillary porosity decreased with the increase of slope length,and 10 m and 20 m were the turning point of the trend.The soil moisture content of different slope length was increased,and the change value of the soil moisture content increased with the increase of slope length,Which the smallest growth was 0.5%~4.8%(10 m),maximum increase was of 5.63%~9.32%(30 m).The variation of soil water content in each plot was larger when the flow discharge was 15 L/min,and the increasing value was decreased with decline of slope position.After discharge,the total storage capacity and the dominant reservoir capacity of different slope length were decreased,and the change was fluctuating with the increase of slope length.The largest decline of the total capacity was 25.58 t/hm~2 where located in 30 m downhill,and the largest decline was 21.92 t/hm~2 in middle slope position of 20 m.To sum up,under the influence of different flow discharges and slope length,the soil erosion stability characteristics of purple soil hilly area are poor and the variation is large.The effect of erosion on the physical characteristics of the new reconstructed soil is largely changeable,which mainly caused the soil structure damage on the new reconstructed soil.There was a positive correlation between runoff and sediment yield and flow discharge,and the effect of flow discharge on the hydrodynamic parameters characteristics of the new reconstructed soil was stronger than the slope length.The slope length of 20 m and 30 m are the critical values of new reconstructed soil erosion at slope 10°. |
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