| The Loess Plateau is one of sloping farmland area in China.Due to the concentrated rainfall,steeper slope,and farming management,it makes the farmland more vulnerable to soil and nutrient loss causing eutrophication of water body,reducing soil fertility and land productivity.The sloping farmland formed rugged microtopography after tillage and management practice,called soil surface roughness,which its features closely related to the soil erosion process in sloping farmland.In order to clarify the soil surface roughness impacts on soil erosion and improve the understanding of rational sloping land management,it is important to conduct experiment on the role of soil surface roughness on infiltration,runoff generation and concentration,and soil nutrient loss process.Therefore,this research is valuable for theoretical and practical recognization of soil surface roughness.Based on systematical summarized on the relevant researches,this study was conducted on the soil rough surface after four common tillage and management methods in the Loess Plateau region,which concluded the artificial digging(AD),artificial hoeing(AH),Contour Plough(CP),traditional plow(TP)and chosen the smooth surface as a control group.Experiments were conducted on indoor soil box and outdoor runoff plots under rainfall simulation condition to investigate the microtopography characteristics,infiltration,runoff generation and sediment yield process,nutrient loss and particle size distribution.The main conclusions as follows:(1)The microtopography characteristics,including slope and aspect,collected from rough surface digital elevation model(DEM),which obtained from laser scanner.The results showed that the microtopography slope was in the range of 0°-80°.For AD and AH surface,the slope concentrated in 10°-15°,20°-40° with the critical micro slope was 20°.The TP surface was concentration in 5°-30°,the critical micro slope was 15°.The CP surface was concentration in 5°-30°,the critical micro slope was 30°.The grid number of lesser than critical slope decreased with increased plot slope,and the grid number of greater than critical slope increased with plot slope increased.The tillage direction and its tool affects the slope aspect of microtopography.The slope aspect of rough surface was concentrated on south,southwest and southeast aspects,which were same as the runoff plot outlet.The grid number of dominant slope aspect increased with plot slope and other slope aspect grid number decreased.(2)The soil moisture dynamic changes in microtopography position(Mound,Depression,Smooth),micro slope(cm~2)and plot slope(m~2)in the rainfall process by indoor rainfall simulation and the soil moisture obtained by the system of soil moisture sensors buried in soil.The results showed that the stable soil moisture content in depression is higher than mound,smooth position among the position in microtopgraphy,which supported that depression enhanced infiltration.The soil moisture dynamic changed in different soil depth in rough and smooth slope were 0-15 cm,0-10 cm,respectively,which indicated that the roughness enhanced soil infiltration.Furthermore,the soil moisture of micro slope(cm~2)and plot slope(m~2)was varied similar during rainfall simulation,it showed that the runoff generation mechanisms in micro slope(cm~2)and plot slope(m~2)was the same type.However,the runoff generated in micro slope will flow into depressions and provide more opportunity to infiltration,thus delay runoff initiation time in slope.(3)The runoff generation point along the runoff donward direction on the rough and smooth surface was 12-181 cm,40-180 cm,respectively,through indoor rainfall simulation.The coefficient variation of both slopes was 34.4-52.1%,15.15-31.1%.These results showed that the runoff generation points in rough surface were more dispersal compared with smooth surface.The roughness delayed slope runoff generation time,however,this impact will decrease as increasing rainfall intensity and slope gradient.The roughness affects the runoff generation time by two aspects,one is the surface depression storage,called the direct impact,and another is increased infiltration water head through the depression storage,called the indirect impact.In order to separate the direct and indirect impact,the ratio of predicted time to runoff generation and measured time to runoff initiation was 2.2-36.2%.The results showed that the indirect impact was the dominant process to delay runoff generation time,which means the depression storage improved slope infiltration capacity.(4)The overland flow direction and flow density obtained from DEMs after rainfalls.These results showed that the flow direction was continuous along the slope downward and flow density is larger than rough surface in smooth surface slope.The flow density was in the range of 13.08-17.06 m/m~2.The flow density increased with the increased of slope gradient and rainfall intensity.In rough surface,the flow direction was multiple.The roughness increased the overland flow tortuosity and decreased overland flow density,which was in the range of 6.85-11.44 m/m~2.Compared to the flow density in smooth slope,the flow density was decreased 31.7-51.5% in rough surface.Combined with the runoff coefficient processes,the average runoff coefficient in rough surface was smaller than the smooth surface,the rough surface can storage precipitation,increase infiltration and overland flow direction,thus leading to reduce the overland flow density and runoff coefficients.These results indicated that the roughness can reduce the overland flow connection.However,the roughness impacts on overland flow connection can decreased as increased rainfall intensity and slope gradient,due to roughness would concentrated overland flow,which accelerate overland flow connection.(5)Laboratory experiments were conducted using soil box and rainfall simulator with three slope surface conditions: Mound,Depression and Smooth,which aimed to investigate the effect of soil surface roughness on runoff and sediment,nutrient loss and particle size distribution in sediment processes.The results showed that the soluble nutrient loss was changed as Mound>Depression>Smooth;absorbed nutrient was Mound>Smooth> Depression.The absorbed nutrient was the dominant nutrient loss.The relationship between total soluble nutrient loss and total runoff volume is power function,and the total absorbed nitrogen is power function with total sediment loss and the total absorbed phosphorus is a positive linear function.In the sediment,the clay content from Depression slope is higher than that from Mound and Smooth slope and the silt,sand content from Mound slope is higher than that from Smooth and Depression slope.During rainfall simulation process,the clay content was gradually decreased,the silt and sand content was increased.Furthermore,the sediment particle size features,including clay enrichment(Er),mean diameter(d50),specific surface area(SSA),correlated to absorbed nutrient loss.Therefore,these results showed that roughness affects runoff and particle size distribution,leading to the different nutrient loss between rough slopes. |
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