| Soil surface roughness is an index for describing the surface microtopography induced by tillage and soil management at the agricultural hillslope,which obviously affects the surface runoff and erosion process.Rill erosion is one of the main patterns of soil erosion,and it has been proved to be the main driving force of soil loss.It is of great significance for us to explore the soil and water conservation of surface roughness and the control of soil and water loss on sloping farmland through studying the effect of surface roughness on soil erosion and the rill erosion.Therefore,on the basis of summarizing the previous research results,rough surfaces were prepared manually according to the surface microtopography of plateau sloping farmland employing four tillage practices(contour plowing CP,manual hoeing MH,artificial digging AD and contour drilling CD)with a smooth surface as control.Rainfall simulation experiments were performed on the outdoor runoff plot.We have observed the change characteristics of the surface roughness before and after rainfall under different tillage practices,and the interaction between the surface roughness,runoff volume,sediment yield and the morphology and distribution of rill erosion.The main conclusions as follows:1.For the estimation of soil surface roughness(RR),surface elevations of runoff plots were measured with a laser scanner based on the difference of average absolute elevation.Different tillage practices provided substantially different values of RR while average RR value of each tillage practice at different slope gradients was calculated as CD > AD >MH > CP > CK.However,the RR after rainfall had no obvious regular pattern.With the increased of rainfall intensity and slope gradient,the RR after rainfall changed from less to more than that before rainfall.For the variation characteristics of infiltration amount and infiltration processes of different tillage practices,the slope with tillage practices had higher infiltration amount,but as the rainfall intensity and slope gradient increased,the difference between the infiltration processes of the tillage practices and the smooth surface decreased gradually.2.Based on the analysis of the effect of surface roughness on runoff and sediment yield,the surface roughness played a delaying role on the start time of runoff.On the whole,the surface roughness had an inhibitory effect on runoff volume and sediment yield.The interaction between rainfall intensity and surface roughness always had a runoff reductioneffect.Conversely,there was a critical slope gradient between 5° and 10° for sediment yield,where rainfall intensity effect counteracted surface roughness effect.The interaction between slope gradient and surface roughness also had a runoff reduction effect,which diminished by increasing rainfall intensity.However,their interactive influence on sediment yield was inconsistent,with a critical slope gradient between 10° and 15°.Based on the comprehensive interactive effects among all three factors,we concluded that rainfall intensity,slope gradient,and surface roughness collectively played a crucial role in promoting runoff and sediment generation under tillage.3.The influence of the surface roughness on the formation of rills.Most of the rill production points of different tillage practices and the smooth surface were located at the middle and lower part of the slope.The distribution of the points of the rough slope was more scattered than that of the smooth surface.The start time of points was also later than that the CK,But As the rainfall intensity and slope gradient increased,this kind of delay gradually weakened.The length of the longest rill,the average width and depth of the rills,the density and the plane density of the rills were all increased with the increased of the rainfall intensity and the slope gradient,but the inhibition effect of the surface roughness on the length,the average width,the density and the plane density of the rills would gradually decreased or even transformed to the promotion effect,and the surface roughness always played a promotion role on the average depth of the rills.The plane density was the best morphological index of rills,and was also the best morphological index of runoff volume and sediment yield.4.The spatial distribution of rills was extracted and based on Strahler's stream order classification in the Arc GIS software.The rill distribution was best described as an "arborization" pattern under the non-tillage and contour plowing treatments.This pattern was mostly absent like a "network" pattern under manual hoeing and artificial digging,while the rill distribution of the contour drilling treatment exhibited a "flat" shape.The effect of surface roughness on the number and length of the first-order rills,second-order rills and the total rills had gradually changed from inhibition to promotion.However,for the third-order rills,surface roughness had no obvious effect.The longitudinal river slope of rills had similar variation characteristics.The mean bifurcation ratio of different treatments did not differ considerably,especially above the 10° slope gradient.With the increase of the surface roughness,the angle between the rills increased gradually from the average 30° under the smooth surface to 90° under the contour drilling.As the increased of rainfall intensity and slope gradient,the effect of surface roughness on total rill erosion alsochanged from inhibition to promotion.For different orders of rill erosion,the proportion of the first-order rill erosion on the cultivated slope was higher than that on the smooth surface,and the proportion of the second-order rill erosion was lower,while the proportion of the third-order rill erosion gradually increased from less than that of CK to be greater as the rain intensity and slope gradient increased.5.Based on the analysis of the relationships among different morphological indicators,rill classification indicators,rill erosion of every order,runoff volume and sediment yield,it could be concluded that rill plane density,second-order rill distribution indicators and second-order rill erosion had significant correlation with runoff volume and sediment yield,and the correlation coefficient was the largest,which provided theoretical support for us to explain the benefits of surface roughness on water and water conservation better. |
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