| Gravel is widely distributed on a global scale,and the presence of gravel in soil will undoubtedly affect soil hydrologic cycle processes.The effect of gravel on hydrological processes depends on its position in the soil.Different gravel layout patterns have a significant impact on the hydrological process of the slope,especially for the gravel buried above the soil surface,which is often neglected because it does not directly participate in the hydrological process of the surface soil,but this gravel layout pattern is often the main factor causing major natural disasters.In the view of the particularity of gravel in hydrological cycle,this paper collects a large number of typical literature on gravel research,summarizes the trend change of gravel research and the emphasis and classification change of gravel research in different disciplines.At the same time,the influence mechanism of different gravel distribution patterns on hydrologic process is discussed by combining field investigation and laboratory simulation experiment.Field investigation is to investigate vegetation factors by arranging sample points,sampling survey,sampling soil.The effects of gravel on infiltration,slope runoff,sediment yield and water absorption were studied through soil column simulation and rainfall simulation.Combined with data analysis software such as Excel 2013,SPSS 18.0,Arc GIS 10.5,R language(version:3.3.3),ggplot2,Origin9.1,GS+9.0 and Image J software,the above experiments statistically analyzed the value and significance of slope scale gravel distribution pattern in controlling slope erosion.The spatial distribution pattern and influence factors of gravel under watershed scale were analyzed to establish the prediction model of gravel spatial distribution.Influence of different embedded gravel content on slope runoff and sediment concentration under rainfall simulation.Through the above discussion,the main research results are as follows:(1)Caliche nodule mass fraction can be used as an indicator of soil erosion status on slopes.The volume fraction of soil clay decreased gradually along the slope.The mass fraction of Caliche nodule is distributed along the slope in a single peak curve and reaches a peak value(10%-15%)at 1/4 to 1/3 of the slope(20-30m from the top of the slope).There was a linear positive correlation between Caliche nodule mass fraction and slope,and a single peak curve between Caliche nodule mass fraction and vegetation aboveground biomass(coverage).The peak value was the critical point at which vegetation affected Caliche nodule mass fraction,and the vegetation coverage at the critical point was between11%and 16%.Slope and vegetation were the most important factors affecting the distribution of Caliche nodule.Small size Caliche nodule are the most widely distributed and the most numerous on the slope,and the decisive size of Caliche nodule is between 5 and 15 mm.(2)High Caliche nodule are mainly distributed in hilltops,ridges,valley edges and other high-altitude areas,while low calcareous nodules are mainly distributed in terraces and barrage.Caliche nodule coverage was related to slope and vegetation coverage,and Caliche nodule diameter was significantly related to altitude.Caliche nodule concentrated distribution in diameter between 10-50mm,accounting for more than 65%of the total Caliche nodule,Caliche nodule of more than 50mm rarely appear on the soil surface.The dominant diameter is mainly distributed in 10-20 mm;we proposed a caliche nodule index prediction model based on slope,vegetation coverage and altitude.(3)Caliche nodule can affect the distribution of soil water in the soil column.The distribution pattern of Caliche nodule water content in soil column section is opposite to that in soil,and the soil water content is obviously higher than that of Caliche nodule.At high Caliche nodule content(0.2,0.3 and 0.4 kg·kg-1),there was no significant difference in soil water storage between the top and bottom layers(P>0.05);at low calcareous nodules(0 and0.1 kg·kg-1),there were significant differences in soil water storage between the top and bottom layers(P>0.05).The quantity and effective quantity of Caliche nodule increased with the increase of Caliche nodule.The effective water storage capacity of Caliche nodule can account for about 50%of its own water storage capacity.If soil water storage capacity under four types of Caliche nodule contents(0.1,0.2,0.3 and 0.4 kg·kg-1)is not considered,soil water storage capacity under four types of Caliche nodule contents(0.1,0.2,0.3 and 0.4kg·kg-1)is respectively underestimated by 3.02%,7.96%,10.49%and 16.88%.(4)Embedded-rock fragment in the soil can change the hydrological process of slope soil,which mainly depends on the slope,embedded-rock fragment content in the soil and slope.Embedded-rock fragment content and slope had little effect on infiltration and runoff,but had great effect on sediment concentration and sediment concentration.The amount of sand and soil erosion modulus increased significantly with the increase of slope.At the same time,embedded-rock fragment in the soil can change the relevant hydraulic parameters.Embedded-rock fragment in the soil can change the hydrological process on the slope mainly during the subsequent stage of rainfall(10-60 minutes).The change of surface roughness and the relationship between soil and water loss caused by rock fragment and runoff dominate the slope hydrological process,which is closely related to the slope slope.Our study suggests that embedded-rock fragment in the soil accelerate sediment yield and cause severe soil erosion,especially on steep slopes.(5)To have accurate runoff velocity,there is need to improve dye tracer method for estimating surface runoff velocity.This can enhance the calculations of runoff and infiltration coefficients and other hydrologic parameters that will lead to a better understanding of hydrological processes and soil erosion.In this study,an integrated dye tracer and image processing method was used to estimate surface runoff velocity under three slope gradients(5°,10°and 15°)and three slope positions(up-slope,mid-slope and down-slope).A digital camera was used to record the processes of runoff flow on the slopes in combination with image processing to detect runoff front under simulated rainfall conditions.Surface runoff velocity determined by the method(IPV)was compared with that by dye tracer method(AOV)and the difference computed.The results showed more variation in in runoff under IPV than AOV during the rainfall event,indicating that the method correctly captured surface runoff velocity variation.IPV was positively correlated with slope gradient,which was the same for AOV.IPV values were close to AOV ones for slope gradients?5°,but were significantly different for slope gradients?10°.The mean AOV value was 10.6%higher than that of IPV.Linear regression analysis showed that compared with AOV,IPV overestimated runoff under low runoff velocity conditions and underestimated it under high runoff velocity conditions.The use of image processing in the integrated method was advantageous because of its ease of use with less artificial errors and its suitability for lateral diffusion of runoff.Irrespectively,additional studies were needed to verify and/or improve further the use of this method in runoff velocity analysis.Based on the above research,we obtained the value and significance of slope scale gravel guiding erosion.The spatial distribution pattern of gravel cover and particle size in watershed scale was obtained by using relevant environmental factors.The effects of gravel on water redistribution and water absorption were obtained through indoor soil column simulation experiment.The results show that gravel embedded or semi-embedded in soil can induce soil erosion on slope surface.Under steep slope conditions,it can accelerate sediment yield and cause serious soil erosion.At the same time,we found that there was a certain error in the determination of slope runoff velocity by manual dyeing tracer method,and we adopted the image processing technology to make the determination of slope runoff velocity by dyeing tracer method more accurate.These conclusions provide reliable reference value for slope soil erosion control and major natural disaster prevention. |
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