| Globally,more than 70 percent of the areas are suffering the soil erosion,and the loss amount of surface soil arrives at 25 billion ton per year.As one of the countries which is suffering the most serious erosion,the preexisting study about the soil loss is still inadequate to meet the national development and ecological construction in China.Due to the scale effect,the characteristics of soil loss in the different scales are combined with each other and also different from each other.The previous studies were mostly focused on the soil erosion developing in the single scale(e.g.soil slot, runoff plot,slope etc.),however,the comparisons among these scales are relative lacking.At the same time,the effects of soil loss appear the nonlinear influences along with enlarging study area.In the regional or even worldwide scale,the change of different influence factor's weight is the critical content in order to reveal the mechanism of soil loss in different scale.Most of all,the soil erosion in the small scale is the base.So,it has important theory and practice significances to illustrate the effects of soil erosion in three scales(runoff plot,toposequence and catchment) and the internal mechanism.The sediment in the Yangtze River mostly originates from the purple soil in the drainage area. As a special soil type of China,the purple soil is characterized by soft texture and high erodibility. Despite high productivity,the purple soil is very vulnerable to weathering and soil loss.Among all China soil types,the erosion area and erosion intensity of purple soil ranked second behind the Loess. In the Three Gorge region,the human activities also accelerate the soil erosion process.The soil loss problem is getting worse year by year and has become a threat to the ecology safety and sustainable development in this region.So,the study focused on the purple soil area,and aimed at revealing the effects of soil erosion in different scales,establishing the foundation for controlling soil erosion and constructing regional soil loss predicted model.Three scales in the Daijiagou catchment,including catchment,toposequence and runoff plot,were applied in our research.With the aids of monitoring equipment,the sediment and runoff were observed,the nutrient contents were tested.1.Within the runoff plot scale,the method of simulated rainfall was applied.Consequently,the changes of soil particle and nutrient loss on the different slope positions and different rainfall intensities were investigated.Moreover,the relationships among the soil particle,runoff and nutrient were analyzed.It is found,under the effects of splashing by the raindrop,detaching/dispersing, transporting and scouring by the flow,the contents of clay and silt decline with the increasing content of the sand in the upper-slope.Different from the upper-slope,the contents of clay in the mid- and lower-slope increase,and the content of sand decreases in the lower-slope.Generally,the losing rates on the different slope positions present the increasing trend with the rainfall intensity enhancing.The difference of soil particle between pre- and post-rainfall is positive correlated with the nutrient's loss in various degrees.When comparing with the changes,the soil aggregate with＜0.25mm and＞2mm increase significantly after rainfall,and the soil aggregate with else sizes increase.The correlation between fractal dimensions of soil aggregate(D) and nutrient losing demonstrates that,the greater difference in D(pre- and after-rainfall) follows with the more much nutrient's losing rate.All sediment loads under the different rainfall intensities exhibit the processes from rising to declining.The time appearing the maximum concentration of sediment is shorter with more enhancive rainfall intensity,and the similar trend still occurs in the maximum nutrient concentration of the sediment.However,not all the nutrient content of sediment increases along with the rainfall intensity enhancing,due to the different adsorbability of nutrient on soil particle.The cumulative quantity of runoff's nutrient increases correspondingly along with the more enhancing rainfall intensity and longer duration of runoff generation.The velocity of runoff and the duration of runoff generation are related with a power function under the relative larger rainfall intensity,and the relation is transfer into the quadratic curve under the weak rainfall intensity.The relations between the runoff intensity/runoff velocity and the concentration of total nitrogen(TN)/total phosphorus(TP) are unremarkable,however,significant correlations exists when comparing with the corresponding capacity of losing TN and TP.In general,fine soil particle decreases and coarse soil particle increases in the upper-slope during the erosive process,and opposite phenomena appears in the lower-slope.The changes of soil aggregates with＜0.25mm and＞2mm increase after rainfall,and losing nutrient is related with soil texture,stability of soil aggregate;The sediment load,losing nutrient is increasing along with the enhancing rainfall intensity and prolonging duration of runoff generation,but the characteristics are distinguishing for different nutrient elements.2.Within the toposequence scale,with the help of 137Cs tracer technique,the soil erosion modulus in different topography positions and different land use were detected.The source of sediment was analyzed,and discussed the difference of the soil erosion modulus in different conditions(topography and land use).At the same time,the changes of particle size and nutrient content in the slope surface,sediment and runoff were analyzed;the relationships among them were investigated,based on the observed data from the outlet of toposequence.It was found,the position and the land use significantly influences the spatial distribution of soil particle.The sequence of soil clay content in the different land is waste-grassland＞forest land(open forest land,orange forest land,peach forest land)＞cropland.The order of soil clay content in the different position with same land use is upper-slope＞mid-slope＞lower-slope.The sand content of sediment particle increases firstly and reduces afterward along with the erosion process,and the content of clay presents the reverse phenomena.Both of above mentioned textures show the quadratic curve relation with the runoff duration.In the toposequence,the average content of every nutrient element in different land use present the same characteristics:upper-slope＜mid-slope＜lower-slope.The organic matter, total nitrogen and available phosphorus in the sediment appear the same trends,which is increasing firstly and reducing afterward along with runoff duration.In addition,their contents exhibit the quadratic curve function with the runoff duration.The total phosphor and the available nitrogen in the sediment present the declining trend along with the runoff duration,and their contents exhibit the power function with the runoff duration.There is a significant negative correlation between the soil erosion modulus and the contents of organic matter,total nitrogen,total phosphorus.The content of organic matter and available phosphor presents significant positive correlation with the content of clay,but the opposite relationship occurs when facing the content of sand.There is a significant correlation between the content of total nitrogen and soil particle.Moreover,the nutrients what aforementioned correlate with the corresponding soil particle by linear function.Collectively,in the toposequence scale,the clay content is highest in the waste grassland,and lowest in the cropland.Lower-slope appears the high concentration of clay and the upper-slope appears the low concentration.The sand content of sediment increases firstly and reduces afterward along with the erosion process,and the content of clay presents the reverse phenomena.The nutrient content in the sediment correlated with the runoff duration by the quadratic curve or power functions, and is linear with the soil particle in varying degrees.3.Within the catchment scale,with the aids of 137Cs tracer technique and combining the monitoring data of precipitation,the influences of the land use types and the spatial variability effecting on the soil erosion modulus,soil particle and the nutrient loss were discussed.It was found, the average soil erosion modulus are 4205.74 t·km-2·a-1,2466.26 t·km-2.a-1 and 1552.91t·km-2·a-1 respectively in the upper,middle and lower reaches of Daijiagou catchment.In the upper reaches, the soil erosion modulus in the masson pine land,cropland and orange forest land are 2839.92t·km-2·a-1,5343.74 t·km-2·a-1 and 3837.1 t·km-2·a-1 respectively.For the masson pine land,the average soil erosion modulus are 2839.92 t·km-2·a-1,2103.64 t·km-2·a-1 and 268.17 t·km-2·a-1 respectively in the upper,middle and lower reaches.For the cropland,the average soil erosion modulus are 5343.74 t·km-2·a-1,2726.40 t·km-2·a-1 and 1830.05 t·km-2·a-1 respectively in the upper, middle and lower reaches;In the orange forest land,the average soil erosion modulus are 3837.10 t·km-2·a-1,2354.05 t·km-2·a-1 and 1291.95 t·km-2·a-1.Due to the effects of splashing by the raindrop, detaching/dispersing,transporting and scouring by the flow,the clay and silt contents on the slope surface increase from the upper- to lower-slope,and the sand content in upper-slope is higher than that in mid- and lower-slope.With the same position(upper-,mid- or lower-slope) of the whole slope,the clay and silt contents increase firstly and decrease afterward from the surface soil to deep soil.The different positions and land use types in the catchment have deeply influenced the soil mechanical composition.In the waste-grassland and the orange forest land,the clay content decreases from the upstream to downstream of catchment;but the reverse trend is presented by the sand content;In the cropland,the clay content is the most in mid-reaches of the catchment,and the clay content is lest in the lower reaches.The soil specific area(SSA) in the lower reaches is larger than that in middle and upper reaches.In the different rainfall conditions and land use types,the coarse sediment be settled down in some areas of catchment and the fine sediment can be transferred into the outlet of catchment following with the runoff.So in the outlet of catchment,the specific gravity of the sediment with＜0.05 mm is 80%,equals to 4 times the specific gravity of the sand. According to the correlation analysis,the organic matter and total nitrogen in the sediment have significantly correlated with the clay content(correlation coefficient R is 0.921 and 0.860 respectively).And the available nitrogen in the sediment had positive correlation with the clay content.The nitrogen in the sediment had significant negative correlation with the sand content,and the available phosphorus and total phosphorus in the sediment had highly significant correlation with the silt content,the correlation coefficient is 0.925 and 0.968 respectively.Therefore,in the catchment scale,the highest and lowest soil erosion modulus appears in the upper and lower reaches respectively.The soil specific area(SSA) in the lower reaches is larger than that in the middle and upper reaches.The sediment particle is mostly composed by the fine particle with＜0.05mm,and there are some different correlations between the sediment nutrients and soil mechanical compositions.4.Comparison and analysis of the soil erosion's characteristics in the different scales. According to the changes of soil/sediment particle size and the nutrient loss,the relationship between them in different research scales,the mechanism of soil loss and the scale effect transferring were discussed.It was found,in the runoff plot scale,the variations of soil particle in the different slope positions are unobvious,but remarkable variation appears in the scale of toposequence and catchment.Because of the complex factors that influence the soil erosion and the complicated relationships among different influencing factors,the variability of soil particle becomes more and more obviously along with the scales enlarging.Under the same rainfall condition,in the runoff plot area,the relationship between clay content and runoff generation time is not obvious,but in the scales of toposequence and catchment,the relationship between the clay content in sediment and runoff generation time is regressed by the quadratic curve.In the scales of runoff plot,natural slop and catchment,the relationships between the sand content in sediment and the runoff generation time demonstrate the functions of power,quadratic and quadratic respectively.And the relationships between silt content and runoff generation time in the runoff plot is power function,the relationship is changed into the quadratic function in the catchment.The relationships between all kinds of sediment particles and the runoff generation time tend to appear the quadratic function when the scale is enlarged from the runoff plot to catchment,and the relationship between soil organic matter and runoff generation time is transferred from power function into quadratic function,same variety is appeared in the relationship between total nitrogen,available nitrogen and the runoff generation time.In addition,the relationship between total phosphor and runoff generation time is varied from the power function into linear function,the same changes is occurred between the avail phosphor and the runoff generation time.According to the relationship between soil particles and the sediment, under parallel rainfall conditions,the correlation between the total potassium and clay content,and content in sediment is significant,in the runoff plot.In the scales of toposequence and catchment, the linear relationship between the sediment particle size and the nutrient content(soil organic matter, total nitrogen,total phosphor,available nitrogen,and available phosphor) do exist.According to the comparison and analysis,the number of nutrient element which is related with the soil particle size is enlarged.In the scale of runoff plot,there is a significant correlation between soil organic matter and sediment load,and the soil organic matter,total nitrogen and available nitrogen in the sediment significantly correlates with the soil erosion modulus when facing the scales of natural and catchment.The soil nutrient loss become more seriously along with more serious soil erosion modulus.It has degrees of difference between soil/sediment particles composition and nutrient loss,as well as their relationship in the scales of runoff plots,toposequence and small catchment.That is because of differing the impact of soil erosion at different scales,as well as primary and secondary effects of the different factors that result in a different order.In the scale of runoff plot,soil erosion is mainly impacted by the characteristics of rainfall and soil.However,topography and vegetation are the main factors affecting soil erosion in a toposequence scale.Then it needs to consider these factors of spatial variability in the small catchment scale.To sum up the above arguments,we studied the soil erosion effect in different scales,and discussed their transformation.Through the monitoring outside and experiment inside,the changes of soil/sediment particle size,nutrient loss and their relationship in different scales were detected, which also is the critical reason causing the scale effects of soil erosion.The research partly revealed the soil loss mechanism and scale effect in the purple soil area.However,it still needs more theory study about the scale effect.We should e develop the research about the scale effect of soil erosion further,which will provide the basis for establishing quantitative soil erosion predicted model,and realizing the quantitative evaluation of soil erosion effects for spatial scale and scale transformation. |
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