| Red soil cover hilly area in the southern is the main citrus producing area in Chi-na.The development of orchard,especially in the early stage of large-scale planting,intensive disturbance leaded to serious damage to the ground vegetation cover and soil structure.In addition,simplified ecological components(e.g.the less diversity of plant species),combined with concentrated rainfall with high intensity leaded to se-vere water and soil loss.Compared with other regions,the red soil region of south China has obvious rainfall fluctuation and complex processes.Previous studies ig-nored the impact of rainfall fluctuation(the variation of rainfall and rainfall intensity in a rainfall event)on soil and water loss.Alternatively,it is characterized by a few indicators such as rainfall amount,average rainfall intensity,and maximum 30 min rainfall intensity,with less consideration being given to the change of rainfall process.This situation limited the accurately reflection on how rainfall affects water and soil loss in red soil area.The changes in surface cover with the planting years and the soil and water conservation measures are another important factor affecting soil and water loss in orchards.However,it is still unclear how the orchard soil erosion and runoff generation response to rainfall fluctuation and surface cover on the perspectives of long-term field observations.By adopting the long-term observation data of the modern ecological orchard ex-perimental platform in Jiangxi Soil and Water conservation Ecological Science and Technology Park from 2001 to 2020,this study addressed the three objectives.The first objective is to reveal the relationship between rainfall fluctuation and soil and water loss in orchards,and determine the relative contribution rate of rainfall fluctua-tion and their changes with orchard planting years.The quantitative characterization parameters of rainfall fluctuation were introduced to decompose the rainfall process.The second objective addresses the changes in resistance of soil in orchards to rainfall erosion under different surface cover along with planting years,and their impacts on soil and water loss.Furthermore,the indices quantifying cover patterns were intro-duced to clarify their impacts soil and water loss under different rainfall conditions in orchard.Thirdly,the responses of soil and water loss in the orchards to rainfall fluctu-ation under different surface cover conditions,and their variations with orchard plant-ing years,were also investigated.The main results of this study are as following:(1)The influence of rainfall fluctuation on orchard sediment yield largely de-pends on the erosion resistance of orchard soils.The rate of structure damage and dis-persion of soil aggregate in different surface cover management orchards decreased with the increase of planting years,and the important percentage of water stable ag-gregates(WSA)and the average weight diameter of aggregates(MWD)over 0.5 mm increase with the increase of planting years.Along with the increasing planting year,the relative contribution rate of conventional rainfall indices(rainfall depth,rainfall intensity et al.)to runoff coefficient and soil loss rate decreased from 63.9%to 20.4%and from 79.8%to 32.1%,respectively.In contrary,the contribution rate of rainfall fluctuation indices(e.g.PD＿p,D＿p,I＿p,PN,Location,IERI,AIR,FIRP,RFRI,and N)presented an overall increasing trend,with the relative increasing from 36.1%to79.6%,and from 20.2%to 67.9%,respectively.(2)The rainfall events with great rainfall intensity and short duration had strong-er sediment yield capacity.The appearance of the location of the maximum rainfall intensity peak(Location)in the middle stage of a rainfall process is more conducive to the runoff yield.Under all soil surface cover in this study,the runoff amount and sediment yield of orchards decreased along with the increasing planting years,with a decrease rate of 49.8%and 45.0%,respectively.The measures that with understory grass in orchard had the best effect on reducing runoff and sediment yield,which de-creased by 86.2%and 98.4%,respectively,compared with the measures of clearing tillage,followed by the interplanting with cash crops,which decreased by 49.0%and38.6%compared with orchard clearing measures,respectively.These results imply that reasonable surface cover can significantly reduce soil and water loss in orchards,and grass growing measures are better than inter-planting.(3)The responses of soil and water loss to rainfall characteristics are affected by the soil surface cover and its distribution pattern.Compared with slope runoff(R~2range is 0.053-0.6138 with the average value of 0.1644),sediment yield is more sensi-tive to the distribution pattern of surface cover(the range of R~2 is 0.044-0.6358 with an average value of 0.1877).Besides,the impacts of soil surface cover and distribu-tion pattern on runoff and sediment were also affected by rainfall characteristics.With the increase of rainfall amount,the relationship between distribution pattern soil of surface cover and runoff and sediment yield on was strengthened.Under heavy rain-fall events,the regulating effect of surface cover on runoff and sediment production was more obvious.(4)The relative contribution rates of rainfall fluctuation to runoff were positively correlated with soil surface cover distribution pattern(indicated by DLI and FDLI).On the contrary,the relative contribution of rainfall fluctuation to sediment yield was negatively correlated with DLI and FDLI.With the increase of planting years,the relative contribution rate of rainfall fluctuation parameters to runoff and sediment yield increased to a certain extent,with a range of 3.3-99.5%and 15.2-248.0%,re-spectively.The rainfall fluctuation in red soil hilly area is obvious.This study argued the rainfall fluctuation affected soil erosion and runoff,and the relative contribution rates to soil erosion differentiate significantly with the planting years.Detailed analysis of the rainfall process by emphasizing the important relationship between rainfall fluctu-ation parameters and orchard water and soil loss will be more helpful to improve the control and prediction of orchard water and soil loss.When controlling soil erosion in orchards through regulating soil surface cover,it is also necessary to focus on the role of surface cover distribution patterns in regulating soil erosion.Specifically,not only the appropriate type of surface cover should be selected,but also the distribution pat-tern of soil surface cover should be optimized so as to improve the regulation and con-trol of the soil and water loss,which will promote the high-quality development of the orchard industry in the red-soil covered hilly area. |
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