| Soil erosion is an important factor that influences environmental change and ecological stability.Aeolian-fluvial interplay regions in northwestern China experience serious soil erosion,and are the main source of coarse sediments entering the Yellow River.Owing to the superposition of erosive power and erosion time in the aeolian-fluvial interplay regions,eroded material and erosion time are extended,resulting in greater ecological impact.Therefore,it is of important scientific significance to analyze the composition,distribution,and transport processes of eroded materials in these regions for the control of soil erosion.The study area are two typical aeolian-fluvial interplay erosion watersheds-the Dongliu Gully and the Xiliu Gully,both belong to the ten watershed of Inner Mongolia.Data were collected from hydrological station,filed sampling and monitoring,and laboratory rainfall simulation.The erosion situation of the study area were analyzed,the energy of erosive wind and precipitation were calculated,the distribution of deposited sediments were analyzed,the dynamic and physical source of deposited sediment in the stream channel were determined.The variations in composition of the eroded materials under different sand cover patterns and rainfall conditions were further analyzed through laboratory rainfall simulation of a sand-covered slope to reveal the influence of wind erosion on the water erosion.The results are conductive to provide scientific reference for control and management of soil erosion in aeolian-fluvial interplay erosion areas.1)The land use,annul runoff and annual sediment load characteristics and variation of the studied aeolian-fluvial interplay erosion watershed were analyzed.Grassland,desert and farmland are the main land use types in the ten watershed of Inner Mongolia,their area percentages are 51.22%,21.42,and 15.81%respectively.The areas of farmland,grassland and construction land have increasing trends,while forest land and desert have decreasing trends since 1980s,and the variation of land use mainly take place in the upper region.The statistics of annual runoff and sediment load also change greatly,and both the change points of annual runoff and sediment load take place around 1998.Annual sediment load is closely related to annual runoff before 1998,and annual sediment load decrease significantly.The soil erosion modulus range from 2180.23 t/(km2·a)?17624.62 t/(km2·a),and most regions belong to intense erosion and strongly intense erosion,while the soil erosion modules of the desert and stream channel are highest,and the farm land is lowest among different land use types.On the other hand,the soil erosion module have a decreasing trend from the upper region to the lower region.2)The energy of erosive wind and erosive precipitation in the study area were calculated.The intra-annual distribution of erosive energy showed that there are two peak points,i.e.,windy season from March to May,and rainy season from July to September.The average wind speed from March to May is 5.4 m/s?6m/s,and the average monthly precipitation from July to September is 31.67mm?74.06mm.Annual energy of erosive wind is 813.67 J·d/m2,and annual energy of erosive precipitation is 1425.0 J·d/m2.The erosive energy in the windy season accounts for 23.71%,and the erosive energy in the rainy season accounts for 54.52%.The composition of suspended load in the stream channel in the windy season is different from the rainy season.The main material of suspended load in the rainy season is silt,which takes up 89.45%.Powder sand and fine sand are included in the suspended load in the windy season,taking up 33.93%and 28.56%respectively,and the proportion of silt is 36.36%.3)The distribution of deposited sediment in the stream channel were analyzed and the sources of deposited sediment in the stream channel were determined based on the law of the conservation of matter.The sediment grain sizes in the watershed were concentrated within two ranges:relatively coarse deposited sediments(0.25 mm to 0.6 mm,mean median grain size(d50)of 0.47 mm)mainly distributed in the hilly zone in the upper watershed;and relatively fine deposited sediments(0.1 mm to 0.2 mm,mean d50 of 0.18 mm)mainly distributed in the desert zone located in the middle watershed and in the farmland zone located in the lower watershed.Sediment entering the Yellow River was dominated by hilly zone material with a d50 of 0.093 mm and desert zone material with a dso of 0.01 mm.The frequency distribution curves of sediments deposited in the stream channel were fitted using the Weibull function,the sources of deposited sediment in the stream channel showed that the hilly zone accounts for 78%of deposited sediments,and the desert zone accounted for 22%of deposited sediments.Wind erosion dynamics accounts for 72%of deposited sediments,while water erosion dynamics accounted for only 28%.However,the dominated erosion dynamic in the hilly zone is water erosion,which accounts foe 69.78%,and the dominated erosion dynamics in both the desert zone and farmland zone are wind erosion,which accounts for 74.54%and 94.14%respectively.Therefore,it can be concluded that the intense erosion in aeolian-fluvial interplay erosion take place in sand-coverd hilly during rainstorm.4)The sediment transport modes of the sand-covered slopes were explored through laboratory rainfall simulation.Water erosion is proved to be intensified under sand-covered slopes.The initial time producing runoff will be greatly delyed as the sand-coived mass increase.The prominent eroded material of sand cover slope is loess,and take up 83%in average.Sand erosion mainly take place in the initial stage of erosion.Sand-covered slopes aggravated loess erosion,not only added sand as additional erosion material,but also increased loess erosion amount,compared with the loess slopes.Both sand cover pattern and sand cover amount have influence on the erosion increment.Depending on the pattern,the increment of erosion material is 64.2%-317.6%of the sand cover amount.Under the same rainfall intensity and and-covered mass,the increment of eroded materials is 87.11%from the sand-covered half slope to thin but total sand-covered slope,and the proportion of loess increase from 77.99%to 92.37%.The relationship between runoff erosion power,erosion sand amount,and erosion loess amount were fitted by multiple linear regression,and it is indicated that the consumed erosion power of per erosion sand amount is 1.19 times per erosion loess amount generally,and the consumed erosion power is influenced by rainfall intensity,initial time producing runoff,sand cover mass,and sand cover pattern.The resluts indicate the thin sand cover induced the largest erosion amount compared with other sand cover patterns and explaine the cause of intense erosion for thin sand-covered slopes in the aeolian-fluvial interplay erosion regions. |
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