Study On Sediment Sources In Small Catchments On The Loess Plateau Using Composite Fingerprinting

To study the sediment source of small catchment,the basic unit of soil erosion on the Loess Plateau,is of great significace.This study used the composite fingerprint recognition technology to discriminate the sediment source of small catchment on the Loess Plateau.QiaoziXiGou,a small catchment in the Luoyu-Gou watershed in the middle of the Loess Plateau was selected in Gansu Province and soil samples were collected across the whole catchment and flood sediment samples were collected from eight erosive rainfall events.Besides,a check dam with 31 years deposition(1960^-1990)was selected in Wangmaogou watershed in Suide County in Shaanxi Province,and the soil samples were collected on the check dam profile and the surface of the slopes.The results are as following.The tracer properties of soilχHf,TN,SOM,Mg,Na,Ca,Mn and TP could provide the best composite fingerprint for discriminating source materials on three source types of gully wall,gully slope and farming land in QiaoziXiGou with discriminating accuracy 100%.Sediment contributions from the gully wall,gully slope and farming land to all collected flood sediment were 18¹00%,0⁵2%and 0⁵7%,respectvly.During the flooding process,gully wall was always the main sediment source,and at some time the gully slope and farming land sediment contributions were 0,that is,soil erosion on gully slope and farming land was intermittent during runoff-yielding time.Combying the sediment source and sediment yield per unite time during rainfall process,we found that sediment yield per unite time from gully wall,gully slope and farming land had a similar tendency of change and the time to peak was near to each other.For the sediment yield from 8 rainfall runoff,the range of gully wall,gully slope and farming land sediment contribution were 41⁵7%,5³6%and 3²4%,respectively,and the mean value was 63%,13%and 24%,respectively.By curve fit method,we found that among rainfall,rainfall energy,rainfall erosivity and rainfall maximum 30min intensity,sediment yield from the catchment as well as it from gully wall,gully slope and farm land all had the best correlation with rainfall erosivity,so the conclusion is that soil erosivity can be used to forecast the soil erosion of both sources and catchment on the Loess Plateau,and that the effects of rainfall on sediment yield from gully wall is more obvious than on sediment yield from slope land.In the NianyanGoou catchment,The tracer properties of soilχfd,SOM,TP and Mg could provide the best composite fingerprint for discriminating three types of sources,each flood couplet sediment yield mainly came from gully wall.The sediment contribution showed that 32¹00%from the gully wall,0⁶8%from the gully slope and 0⁶6%from the farming land,and that the mean value was 69%,10%and 21%,respectively.The sediment yield of the check dam was 67190 t.The range of sediment yield from rainfall event was 67⁶748 t,with a mean of 896 t and the range of soil erosion modulus was0⁵3880 t km^-2 a^<sup>-1,with a mean of 11970 t km^-2 a^<sup>-1.Sediment yield of gully wall,gully slope and farming land was 45990 t,3318 t and 17878 t.According to the double accumulative curve of annual sediment versus annual pricipitation,the working time of the check dam could be divided into three stages:from1960 to 1964(soil erosion modulus of 26220 t km^-2 a^<sup>-1);from 1965 to 1983(5590 t km^-2a^<sup>-1)and from 1984 to 1990(19130 t km^-2 a^<sup>-1),at diferent stages,sediment yield from diferent sources had a significant diference.From the first stage to the third stage,the sediment yield from gully wall was 3548 t a^<sup>-1,752 t a^<sup>-1 and2102 t a^<sup>-1,respectively,from gully slope was86 t a^<sup>-1,56 ta^-1 and 268 t a^<sup>-1,and from farming land was 1113 t a^<sup>-1,259 t a^<sup>-1and 1092 t a^<sup>-1According to the double accumulative curve of annual sediment of each source versus annual pricipitation,the sediment yield of gully wall and farming land had a similar periodic change to the catchment.The relationships between event sediment yield and sourse sediment yield and event rainfall erosivity for these three periods indicate that the period 1960¹990 was characterized by higher sediment yields and gully sediment yield for a given event rainfall erosivity.However,the relationships between farming land sediment yield and event erosivity,suggest that the period 1984¹990 was characterized by higher event sediment yields for a given event erosivity than during the period 1960^-1964.The grey relational analysis method was applied to calculate the average proportion of soil erosion attributable to human activity,and the results showed that the proportions were 35%for a combination of the first(1960^-1964)and second stages(1965^-1983)and 83%for a combination of the second(1964^-1983)and third stages(1984^-1990).Therefore,we concluded that the driving force affecting soil erosion in the catchment was human activity.