| Taking the cultivated land/woodland depression watershed in Qingmuguan karst trough valley area as the research subject,this paper traces the variation characteristics and driving factors of sediment yield intensity in the past 60 years based on 137Cs and HCHs dating,combined with the analysis of land use change in the basin.The elemental characteristics of soil(slope,gully)and erosion sediment(depression sediment,surface suspended sediment)in potential sediment sources of the basin were analyzed,and the relative contribution rates of different sediment sources to sediment yield were quantitatively analyzed by using compound fingerprint.It is expected to provide reference for the quantitative study of erosion and sediment yield and the formulation of water conservation measures in small watershede of karst trough valley.The main research results are as follows:(1)The sediment profiles of cultivated land-type depression in Longdongcao and forest land-type depression in labor village were obtained.Peak value of 137Cs in in 1963 and the dating point of HCHs forbidden in 1983 were obtained.The sediment yield modulus of Longdongcao and labor village depression small watershed in 1963-1983 and 1984-2019were calculated.The sediment yield modulus of Longdongcao cultivated land type and labor village forest land type depression watershed from 1963 to 1983 were 232 t/(km2·a)and 69t/(km2·a)respectively.That of Longdongcao cultivated land type and labor village forest land type depression watershed in 1984 to 2019 is 82 t/(km2·a)and 39 t/(km2·a)respectively.The variation trend of sediment yield modulus is similar in the last 60 years,and the sediment yield modulus decreases,which indicates that the ecological environment of the watershed is improved significantly.Compared with the period from 1963 to 1983,there was no significant change in the mean annual precipitation in the period from 1984 to 2019,and the frequency variation rate of annual precipitation,and inter-annual heavy rain/rainstorm increased,but the sediment yield modulus of the two small basins decreased significantly.The results show that in recent 60 years,the intensity of sediment production is mainly controlled by human activities such as land use mode and water and soil conservation measures.(2)The peak values of 137Cs and HCHs in sediment profile,137Cs and HCHs in surface sediment and the sediment yield modulus in different periods are significantly higher than those in the small watershed of laber village depression,which is mainly caused by the difference of sediment yield intensity between cultivated land type and forest type watershed under the influence of human activities.The amount of soil loss in the small watershed of Longdongcao depression is higher than the allowable amount,while the small watershed of labor village depression is close to the upper limit of the allowable amount,and the risk of soil erosion in the region is still very high.In addition,137Cs and HCHs can be used to study the sediment yield history of karst basin in southwest China,which provides a new approach to study the sedimentation rate and sediment yield modulus of karst depression.(3)The mechanical composition of the soil in the source land of the two depressions,Longdongcao and laber village,is similar.The content of silt particles is large,and the soil texture belongs to silt loam;the average carrying ratio of silt particles is>1,and the soil with higher silt content is more easily eroded.The contents of organic matter,total nitrogen and available nitrogen in forest land and grassland in two small watersheds were higher than that in slope farmland and channel,mainly due to the supplement of organic matter and nitrogen by litter in forest land and grassland and less disturbance by human.The content of total and available phosphorus in slope cultivated land is higher than that in forest land,grassland and channel,which is influenced by farming,fertilization and straw returning.The total potassium and available potassium content of grassland were higher than those of the other three types of sediment sources.The differences between 137Cs and?in different sources of soil in two depressions are relatively obvious,which provides a strong basis for the identification and analysis of different sources of erosion sediment.(4)In the small watershed of Longdongcao cultivated land type depression,21 of the 35elements in the source soil entered stepwise discriminant analysis through non-parametric test,and the best fingerprint factor combination consisting of 137Cs,TP,Co,SOM,Li,Rb,and?was selected.The relative contribution of slope and channel to sediment in small watershed is87.55%and 12.45%respectively,and the mean contribution to surface suspended sediment in small watershed is 80.36%and 19.64%respectively,which illustrates that slope is the main source of erosion sediment in watershed,and the contribution of erosion sediment in small watershed on slope has the following relationship:slope farmland>forest land>grassland.In the small watershed of the forest land type depression,22 elements of the 35 elements in the source soil were analyzed step by step by nonparametric test,and the best fingerprint factor combinations of 137Cs,Li,SOM,?,Rb,and Tl were selected.The relative contribution of slope and channel to sediment in small watershed is 76.35%and 23.65%respectively,and the mean contribution to surface suspended sediment in small watershed is 67.54%and32.46%respectively.Slope is the main source of erosion sediment in small watershed.The contribution rate of erosion sediment in small watershed on slope is ordered as follows:forest land>sloping farmland>grassland.(5)According to the percentage of sediment contribution per unit area,the sediment contribution per unit area of slope cultivated land in the small watershed of Longdongcao depression is 3.44 times that of forest land and 1.67 times that of grassland.The slope cultivated land in the small watershed of labor village depression is 3.05 times that of forest land and 3.28 times that of grassland,which indicates that the slope cultivated land is the most erodible part of slope surface in the small watershed.The slope farmland is affected by agricultural activities such as tillage,fertilization and weeding.The ground coverage is low,the surface soil is relatively loose,and the soil is prone to runoff.The vegetation coverage of forest land and grassland is relatively high,which has the advantages of weakening raindrop splashing,increasing infiltration,intercepting sediment and improving soil structure by root system.It effectively reduces soil erosion,so the contribution value of sediment per unit area of forest land and grassland is small. |
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