Using Depression Deposits To Access Sediment Yield From A Karst Catchment In The Three Gorges Reservoir Region

Soil erosion and rocky desertification have become prominent environmental problems in karst areas,which seriously threaten regional ecological security and sustainable development of social economy.Accurate and reliable data of erosion and sediment yield are the basic basis for understanding the evolution process of karst rocky desertification under the influence of human activities.Taking the typical karst depression and its small catchment in the Three Gorges Reservoir Region as the research object,12 sampling points were evenly distributed in the depression,and sediment cores were collected in layers.¹³⁷Cs and ²¹⁰Pb_ex nuclide tracer techniques were used to determine sediment age,so as to invert the nearly 100 years history of erosion and sediment production in karst depressions catchment unit,and find out the evolution process and law of erosion and sediment production.By analyzing the depth distribution characteristics of ¹³⁷Cs,²¹⁰Pb_ex,organic matter,magnetic susceptibility,particle size and bulk density in the sediment profiles,the time scales of the past 50 years and the past100 years were obtained,and the corresponding relationship between the changes of various indicators and relevant environmental events was established.Combined with the environmental evolution,the depression sedimentation was divided into two periods,which were 1917～1963 and 1963～2017,and the sediment deposition rate and specific sediment yields of the catchment were calculated respectively in different time periods.Referring to the history of human activities and environmental evolution of the catchment,the trend and reason of the change of sediment deposition rate and specific sediment yields were explored.Karst rocky desertification was not only the result of human disturbances in the last 50 years,but also related to human activities with nearly 100 years and even longer.The main conclusions of the study were as follows:（1）The bulk density and particle composition of the sediment profile had no obvious change with the depth.On the one hand,it indicated that the soil in the small catchment of the depression was evenly mixed in the process of erosion,transportation,deposition and cultivation;on the other hand,it reflected that the soil texture in the karst area was relatively uniform,and there was no wind crust between the karst area and the bedrock,which belonged to a"mutation"contact.（2）There was a significant correlation between organic matter and magnetic susceptibility.The increase of organic matter and magnetic susceptibility in the sediment profile from the bottom to the top may be related to the application of organic fertilizer and chemical fertilizer and the return of straw to the field in recent 50 years,but not directly related to the erosion and sediment yield of the catchment.（3）The spatial distribution characteristics of ¹³⁷Cs inventories in Changjiawa depression were high in the middle and low in the surrounding areas,the spatial distribution of sediment deposition rate were high in the surrounding areas and low in the middle since 1963,which were related to the micro-topography,rainfall and man-made tillage disturbance of the depression,and reflects the spatial differences of the sedimentation in the depression.（4）The average sediment deposition rate in the depression is 1.00 cm/a（1917～1963）and0.27 cm/a（1963～2017）,the average specific sediment yields（SSY）in the catchment is 609t/km²·a（1917～1963）and 155 t/km²·a.（5）The higher values of sediment yield intensity during 1917～1963 were related to the gradual enhancement of human activities since the beginning of last century and the large-scale deforestation in 1958.The sharp decreased values of 1963～2017 were due to the fact that the previous erosion had led to less soil and exposed local bedrock,combined with the vegetation recovery,the decrease of human activity intensity and conservation measurements.