Modeling Sediment Yield Using WaTEM/SEDEM For A Small Depression Catchment Of Karst In The Three Gorges Reservoir Region

The formation of soil in karst area is slow,the soil layer is shallow and spatially heterogeneous,and affected by human activities,soil erosion is increasingly serious.The carbonate rocks in the Three Gorges Reservoir area are widespread and the terrain is complex.Human disturbance has been intense,and the problem of rock desertification has become prominent.Obtaining reliable data on sediment production in small watersheds can help scientifically understand the process of rock desertification.In this study,the typical karst depression catchment in the Three Gorges Reservoir area,namely the Changjiawa small catchment,was taken according to the topographic characteristics of the 12-hole sediment core in the depression.According to the evolution of the catchment environment and human activities,the ¹³⁷Cs,bulk density and grain size of each hole depth distribution characteristics of the diameter composition;use the WaTEM/SEDEM model to simulate the sediment yield intensity of the catchment,use ¹³⁷Cs to determine the sediment deposition rate and the sediment yield modulus of the depression,and use it to correct the model parameters,and use the reported erosion and sediment yield data of the Karst catchment verification.The combination of nuclear tracing dating technology and the WaTEM/SEDEM model to obtain the erosion and sediment yield of the river basin has certain reference significance in karst areas lacking monitoring data,and can become an effective tool for evaluating soil erosion in the area in the future.The main results are as follows:（1）The distribution of ¹³⁷Cs activity in all sediment profiles in Changjiawa depression is roughly the same,all from bottom to top,and it increases rapidly after reaching a certain depth range.The sediment profile of Changjiawa depression is a typical accumulation soil profile.The high-concentration ¹³⁷Cs depth distribution range is 0～36 cm.In 1963,the ¹³⁷Cs time scale depth averaged 29.83 cm,the sediment thickness of the depression was 15 cm,and the deposition rate was 0.27 cm/a The total sediment deposition in the watershed is 703t,and the sediment yield modulus in the watershed is 155 t/（km²·a）.（2）The particle composition and bulk density of sediment profiles in the Changjiawa small basin have no obvious changes with depth,and the overall trend is relatively stable.The sediment sediments in depressions are mainly composed of sandy silt and clay silt,of which the proportion of powder particles is the highest,followed by sand particles,and the clay is the lowest.This result is consistent with the particle size characteristics of eroded sediments in karst slopes.The variation range of soil bulk density is small,ranging from 1.18to 1.35 g/cm³,with an average value of 1.25 g/cm³.（3）The ¹³⁷Cs sediment yield data of Changjiawa depression sediments are used to obtain the best K_TC-high and K_TC-low of 18 and 7 after model correction.At this time,the efficiency coefficient c _NS reaches a peak value of 0.79,which shows that the model simulation effect is the best.The WaTEM/SEDEM model was run to find that the total sediment deposition in the Changjiawa small basin since 1963 was 557 t,and the watershed sediment yield modulus was 122.91 t/（km²·a）.（4）Using the reported erosion and sand production data in the Karst basin to verify the simulation results of the WaTEM/SEDEM model,the correlation coefficients between the data of the sand production modulus（SSY）and the sand production（SY）and the measured values are 0.93 and 0.94,respectively.The obtained ¹³⁷Cs data can reflect the erosion and sediment production in the area,and the data also has a good correction effect on the parameters of the WaTEM/SEDEM model.（5）When the basin area is similar,the erosion and sand production intensity in the small basin of Changjiawa is high,and the sand production modulus of the small basin in the Karst area gradually decreases with increasing area.This is due to the connectivity and sediment transport ratio of the basin.The area increases and decreases,which is basically consistent with the non-karst area.