| Since the 1980s,the water and sediment of the Yellow River has gradually decreased.This change has attracted the attention of a large number of scholars.It is generally believed that the combined effect of climate change and human activities is the main reason for the decrease of water and sediment in the upper and middle reaches of the Yellow River.These measures have caused major changes in the underlying.surface of the basin,thereby affecting the water and sediment change process in the basin.This study selects the middle and upper reaches of the Fenhe River as the research object.It first clarifies the inter-annual and intra-annual variation characteristics and driving factors of hydrological elements in the study area,quantitatively analyzes the contribution of climate change and human activities to the water and sediment changes in the study area,and then constructs the study area SWAT model,evaluate the applicability of the model in the middle and upper Fenhe River basin,and then use the SWAT model to analyze the distribution characteristics of runoff,sediment transport and runoff erosion power in the middle and upper Fenhe River basin,and finally,borrow the model simulation module to set different scenarios,To predict the trend of water and sediment process in the study area.The main results of this study are as follows:(1)Clarified the inter-annual and intra-annual variation characteristics and driving factors of hydrological elements in the study area,and quantitatively analyzed the contribution of climate change and human activities to water and sediment changes in the study area.The annual precipitation,annual runoff and annual sediment transport in the middle and upper reaches of the Fenhe River are unevenly distributed during the year,mainly concentrated in the flood season from June to September,accounting for 73%,64%and 90%of the multi-year average respectively.The year of abrupt changes in annual runoff and annual sediment transport in the study area was 1980.The total impact of climate change and human activities on the annual runoff in the study area is 412 million m3,with a contribution rate of 29%and 71%,respectively;the total impact on the annual sediment transport volume is 11.508 million tons,with a contribution rate of 17%respectively And 83%.Generally speaking,human activities have played a leading role in the process of water and sediment changes in the middle and upper reaches of the Fenhe River.(2)The SWAT model of the middle and upper reaches of the Fenhe River was constructed and the applicability of the model in the study area was evaluated.Based on DEM,land use data,soil type data and meteorological and hydrological data in the middle and upper reaches of the Fenhe River,a SWAT model was built,and Yitang Station was used as a simulation station.The LH-OAT method was used to identify the sensitivities affecting the runoff and sediment transport processes in the middle and upper reaches of the Fenhe River The SWAT model is used to quantitatively evaluate the water-sediment process simulation results of the SWAT model with the determination coefficient R2 and the Nash efficiency coefficient.The evaluation results show that the SWAT model is suitable for the simulation of the water-sediment process in the middle and upper reaches of the Fen River,and the simulation accuracy is high.(3)The spatial convergence process of annual runoff,sediment transport and runoff erosion power in the middle and upper reaches of the Fenhe River is explained.The annual runoff in the upper and middle reaches of the Fenhe River presents the characteristics of "small upstream,large downstream,small tributaries,and large main stream",and the sediment load presents the characteristics of "small main stream,large tributary,large river head,small estuary".The runoff erosion power presents the characteristics of "large river source,small estuary,larger control area,and smaller runoff erosion power".The runoff erosion power has a spatial scale effect,and the runoff erosion power has a good power function relationship with the watershed area.The runoff erosion power reaches a clearly stable threshold of the watershed area of 161km2.(4)The changes of water and sediment trends in the middle and upper reaches of the Fenhe River basin in the next 30 years under different scenarios are simulated.The SWAT model simulates that the average annual runoff in the middle and upper reaches of the Fenhe River from 2021 to 2050 will be between 752.9 and 812.3 million m3,and the average annual sediment transport will be between 631,141 and 690,200 tons.The water production mainly comes from the Fenhe Plain in the middle reaches,and the area with a larger sand transport modulus is mainly at the source of the tributaries of the Fenhe River and the middle reaches of the plain.Comparing the simulation results of various scenarios,the forest land has a good effect of reducing water and sediment,especially when the study area is above 15 degrees and is forest land.The construction of check dams has little impact on the aquatic and sediment production within the dam-controlled area,and has a greater impact on the aquatic and sediment production in the downstream sub-catchments. |
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