Variation Characteristics And Process Simulation Of Water And Sediment In The Source Region Of The Yellow River

Affected by global climate change,extreme precipitation events have increased significantly.The source area of the Yellow River,as an important runoff area and water conservation area in the Yellow River Basin,has become a research hotspot in recent years.Therefore,studying the temporal and spatial characteristics of water and sediment changes in the source area of the Yellow River,the degree of the impact of extreme precipitation on water and sediment,and revealing the evolution of the water and sediment process can provide a basis for water and sediment control in the source area and downstream.Based on the measured data from the meteorological and hydrological stations in the source area of the Yellow River from 1960 to 2019,this paper adopts M-K trend test,double cumulative curve method,sliding t test and other methods,and selects the precipitation in the flood season(FSPTOP),heavy precipitation(R95p),Six representative extreme precipitation indexes,including daily precipitation intensity(SDII),extreme heavy precipitation(R99p),maximum precipitation for five consecutive days(R5d),and maximum daily precipitation(R1d),are analyzed for nearly 60 meteorological and hydrological elements.The temporal and spatial trends of the year,quantitatively assess the impact of extreme precipitation indexes on runoff and sediment transport,and use the SWAT(Soil and Water Assessment Tool)model to explore the degree of soil erosion in the source area from 2006 to 2019.The main research conclusions are as follows:(1)The precipitation in the source area of the Yellow River has shown a significant increasing trend in the past 60 years,while the amount of sediment transport and runoff have shown an insignificant downward trend.The abrupt years of precipitation and runoff were 1989 and 2002,and the abrupt year of sediment transport was 1989.Taking the period from 1960 to 1989 as the base period,it was found that the relationship between water and sediment was better;taking 1990-2002 as the early period of the abrupt change,the annual runoff and sediment transport decreased simultaneously with the decrease in precipitation;the period from 2003 to 2019 was the late period of the abrupt change.The runoff gradually increases with the increase in precipitation,but the change in sediment transport is not obvious.(2)The precipitation,runoff,and sediment discharge in the source area of the Yellow River vary unevenly during the year,mainly in the flood season,accounting for 77%,71.5%,and 93.8% of the annual total respectively.In addition,the average annual precipitation gradually increased from the northwest to the southeast.The precipitation at Maduo,Guinan and Xinghai stations increased significantly,and the southeast region showed a downward trend.(3)All extreme precipitation indexes in the source area of the Yellow River showed a non-significant increase trend.Except for R99 p,other indexes showed a very significant increase trend from 1990 to 2019.The spatial variation of extreme precipitation indexes gradually increased from northwest to southeast.FSPTOT,R95 p,and SDII in Xinghai and Guinan showed a significant increase,while Jiuzhi decreased significantly compared with other stations in the southeast.(4)The extreme precipitation index is an important factor affecting runoff changes.R95 p and FSPTOT had a greater impact on runoff in the early and late stages of the mutation,while SDII had a smaller impact on runoff.Especially in the late stage of the mutation,the contribution rates of R95 p and FSPTOT to the increase in runoff were 85.4% and 75.7%,respectively.Due to the decrease in precipitation from 1990 to 2002,R95 p and FSPTOT have a greater impact on the reduction of sediment transport,with contribution rates of 93.6% and 76.9%,respectively.After 2003,the extreme precipitation index has a relatively small impact on the sediment transport.,All do not exceed 26%,and non-precipitation changes have become the main factor affecting changes in sediment transport.(5)The SWAT model can better simulate the discharge and sediment transport at Tangnaihai Station at the outlet of the basin.Among them,the Nash-Sutcliffe Efficiency Coefficient(NSE)and Coefficient of Determination(R2)of the monthly average runoff during the verification period Both are above 0.9,the NSE and R2 of the daily runoff are both 0.88,and the deviation percentages(Percent Bias,PBIAS)are 0.39% and 1.81%,respectively;the sediment discharge rate during the verification period has increased regularly compared to the three evaluation indicators.(6)The areas with a large amount of sediment transport in the source area of the Yellow River are mainly the No.24 and No.28 sub-basins,with a total sediment transport volume of 3.0723 million tons;the average annual sediment transport volume of the northern and northwestern sub-basins ranges from50,000 to 1 million tons..At the same time,the soil erosion level in the source area is slight and light,and the erosion intensity gradually increases from west to east,which is consistent with the spatial variation characteristics of extreme precipitation.Especially in the southeast region,the 24 th and 28 th sub-basins and the source area have a high degree of erosion,which is a key prevention area for soil erosion.