Variations Of Runoff And Sediment And Their Responses To Climate Change And Human Activities In The Yellow River

The Yellow River is the mother river of the Chinese nation,and the ecological protection and high-quality development of the Yellow River Basin has been elevated to a national strategy.On 22 October 2021,President Xi Jinping stressed at the Symposium on Deepening the Ecological Protection and High-Quality Development of the Yellow River Basin,held in Jinan,that "we should scientifically analyze the current situation of ecological protection and high-quality development of the Yellow River Basin,and grasp the major issues of promoting ecological protection and high-quality development of the Yellow River Basin.We should bite the target,be practical,work hard and make long-term efforts to ensure that the ecological protection and highquality development of the Yellow River basin during the 14 th Five-Year Plan period will achieve obvious results,and make unremitting efforts for the Yellow River to benefit the Chinese nation forever".The President Xi's speech highlighted the importance of the long-term stability of the Yellow River basin for the well-being of the people,and pointed to the need for in-depth research on the long-term evolution of the relationship between runoff and sediment in the Yellow River.Therefore,this paper,in the context of the urgent needs of current national strategies,investigated the changes in the runoff and sediment of the Yellow River and their responses to climate change and human activities,which is of great theoretical guidance for optimising the allocation of water resources in the Yellow River basin,coordinating the relationship between people and land,runoff and sediment transfer,flood prevention and mitigation.Based on the long time series(1950-2019a)of measured runoff and sediment data from representative hydrological stations in the upper,middle and lower reaches of the Yellow River,the variation coefficient method,sliding average method,cumulative distance level method,Mann-Kendall test method and wavelet analysis are used to explore the trend,abrupt change,stage and periodicity of the Yellow River basin,respectively.The variability of runoff and sediment in the Yellow River basin was investigated in terms of trends,abrupt changes,phases and cycles,and the differences in the variability of runoff and sediment between the upper,middle and lower reaches of the Yellow River basin were clarified.The contribution of precipitation and land use to runoff and sediment in the upper,middle and lower reaches of the Yellow River and the whole river basin was quantitatively assessed.The dynamic evolution of land use in the Yellow River basin was analysed from the perspectives of the extent of land use,the rate of land use change and the direction of land use transfer.The parameters of SCS runoff curve(CN2),main channel hydraulic conductivity(CH-K2),soil saturation hydraulic conductivity(SOL-BD),groundwater delay factor(GW-DELAY)and effective surface soil water content(SOL-AWC),which have a large influence on runoff,and groundwater sediment concentration(LAT-SED),linear sediment transport coefficient(SPCON),average sediment transport coefficient(SPCON),were selected to have a large influence on sediment.A total of 27 parameters were used to rate and validate the results of the SWAT model simulations.A global sensitivity analysis was conducted to compare the P-value and the statistic T for each parameter,and based on the principle that the smaller the P-value and the larger the statistic T,the higher the sensitivity of the model parameters,the parameters of the SWAT model for the Yellow River Basin were optimised to better match the simulated values with the measured data.precipitation data under the climate model,combined with the optimised SWAT model,the trends of runoff and sediment in the Yellow River under different scenarios were analysed.After the above series of studies,the following main conclusions were obtained.(1)The trend of runoff and sediment in the upper reaches of the Yellow River is relatively stable,while the runoff and sediment in the middle and lower reaches show a significant decline.In the upper reaches of the Yellow River,the trend of runoff and sediment at the Tangnahai hydrological station is stable,while the trend of runoff at the Lanzhou hydrological station is stable and the sediment has decreased significantly.The rate of change in runoff and sediment at the Middle reaches of the Yellow River decreased by 2.53% and 15.11%,and the rate of change in sediment decreased by 42.27%and 80.21% respectively,except for the Tangnaihai hydrological station,where the trend of runoff and sediment was stable.The rate of change in runoff at the lower reaches of the Yellow River decreased by 17.34% and 32.55% respectively,and the rate of change in sediment decreased by 85.86% and 80.67% respectively.This indicated that the runoff in the source area of the Yellow River is less disturbed by the outside world and maintains the relationship between precipitation and runoff in its natural state,with the change in runoff mainly influenced by precipitation.The runoff and sediment in the middle and lower reaches of the Yellow River have been declining in recent years due to the disturbance of human factors,such as the influence of policies to divert runoff and sediment.(2)The Yellow River is currently at the stage of dry runoff and less sediment.The runoff stages at the Tangnahai and Lijin hydrological stations are divided into the abundant runoff stage(1951-1975a),the flat runoff stage(1976-1984a)and the dry runoff stage(1985-2019a).The runoff stages at Lanzhou,Toudaoquan,Tongguan and Huankou hydrological stations are divided into the abundant runoff period(1951-1967a),the flat runoff period(1968-1984a)and the dry runoff period(1985-2019a).1985 a is a turning point in the change of the runoff of the Yellow River,which is mainly related to the construction of water conservancy.2000 a is a turning point in the change of sediment in the Yellow River,which is mainly related to the implementation of the project of returning farmland to forest and grass;the runoff is more obvious on the scale of 12-29 a,and the extreme value of wavelet variance of runoff is most significant at the scale of about 29 a.The sediment is more obvious on the scale of 5-30 a,and the extreme value of wavelet variance of mud flow is most significant at the scale of about 17 a.(3)The timing of abrupt changes in runoff and sediment varies between the upper,middle and lower reaches of the Yellow River,but the timing of abrupt changes in different river sections remains largely consistent.The abrupt changes in runoff at the Tang Naihai and Lanzhou hydrological stations in the upper reaches of the Yellow River Basin are 1993 a and 1981 a,respectively;the abrupt changes in runoff at the Toudaoguai and Tongguan hydrological stations in the middle reaches of the Yellow River Basin are 1985a;the abrupt changes in runoff at the Huayuankou and Lijin hydrological stations in the lower reaches of the Yellow River Basin are 2002 a.This is due to the fact that the Yellow River basin is affected by different climatic conditions,substratum and human activities,resulting in different mutation times.(4)Human activities are the most important factor influencing changes in runoff and sediment in the Yellow River basin.The changes in runoff in the upper reaches of the Yellow River are mainly influenced by precipitation,while the changes in runoff in the middle reaches,lower reaches and the whole basin of the Yellow River are mainly influenced by human activities;the changes in sediment in the upper reaches,middle reaches,lower reaches and the whole basin of the Yellow River are mainly influenced by human activities.from 2000 to 2019,the proportion of human activities influencing both runoff and sediment in the Yellow River basin was 83%,indicating that human activities are still the most important factor influencing the changes in runoff and sediment in the Yellow River at this stage.(5)Land use types in the Yellow River Basin are mainly arable land,forest land and grassland,with the sum of the areas of the three being close to 90% of the entire basin;the direction of land use types turning out in the Yellow River Basin is mainly arable land and grassland,with unused land changing the most significantly and decreasing the most in the last 40 years,with a conversion rate of 53.89%.It showed that the utilization rate of unused land is high,the exploitation capacity of unused land is strong and the intensity of human activities is intense.(6)Under the extremely land use scenario,the land use type with the greatest impact on runoff in the Yellow River Basin is arable land,followed by grassland and finally forest land.Based on the CA-Markov model land use scenario,the reduction in runoff and sediment is smaller under the ecological conservation priority scenario.Under the economic development priority scenario,the largest reductions in runoff and sediment are observed.Under the climate change scenarios,runoff and sediment at the Lijin hydrological station under the Cmip5 climate model RCP6.0 and the Cmip6 climate model SSP1-2.6 scenarios showed the least change,with runoff and sediment increasing by 3.86% and 5.45% for RCP6.0 and 3.81% and 5.12% for SSP1-2.6,respectively.The largest changes in runoff and sediment were observed at the Lijin hydrological station under the Cmip5 climate model RCP8.5 and the Cmip6 climate model SSP5-8.5,with runoff and sediment increasing by 8.04% and 10.97% for RCP8.5,and by 9.19% and 12.35% for SSP5-8.5,respectively.The scenario with the SSP1-2.6climate model combined with ecological land use protection showed the least reduction in runoff and sediment,while the scenario with the RCP8.5 climate model combined with economic land use development showed the greatest reduction in runoff and sediment.At the same time sediment changes more dramatically than runoff.