Study On Future Extreme Climate Change In The Yellow River Basin And Its Impact On Mid-stream Flood Events

Taking the Yellow River basin as the research area,this paper uses the observation data of climatic factors in the same period of the basin history to evaluate the data of global climate model from the perspective of change trend,statistical characteristics and spatial distribution characteristics,and selects the most suitable global climate model for the Yellow River basin.The spatial and temporal distribution characteristics of extreme climate events in the future scenarios of RCP2.6,RCP4.5 and RCP8.5 in the Yellow River basin were estimated by the method of MME.Finally,taking jingle station of fen river in the middle reaches of the Yellow River as an example,a hydrological model based on land use/cover change was established to analyze the impact of land use change on flood events.In combination with extreme climate data of global climate model,extreme precipitation scenario was set to simulate future flood events in the basin.The main results are as follows:(1)From 1961 to 2005,the annual maximum and minimum temperatures in the Yellow River basin were14.58°Cand2.05°C respectively.The increase rates were 0.26°C /10 a and 0.28°C /10 a,respectively.In terms of spatial distribution,the highest temperature ranges from 25 °C to 45°C,and the lowest temperature ranges from-30°C to-20°C.The average annual precipitation is 471 mm,showing a decreasing trend and showing obvious differences in spatial distribution.The Daily bias-correction method was used to reduce the scale of the global climate model data,and then the observation data of climate factors in the same period of the basin history were used to evaluate the global climate model data from the change trend,statistical characteristics and spatial distribution characteristics,and the three most suitable global climate models of the Yellow River basin,finally selected GFDL,FGOALS and CCSM4 these three is most suited to the Yellow River basin of the global climate patterns.(2)In the future,the maximum temperature,the minimum temperature and the minimum temperature in the Yellow River basin are all on the rise,among which the temperature increase rate of RCP8.5 is the highest,while that of RCP2.6 is the lowest.In terms of extreme temperature,the highest daily temperature(TXx),warm day days(TX90),lowest daily temperature(TNn),cold night days(TN10)and frost days(FD0)were selected.In the three scenarios,TXx,TNn and TX90 showed an increasing trend,while TN10 and FD0 showed a decreasing trend.The change amplitude of RCP8.5 was greater than that of RCP4.5 and RCP2.6.Compared with 1961-2005,under RCR2.6 scenario,the maximum increase of TXx from 2055 to 2099 is 2.9°C,the maximum increase of TNn is 2.0°C,the average increase of TX90 is 21.8 days,and the average decrease of TN10 and FD0 is 14.2 and 10.6 days respectively.In the scenario of RCP4.5 and RCP8.5,FD0 decreased by 18.5 and 29.9 days,TN10 decreased by 22.2 and 29.2 days,TNn increased by 3.2°C and 4.2°C,TX90 by 43.8 and 63.5 days,and TXx by 3.6°C and 6.2°C.In terms of spatial distribution,in the future,TNn will mainly fall in the source area of the Yellow River,and rise in the upper lanzhou area and the loess plateau area in the middle reaches of the Yellow River,with little change in the lower reaches.TXx on the whole shows a rising state compared with the historical period,among which the loess plateau area in the middle reaches of the Yellow River has the largest rise,and the rise is more significant with the passage of time.TX90 shows a significant increasing trend,among which the heyuan area and the loess plateau area increase the most obvious;FD0 and TN10 showed a significant decline,which was also the largest in the source region of the Yellow River and the middle reaches of the loess plateau.(3)Daily maximum precipitation(RX1day),five-day maximum precipitation(RX5day),precipitation intensity(SDII),number of heavy rain days(R20),continuous wet days(CWD),extreme heavy precipitation(R99p),and total wet daily precipitation(PRCPTOT)were selected for the study.The total annual precipitation in the Yellow River basin fluctuates and increases.In the three scenarios,the extreme precipitation index shows an upward trend in time.For RX1 day,RX5day,R99 and PRCPTOT,the precipitation range under RCP8.5 is higher than that under RCP4.5 and RCP2.6 on the whole.For SDII,the precipitation intensity fluctuates greatly in the interannual variation.Before the 2050 s there is no significant difference in the precipitation intensity under the three scenarios.After the 2050 s,the precipitation intensity under the RCP8.5 scenario is higher than that under the other two scenarios.R20 and CWD will increase significantly in the future.In terms of spatial distribution,compared with 1961-2005,under RCP2.6,the variation range of RX1 day is-10mm?40mm,the variation range of R20 is 0mm?50mm,the variation range of PRCPTOT is 5mm?25mm,and the increase ratio of R99 is 0?0.4.The overall increase range of the late 21 st century is higher than that of the middle 21 st century.The increase range of RCP4.5 and RCP8.5 is more significant than that of RCP2.6,especially RCP8.5.In this scenario,the variation range of RX1 day is 0mm?45mm,the variation range of R20 is 10mm?160mm,the variation range of PRCPTOT is 5mm?65mm,and the increase ratio of R99 is 0?0.6.However,the whole spatial change rule is consistent in these three scenarios,which all show that the area with a relatively increased probability of extreme precipitation events and a large increase in the probability of heavy precipitation is in the middle reaches of the Yellow River.Finally,RX1 day and RX5 day in different recurrence periods were studied.RX1 day in 10 years was 27 mm,RX1day in 20 years was 30 mm,and RX5 day in 50 years was 33 mm.The amount of water that falls in future scenarios will increase.(4)The change of land use in the upper reaches of fen river increases the regulation and storage capacity of the basin,reduces the surface runoff of flood events,increases the midstream and subsurface runoff,and reduces the peak discharge.The model parameters determined under the land use conditions in 2010 were used to simulate the future flood in the basin by combining the extreme precipitation and evaporation data of the typical basin in the next 10 years under the scenario of RCP4.5.By setting up scenarios of different hourly extreme precipitation proportions,the future flood events in the basin under the two scenarios were analyzed.Compared with the historical flood,in scenario a,the flood process line shows a trend of steep rise and fall on the whole,and the overall flood peak flow rate drops by 37% and the flood volume drops by 13.3%.However,in scenario b,the flood peak of scenario a increased by 52%,and the flood volume increased by 31%.