The Changes Of Future Runoff And Drought And Flood In The Yangtze River Basin Under Climate Change

In the context of increasing global climate change,the global water cycle has changed significantly,causing a series of resources,disasters and environmental problems.The Yangtze River Basin(YRB)is a strategic water source for China’s water resources allocation.The changes of water cycle and water resources in the YRB have a significant impact on China’s social and economic development.However,there is no consensus on the evolution trend of water resources in the YRB in the future.Therefore,this paper takes the YRB as an example.Firstly,this paper analyzes the variation characteristics of climate variables and runoff during the period of 1960-2014.Secondly,the future changes in precipitation and temperature under three SSP-RCP scenarios(SSP1-2.6,SSP2-4.5 and SSP5-8.5)are explored based on the Global climate models(GCMs)from the Coupled Model Intercomparison Project phase 6(CMIP6).Thirdly,the monthly runoff of the YRB in the middle of the 21st century(2045-2064)under the influence of climate change and land use change is predicted by coupling GCMs,Future Land-Use Simulation(FLUS)model,and Soil and Water Assessment Tool(SWAT),and its change compared with the baseline period(1995-2014)is analyzed.Finally,the changes of drought and flood are evaluated based on the above research results.This paper can provide decision support for the adaptive management of water resources and the prevention and control of drought and flood disasters in the Yangtze River Basin under global change.The followings are main works and conclusions of this study:(1)The evolution characteristics of climate and runoff over the YRB during the period of 1960-2014 were systematically expounded using TFPW-MK approach,Pettitt’s Test and EEMD decomposition.The annual precipitation and average daily maximum temperature in the YRB as a whole did not change significantly,while the average daily minimum temperature increased significantly at the rate of 0.11°C/decade.Among the eight hydrological stations in the YRB,only the annual runoff of Wanxian station and Yichang station changed significantly,with the rates of-8.93×10~8m~3/a and-7.44×10~8m~3/a respectively.The trend results of seasonal runoff show that the spring and summer runoff of the eight stations mostly show insignificant trend changes,the autumn runoff mostly show a significant decreasing trend,while the winter runoff all show a significant increasing trend.The mutation test results based on Pettitt’s test show that there is no significant pattern change in the YRB,except for the significant mutation of annual runoff of Wanxian station in 2005.The results based on EEMD decomposition show that interannual oscillation has dominated the long-term variation of runoff in the YRB.(2)The projected changes in precipitation and temperature over the YRB under the three SSP-RCP scenarios were explored based on 23 GCMs from the CMIP6.The bias-correction results based on empirical quantile mapping(EQM)show that this method can effectively correct the bias in the GCM,and the simulation performance of each GCM has been improved after correction.The performance of the multi-model ensemble(MME)is proved to be better than that of any single GCM.Hence,future changes in precipitation and temperature over the YRB were explored based on MME.The prediction results show that the precipitation,daily maximum temperature and daily minimum temperature in the YRB show a significant increase trend from 2025 to2100,and the increase rate increases with the increase of emission concentration.Under the same scenario,the warming rate of daily maximum temperature is expected to be higher than that of daily minimum temperature.By the end of the 21st century(2081-2100),the area-averaged precipitation over the YRB is projected to increase by 10.41%,10.66%,and 15.80%,under SSP1-2.6,SSP2-4.5,and SSP5-8.5,respectively,with reference to the baseline period.And the daily maximum(minimum)temperature under different scenarios is expected to rise by 1.75(1.50)°C(SSP1-2.6),2.72(2.54)°C(SSP2-4.5)and 5.04(4.85)°C(SSP5-8.5),respectively.(3)The runoff change in the YRB under the influence of climate change and land use change were predicted,by coupling GCMs,FLUS model and SWAT model.The performance of SWAT model is evaluated in the monthly scale using multiple indicators(R~2,NSE,PBIAS,etc.),which proves that the constructed SWAT model in this paper can be used for runoff prediction for YRB.Then,8 GCMs with reliable runoff simulation accuracy in the monthly scale are selected from the 23 bias-corrected GCMs,based on the constructed SWAT model.The land use distribution over the Yangtze River Basin in 2050 under three SSP-RCP scenarios is simulated based on FLUS model,after the applicability of the model is proved.Based on the constructed SWAT model,the bias-correction results of 8 GCMs,and the land use simulation results under different scenarios,this paper predicts the monthly runoff of the YRB in the middle of the 21st century under the influence of climate and land use changes,and compares it with the reference period.The results show that under the different SSP-RCP scenarios,the annual runoff of each hydrological station in the YRB will increase,while the change directions of the monthly runoff are different.The month with the largest change in the proportion of monthly runoff is concentrated in June to October,which is mainly reflected in the decrease of the proportion of runoff from June to July and the increase of the proportion of runoff from August to October.Q95(high flow)of each hydrological station will increase.Under SSP1-2.6,Q5(low flow)of each hydrological station will increase.Under the scenarios of SSP2-4.5 and SSP5-8.5,Q5 of each hydrological station in the upper reaches of the YRB will decrease,while Q5 of each hydrological station in the middle and lower reaches of the YRB will increase.(4)The changes of hydrological drought over the YRB under the SSP-RCP scenarios were comprehensively evaluated from the aspect of the variation trend of standardized runoff index(SRI),drought characteristics and frequency.The trends results of seasonal and annual scale SRI in 145 subbasins of the YRB show that both seasonal and annual SRI are dominated by an insignificant wetting trend,under the three SSP-RCP scenarios.In terms of drought characteristics,both the drought duration and drought severity of the YRB as a whole will decrease in the middle of the 21st century compared with the reference period.The drought intensity under different scenarios is projected to rise slightly compared with the reference period.The longest drought duration will be reduced,and the decrease is large in both SSP2-4.5 and SSP5-8.5 scenarios.Under SSP1-2.6,the maximum drought severity during 2045-2064 will be slightly higher than that in the reference period,while the maximum drought severity under the other two scenarios will be significantly lower than that in the reference period.In terms of drought frequency,the future drought frequency in the YRB under different scenarios is slightly higher than that in the reference period,and the increase range increases with the increase of greenhouse gas concentration.(5)The changes of flood disaster risk during 2045-2064 relative to the baseline period,under different scenarios were explored based on the bias-corrected results of GCMs and the runoff prediction results.The results show that the flood disaster risk index in most areas of the YRB in 2045-2064 under each scenario is higher than that in the baseline period,especially in Xiaogan of Hubei Province,Neijiang,Daocheng County and Tibetan Autonomous County of Muli in Sichuan Province.The higher-risk areas and the highest-risk areas in the middle of the 21st century will further expand outward on the basis of the baseline period.Overall,the increase range of highest-risk areas is the largest under SSP2-4.5,reaching 35.16%,and the increase range of higher-risk areas increases with the increase of greenhouse gas concentration.The above results indicate that the risk of flood disaster in the YRB will increase in the middle of the 21st century.It is necessary to make relevant defense preparations for the dangerous areas,and pay more attention to areas with a large increase in the risk index of flood disaster at the same time.