Runoff Patterns Change And Its Responses To Climate Change And Human Activity In The Qinling-Bashan Mountains Region

Water resources are indispensable strategic resources for human survival and development.River runoff is an important form of water resources,which can reflect the abundance and shortage of regional water resources to a certain extent.The impact of climate change on the global water cycle promotes the runoff variation process,and human activities alter the change trend of runoff through land use management and other behaviors.The Qinling-Bashan Mountains Region is the transition zone between Northern and Southern China and is also the water source area of the Middle Route of the South-to-North Water Transfer Project.It is of great significance to analyze the variation of riverine runoff patterns and the response to climate and human activity in the region.In view of this,this thesis takes the Qinling-Bashan Mountains Region as the study area,based on the meteorological,hydrological,land use and GIMMS NDVI data,using multiple statistical methods such as trend analysis,mutation analysis and phase analysis,the temporal and spatial variation of precipitation characteristic indices,potential evapotranspiration,land use/cover and runoff at different timescales in the Qinling-Bashan Mountains Region are comprehensively analyzed.Six typical watersheds in the Qinling-Bashan Mountains Region are selected from west to East,namely the Minjiang watershed(the section controlled by the Zhenjiangguan station),the Xihanshui watershed,the Hanjiang watershed(the section controlled by the Hanzhong station),the Xunhe watershed,the Danjiang watershed and the Duhe watershed.Soil and Water Assessment Tool(SWAT)models which are suitable for typical watersheds are constructed.Combined with the linear regression,double cumulative curve,natural runoff reduction method and elastic coefficient method based on the Budyko hypothesis,the contributions of climate change and human activity to runoff change in the typical watersheds are quantitatively estimated and the applicability of these selected methods was evaluated.The spatial difference of response characteristics of regional runoff to driving factors is analyzed.Finally,the statistical downscaling model called the SDSM model is constructed.The generated daily climate element sequences from 2031 to 2100 under the three future Representative Concentration Pathway(RCP)scenarios,RCP 2.6,RCP4.5 and RCP8.5,of the second-generation Canadian Earth System Model(Can ESM2)by the SDSM model are input into the SWAT model to predict the change of future runoff.The specific results and conclusions are obtained as follows:(1)The variation patterns of precipitation characteristic indices,potential evapotranspiration and land use/cover in the Qinling-Bashan Mountains Region at different time scales are revealed.Rainfall,evapotranspiration and surface cover are momentous factors affecting riverine runoff variations in the region.The annual potential evapotranspiration in the Qinling-Bashan Mountains Region decreased significantly at the rate of 4.9 mm/10a from 1960 to 2019.Among the precipitation characteristic indices,such as precipitation,continuous dry days(CDD),continuous wet days(CWD),moderate rain days(R10),heavy rain days(R25),daily maximum rainfall(RX1day)and rainfall intensity(SDII),only SDII showed a significant upward trend(p<0.05).Except for CDD and CWD,the multi-year average maximum monthly values of the other indices appear in July,while April and June are the months with more significant inter-annual changes in these climatic indicators.The overall performance of the Qinling-Bashan Mountains Region is that the south is wet and the north is dry,the east is wet and the west is dry,and the east and northwest corner are more prone to flood and drought disasters,respectively.The main land use types in the region are forest land,grassland and cultivated land.Forest land is mainly distributed in the east,and the distribution areas of cultivated land and construction land basically coincide.From 1980 to 2020,the area of regional cultivated land and grassland decreased by 2108 km~2 and 2047 km~2,respectively,and the area of the other land use types increased to various degrees,among which the growth rate of construction land was the largest(104.9%).The vegetation coverage in the Qinling-Bashan Mountains Region has been significantly improved at the original high level,especially in the low coverage area.The multi-year average NDVI value shows the general distribution characteristics of low around,high in the middle,low in the northwest and northeast,and high in the middle and south.The regional annual NDVI value increased from 0.79in 1994 to 0.86 in 2015,with a significant increase rate of 0.09%/a between 1982 and2015.Except for September,the NDVI of other months showed an increasing trend,and the increase rate of NDVI in April was the largest(0.5%/a,p<0.01).(2)The temporal and spatial variation characteristics of the observed annual runoff of the main rivers in the Qinling-Bashan Mountains Region are expounded.The annual average runoffs from 1960 to 2018 of the Minjiang River,the Jialingjiang River and the Hanjiang River,the primary tributaries of the Yangtze River,are 128.58×10~8 m~3,181.13×10~8 m~3and 342.37×10~8 m~3,respectively,which are much greater than that of the Taohe River,the Weihe River and the Yiluohe River,the primary tributaries of the Yellow River,with the value being 44.76×10~8 m~3,36.76×10~8 m~3and 22.78×10~8 m~3,respectively.The observed annual runoff of the Taohe River,the Weihe River and the Yiluohe River exhibited significant downward trends(p<0.01),with the change rates of-0.33×10~8 m~3,-0.33×10~8 m~3and-0.67×10~8 m~3/a,and the annual runoff of the Minjiang River,the Jialingjiang River and the Hanjiang River decreased significantly(p<0.05)at the rates of-0.47×10~8 m~3/a,-1.46×10~8 m~3/a and-2.31×10~8 m~3/a,respectively.Except that the annual runoff at the Xianyang hydrological station of the Weihe River changed significantly in 1993(p<0.01),the significant change points of annual runoff at the other hydrological stations of the other main rivers were all in 1990.Compared with the respective reference period(the period before the significant abrupt change year),the reduction rates of the annual average runoff of main rivers in the change period(the period after the significant abrupt change year)are between 12.52%(the Pengshan station of the Minjiang River)and 50.43%(the Xianyang station of the Weihe River),and the year when the annual runoff of the river enters the dry season is consistent with the respective significant abrupt change year.The inter-annual fluctuation of the annual runoff at the Hongqi station in the Taohe River is smaller than that of the Xianyang station in the Weihe River and the Heishiguan station in the Yiluohe River,while among the three primary tributaries of the Yangtze River,the inter-annual fluctuation of the annual runoff at the Huangjiagang station of the Hanjiang River is greater than that of the Tingzikou station of the Jialing River and the Pengshan station of the Minjiang River.The average annual runoff at the Huangjiagang station of the Hanjiang River and the Tingzikou station of the Jialingjiang River are greater than that of the Pengshan station of the Minjiang River,whereas the average annual runoff depth(408 mm)of the drainage area controlled by the Pengshan station from 1960 to2018 and the decadal runoff depths except for the 1980s are the largest.(3)The intra-annual distributions of observed runoff and change trends in season and annual runoff of the hydrological stations at the mainstream and tributaries in main rivers over the Qinling-Bashan Mountains Region are not completely consistent.The distributions of annual runoff depth also vary in different sections.Compared with the period of 1960-1986,the monthly runoff in 12 months at the hydrological stations in the mainstream of the Taohe River,the Weihe River and the Yiluohe River reduced to a certain degree from 2007 to 2018.Among the Minjiang River,the Jialingjiang River and the Hanjiang River,months with increased monthly runoff are concentrated in December,January,February and March.The average runoff at all mainstream and tributary stations of major rivers in September decreased by 3.12%～53%.The annual runoff variation trends of some mainstream and tributary stations of different rivers from 2007 to 2018 are opposite to that from 1960 to 1986.The spatial distribution of average annual runoff depth in different periods in the main rivers is similar to that of the regional precipitation,with the patterns of high in the south and low in the north.The average annual runoff depth from 2007 to 2018 in most sections decreased by 4mm(the section above the Dazhuhe station in the Hanjiang River)～694 mm(the section between the Pingwu station and the Jiangyou station in the Jialingjiang River)compared with that in the period of 1960-1986.(4)The effects of climate and human activity on runoff variation are quantitatively evaluated.Affected by climate change and human activities,the runoff in the Qinling-Bashan Mountains Region shows a downward trend as a whole,but there are spatial differences in the changing trends and the responses of runoff to the influencing factors.The runoff in the middle of the Qinling-Bashan Mountains Region showed a non-significant decreasing trend from 1960 to 2016.The annual runoff of the Duhe watershed in the southeast of the region experienced a significant decreasing trend(p<0.05),whereas the runoffs of the Minjiang watershed(the section controlled by the Zhenjiangguan station)in the southwest,the Xihanshui watershed in the northwest and the Danjiang watershed in the northeast exhibited an extremely significant decreasing trend(p<0.01).SWAT models suitable for six typical watersheds in the Qinling-Bashan Mountains Region are constructed respectively,and the simulation results meet the accuracy requirements.The application effects of different methods in the attribution of runoff change are evaluated.The results show that the four methods,including simple linear regression,double cumulative curve,elasticity coefficient method based on the Budyko hypothesis and hydrological simulation method based on the SWAT model,all produced relatively consistent estimates when applied to the watershed with significant runoff change,while the application effect of natural runoff restoration method is comparatively poor.Compared with the reference period of each watershed,the runoff variation of the the middle of the region is mainly affected by climate change,whereas human activity is the dominant factor in runoff reduction in the Minjiang watershed(the section controlled by the Zhenjiangguan station),the Xihanshui watershed,the Danjiang watershed and the Duhe watershed,with the contributions of 72.78%～93.04%,56.45%～75.03%,54.85%～93.22%and 88.18%～96.89%,respectively.(5)The future climate and runoff changes of the Xunhe watershed whose runoff is mainly affected by climate change in the Qinling-Bashan Mountains Region are predicted.The effect of simulating the highest and lowest temperatures by the SDSM model is better than that of precipitation.The simulation results demonstrated that annual precipitation,maximum temperature and minimum temperature of the Xunhe watershed from 2031 to 2100 will increase under RCP2.6,RCP4.5 and RCP 8.5scenarios of Can ESM2 mode.Compared with the period of 1960-2016,the average annual precipitation of the watershed in the next 70 years will be reduced by 3%under the RCP2.6 Scenario,while under RCP4.5 and RCP 8.5 scenarios,the precipitation will increase by 5.6%and 11.9%,respectively.The average annual maximum temperature and minimum temperature under the three emission scenarios will increase by1.0℃～2.9℃and 1.2℃～3.0℃,respectively.The quantitative prediction results of the watershed SWAT model show that the annual runoff of the Xunhe watershed under the three scenarios of the Can ESM2 mode express increasing trends in the next 70 years.Compared with 1960-2016,the average annual runoff of the Xunhe watershed from2031 to 2100 will increase by 10.3%～50.1%.In conclusion,this study scientificly understands the climatic and surface cover changes’background of runoff formation and change in the Qinling-Bashan Mountains Region,wihch is helpful to determine the distribution and change characteristics of regional runoff under changing environment.The application effects of various quantitative methods are compared,and the quantitative impact of climate change and human activities on regional runoff change is clarified,which provides a reference for selecting appropriate and effective attribution analysis methods of regional runoff change,and is conducive to identifying the difference in runoff responses,so as to formulate scientific local adaptive policies.The runoff change under the future climate change scenario is predicted,which has important guiding significance for the rational planning,management and allocation of regional water resources.