Evolution Of Global Typical River Runoff Under Climate Change

River runoff is an important part of the terrestrial water cycle,and the terrestrial water cycle is an important component of the water cycle,it has multiple-directional feedback with water-sediment,water chemistry,and water ecological processes,its changes will also have a significant impact on global change.Under the combined influence of climate change and human activities,the terrestrial water cycle process and fluxes have changed,and extreme hydrological events such as droughts and floods have occurred frequently.At the same time,they have had a significant impact on water resources and induced a series of ecological and environmental effects,endangering the sustainable development of social economy and ecological environment.Identifying the temporal and spatial distribution regularities of typical river runoff under climate change can lay a foundation for the study of the evolution of terrestrial water cycle and its role in global change.It is also an important foundation and key support for dealing with the global climate change and solving water crisis.In this thesis,40 representative large rivers in the world are selected to analyze the time trends and spatial distribution of runoff coefficient and runoff discharges directed into the ocean.The runoff variation attribution analysis method based on the Budyko framework was improved,and the elastic coefficients and contribution rates of precipitation,potential evapotranspiration,underlying surface,glacial meltwater,and human activities to the changes in runoff were quantitatively calculated.Taking the Huang-Huai-Hai River Basin and the Yangtze River basin as typical areas,using cluster analysis and other methods to analyze the temporal and spatial changes in the water yield coefficient(WYC)and the water yield modulus(WYM).Factors such as precipitation,rainstorm days,rainless days,vegetation cover change and land use/cover change were considered to determine the causes of these changes.Based on the relationship between runoff coefficients and runoff and precipitation,spatial interpolation was performed using runoff coefficients in the 40 representative basin,and then global land surface runoff distributions were obtained based on precipitation data.The temporal and spatial distributions of runoff from continents,latitudes,and globally were analyzed.The main conclusions of this study are as follows:(1)The runoff in typical rivers tends to be in a downward trend.In North America and Africa,the runoff mainly shows a downward trend,and it mainly shows an upward trend in Europe.The runoff coefficient is relatively stable,and it tends to decline in a few basins where human activities have a major impact.Climate factors are the main influencing factors for the changes of runoff.Only about 25%of river runoff is affected by human activities.The runoff in the subtropical monsoon humid climate and the savannah climate zone tends to rise;in the tropical rainforest climate and the tropical monsoon climate zone,there is a downward trend;in the temperate monsoon climate,the temperate continental climate,and the temperate maritime climate zone,the changes are not significant.The runoff in the mid-high latitudes tends to increase,while the runoff in the areas with the most significant human activities in the mid-low latitudes tends to decrease,in these areas,the runoff coefficient also shows a downward trend.(2)The WYC of the Huang-Huai-Hai River basin varied slightly in time,with the western part showing a negative trend and the eastern part showing a positive trend.Spatially,the distribution is fairly regular,exhibiting a gradual increase from the northern part of the Yellow River Basin to the surrounding areas.The WYC is positively correlated with precipitation and rainstorm days and negatively correlated with rainless days.Although normalized difference vegetation index(NDVI)is a positive correlation with the change rate of WYC,the two escalating rates are a negative correlation.For the WYM of the Yangtze River basin,temporally,it showed an upward trend in the eastern part and a downward trend in the western part but not significantly.Spatially,with the exception of anomalies in the central and western regions,it gradually increases from north to south.The temporal and spatial changes in the WYM are closely related to precipitation.(3)In the past 50 years,global terrestrial runoff have shown an indistinctively upward trend,the frequency and amplitude of interannual changes have increased since the 1990s.Among the six continents,only Africa shows a significantly downward trend,while other continents have no significant changes.The high latitudes show a significant upward trend,while the mid-low latitudes show no significant trend.The segmentation trends of each continent and latitude zone are different.The runoff,precipitation,and evapotranspiration are low in the mid-low latitudes which located in central and northern and southwestern Asia.northern and southwestern North America,northern Africa,and Oceania.The runoff,precipitation,and evapotranspiration are relatively high in the mid-high latitudes which located in eastern Asia and Europe.The runoff,precipitation,and evapotranspiration are all high in the low latitudes on both sides of the equator on the northern South America and southeastern Asia.In parts of eastern North America,eastern South America,and central Africa,the precipitation and evapotranspiration are high,but the runoff is relatively low.