Evolution Characteristics And Driving Mechanism Of Generalized Water Resources In The Headwaters Of The Yangtze River Under Climate Change

The response mechanism of watershed water resources under the changing environment has become a hot and cutting-edge issue of global change research.The changing environment can be summarized into two categories: climate change and human activities.To some extent,climate change and human activities have changed the water balance and the state of water resources storage and allocation in time and space.The traditional water resource planning pays more attention to the surface water resources(blue water resources)of physical storage,while the potential green water resources in the transmission channel of water cycle system in river basins are usually ignored.Therefore,from the perspective of blue water green water composite system,it is of great theoretical and practical significance to identify the temporal and spatial evolution of blue water green water resources,and to quantitatively separate the impact degree and contribution rate of climate change and human activities on blue water green water resources,so as to promote the comprehensive management of blue water green water and sustainable development of water resources in river basins.In this paper,the headwaters of the Yangtze River(YRHA)is taken as the research area,and the evolution characteristics of meteorological and hydrological elements and underlying surface in the YRHA are identified by using time series analysis technology and spatial analysis technology.Based on SWAT hydrological model,this paper simulates and evaluates the temporal and spatial evolution characteristics of blue water and green water resources in the YRHA,quantitatively analyzes the impact degree and contribution rate of climate change and human activities on blue water and green water resources,and the Choudhury-Yang hydrothermal coupling model based on Budyko hypothesis is used to verify the attribution results.The main research results of this paper are as follows:(1)From 1956 to 2016,precipitation,temperature,potential evaporation and runoff all showed an obvious increasing trend;from the perspective of spatial distribution,precipitation,temperature and potential evaporation all showed a decreasing spatial distribution feature from south to north in general;the change point detection results showed that 2007 was the change point of precipitation,and 1997 was the most likely change point of temperature It can be seen that the potential evaporationhas changed around the 1990 s,and the change point of runoff series occurred in 2002.From 1990 to 2014,the change of land use type in the YRHA was more obvious,the area of change accounted for 39.99% of the total area,and the change area was mainly concentrated in the northwest.(2)The blue water(BW)in the YRHA shows an increasing trend.Before 1994,the change of the BW was relatively stable and in the state of annual fluctuation.Since1994,it has a relatively obvious upward trend.The spatial distribution of the BW is relatively uneven,and the distribution pattern is more abundant in the south and the east,but less in the northwest.The green water flow(GWF)showed a significant increase trend.Before 1990,it was in the state of interannual fluctuation,basically fluctuating near the multi-year average value,but after 1990,it showed a more obvious upward trend;the spatial distribution of GWF was more abundant in the east and south than in the north and middle.There is no obvious trend change in green water storage(GWS).The change of GWS is relatively stable,in the state of annual fluctuation,basically fluctuating near the multi-year average value.The distribution pattern of GWS also shows the characteristics of more in the south and east.(3)From 1960 to 2016,climate change was the main factor that caused the increase of BW,and the impact of human activities and other coupling factors on BW was also obvious,with the contribution rate of 70.86%,23.4% and 5.74% respectively;Among them,climate change only caused the decrease of BW in the southeast,human activities reduced BW in the southeast,and other coupling factors increased the BW in the west and east,but contributed less.Climate change and human activities increased the GWF,contributing 81.92% and 24.57% respectively,while other coupling factors reduced the GWF,contributing 6.49%,in which climate change increases the GWF in the YRHA,while human activities increase the GWF in most areas,and other coupling factors only increase the GWF in the southeast part of the YRHA,and decrease the GWF in the rest areas.The change of GWS is relatively small.The impact of climate change and human activities on the GWS in the YRHA is to increase the green water storage,while other coupling factors reduce the green water storage,which is mainly affected by climate change.Among them,the spatial distribution of the results of climate change and human activities on the GWS in the YRHA is relatively consistent,that is,the GWS increases in the south of the source area,decreases in most areas in themiddle and northwest of the source area,and other coupling factors increase the GWS in the northwest and part of the central area.(4)By comparing the Nash-Sutcliffe coefficient and correlation coefficient of the hydrothermal coupling equilibrium model and the SWAT model method,it can be seen that the SWAT model method is better than the hydrothermal coupling equilibrium model method,but the simulation results of the two methods are relatively close,which can be mutually verified.The results of runoff attribution obtained by the two methods are relatively consistent,indicating that climate change is the leading factor of runoff change in the YRHA in the past 60 years,the change of underlying surface also play an important auxiliary role,and other coupling factors have obvious impact on the increase of runoff,which can not be completely ignored.