Impacts Of Changing Environment On Blue And Green Water Resources Of The Yalong River Basin

Water resources assessment and research on water management in a changing environment,including natural and abnormal climate and land use changes,are essential for addressing water security issues.Traditional water resources assessment mainly focuses on visible water(BW)and overlooks invisible water(GW).GW plays a crucial role in supporting the functioning of terrestrial ecosystems such as grasslands,forests,and agricultural systems.Incorporating GW into water resource evaluation is of great theoretical,technical,and practical significance in addressing water scarcity and ensuring scientific allocation.BW and GW represent important aspects of the comprehensive impact of climate and land use changes,making them a focal point in global change research.Therefore,a quantitative assessment of the influence of land use and climate change on BW and GW is vital for water security management.The changes in BW and GW are complex and exhibit significant scale effects.Exploring the evolution patterns and influencing factors of BW and GW at the basin scale,particularly in data-scarce areas,is extremely necessary.Such an exploration can provide an important scientific basis for guiding decisions on basin water security management.The YLRB situated in the upper reaches of the Yangtze River,is an ecologically fragile area and a crucial ecological barrier and water source conservation zone.It serves as a significant water source for national water networks like the West Route of the South-to-North Water Diversion Project and regional water network projects such as the Yalong River to Yanyuan and Yalong River to Anning River water diversion projects.Moreover,it is a vital clean energy base for wind power and hydropower generation,contributing to the country’s achievement of the"dual carbon"goal.Although the YLRB is rich in water resources,hydrological and meteorological data are lacking in this area.Climate and land use changes have caused substantial alterations in BW and GW resources over time,leading to increased aridity,grassland degradation,wetland shrinkage,weakened forest function,and declining ecosystem services.Understanding the spatial and temporal distribution and change characteristics of BW and GW in the context of climate and land use changes in both historical and future periods is crucial for the scientific management and sustainable utilization of water resources in the basin.Therefore,this study conducts a quantitative analysis of the spatiotemporal changes in BW and GW in the YLRB from 1980 to 2020 using the SWAT model.The simulation results of the SWAT model are evaluated based on three parameters:R~2,Ens,and PBIAS.Additionally,by applying the theory of seasonal division of climate element fields,the study obtains the seasonal change trends in each sub-basin and the intensity of future changes.The relative change rates of BW and GW in 6 typical years are calculated based on the annual extreme values of rainfall,temperature,and vapor pressure difference.Using the MME method,the study calculates the changes in daily rainfall,daily maximum temperature,and daily minimum temperature for 15 meteorological stations under three radiative forcing scenarios:SSP1-2.6,SSP2-4.5,and SSP5-8.5,from 2021 to 2060.Furthermore,the study incorporates global land cover data for every 10 years from 2020 to2050 under the three scenarios to simulate the corresponding changes in BW and GW.By establishing 169 climate change scenarios,the response and sensitivity of BW and GW to rainfall and temperature are analyzed.The study also examines the main transformation trends in land use types between 1980 and 2020,calculates the volume of BW and GW under different land uses,and develops nine land use change scenarios resulting from the mutual transformation of grassland,forest land,and cultivated land.(1)The SWAT model demonstrates good applicability in the YLRB.It generated 1124HRUs and 24 sub-basins.The SWAT-CUP was used to calibrate and validate the runoff.The simulation results were reliable,as indicated by an R~2greater than 0.6,an NSE greater than 0.6,and an error less than±15%at the Ganzi,Zhuba,Xiaodeshi,and Tongzilin stations.(2)The study clarified the spatiotemporal distribution characteristics and trends of BW and GW in the historical period of the YLRB.Between 1980 and 2020,the annual average BW and GW amounts were 449.5mm and 429.0mm,respectively.BW amounts fluctuated irregularly,while GW showed a slight upward trend.Both BW and GW exhibited similar spatial distribution characteristics,increasing from the northwest to the southeast.In different typical years,BW volume increased the most during rainy years and decreased the most during less rainy years.Conversely,it increased the most during dry years and decreased the most during rainy years.(3)The study quantified the contribution of climate and land use changes to BW and GW.By analyzing the distribution of BW and GW in the periods of 1980-1999 and2000-2020,it was found that climate change played a dominant role in BW,accounting for95.33%,while the impact of land use change on BW was only 4.67%.Rainfall primarily occurred from June to October,while BW and GW were mainly distributed from July to September.(4)Explored of the seasonal variation characteristics and patterns of BW and GW.Based on the"seasonal division theory of climate element field,"the four seasons in the YLRB were roughly divided into spring(April-May),summer(June-August),autumn(September-October),and winter(November-March).The highest distribution of BW and GW occurred in summer,while the lowest occurred in winter.Under the temperature field,the four seasons were divided into spring(April),summer(May-October),autumn(November),and winter(December-March).The highest distribution of BW occurred in summer and the least in spring,while the highest distribution of GW occurred in summer and the least in autumn.The decrease in BW and the increase in GW mainly occurred downstream during summer.The downward trend of BW would continue,but the increasing trend of GW would be strengthened.(5)The MME model effectively simulated rainfall and temperature in the YLRB,using the MME model to effectively simulate rainfall and temperature in the YLRB under SSP1-2.6,SSP2-4.5,and SSP5-8.5 conditions.Most meteorological stations showed a weak upward trend in rainfall,while the minimum and maximum temperatures exhibited a significant upward trend.The frequency of extreme weather increased,resulting in a higher number of days with high and extreme temperatures.This led to an increase in GW and a decrease in BW in the study area.(6)Explored the response patterns and characteristics of BW and GW to rainfall and temperature.According to the results of climate simulation scenarios,there is a positive correlation between changes in BW and GW and rainfall,a negative correlation between changes in BW and temperature,and a positive correlation between changes in GW and temperature;When rainfall decreases more,the increase in temperature leads to an increase in the amount of BW content,but the increase is small.The decrease in temperature leads to a decrease in the amount of change in BW content;As rainfall increases,the increase in temperature leads to an increase in changes in GW,while a decrease in temperature leads to a decrease in changes in GW.(7)The study clarified the impact of land use change on BW and GW by comparing land use data in the YLRB from 1980 to 2020.The findings revealed a decrease in cultivated land,water area,and bare land,while the area of forest land,grassland,and construction land increased.Practices such as rice no-tillage were found to increase GW and decrease BW.Conversely,rotation fallowthe,conversion of farmland to forests and afforestation resulted in a decrease in BW and an increase in GW.(8)The BW and GW management measures in the upper,middle and lower reaches of the Yalong River basin are proposed.The important ecological functional areas in the upper reaches of the Yalong River should actively promote the system of glacier,permafrost,forest,river,lake,wetland recuperation and restoration,as well as returning farmland to forests;Accelerate the implementation of the Yalong River to Yanyuan water diversion project in the middle reaches of the river basin;Develop crop cultivation plans suitable for various agricultural production areas downstream of the watershed while constructing high standard farmland.