Response Of The Water Body To Changing Climate And Ecological Environment In The Qinghai Lake Watershed

Lakes are an important component of terrestrial water resources and one of the sensitive indicators of local climate and global environmental changes.As a lake water body,the changes in lake level,area and water volume directly reflect the process of lake changes and water volume balance changes in the basin,and are more sensitive to regional and global climate change.Qinghai Lake is located in the Northeastern Qinghai-Tibet Plateau,in the convergence zone of the East Asian monsoon,the Indian monsoon and the westerly,and the transitional zone of semi-humid,semi-arid,and arid zone.The changes in the lake water and the evolution of the surrounding environment are indicators and regulators of climate change on the Qinghai-Tibet Plateau.However,the previous studies mainly focused on the short-term qualitative research on the dynamic changes of water area,lakeshore morphology,climate change and ecological environment,etc.The long-term research was relatively less affected by data,methods,which focus on changes in water body(e.g.,area and water level),lake water volume,water balance process,and the impact of ecological environment and cryosphere changes on lakes in basin.Meantime,many studies mainly focus on the single variable study of the area,water level or water quantity,far from reaching the comprehensive perspectives of changes in the basin climate,ecological environment,cryosphere and human activities and their impact on lakes.Therefore,the core scientific question of this study is the characteristics of water changes in Qinghai Lake,such as water level,area and water volume,and their impact mechanisms under the background of regional and global changes.This study combines station observation data,reanalysis data,assimilation data set,satellite remote sensing data,etc.,and uses water balance model,water volume assessment model and mathematical statistics to analyze the characteristics of the water level,area,and volume in the long-term,the water balance and its impact mechanism,and the impact of lake ecological environment changes in the Qinghai Lake Basin in the past 60 years.Based on the impact of snow cover,permafrost and vegetation changes on river runoff,the characteristics of changes in water level,area and volume of Qinghai Lake and their impact mechanisms from a comprehensive perspective were analyzed to provide reliable evidence for the impact of global and regional climate change on lake water bodies,so as to lay a foundation for the study of climate change and its impact on water resources in large lakes in the plateau region,which has important ecological significance and scientific value.Mainly research contents are as follows:(1)Based on station monitoring and assimilation data sets,we analyzed the Spatial-temporal characteristics of climate change in Qinghai Lake Basin.(2)Based on satellite remote sensing data,station monitoring data and data sets,the monthly scale Lake area is reconstructed.The characteristics of monthly and interannual variation of Qinghai Lake area in long-time series are analyzed and discussed,and the relationship between climate change and lake level and area variation is discussed.(3)We collected the satellite remote sensing data of lake water volume changes,based on the lake water level and area data in this study,and analyzed the variation of lake water volume in recent 60 years by using lake water volume estimating models of water quantity change.The main differences between the two-lake water volume estimating models are also analyzed.(4)Combined with the lake water balance model,the variation characteristics of precipitation,river runoff,evaporation and other factors that impact the change of water level and their relationship with runoff and water level are studied;(5)Using satellite remote sensing data and model output data,the characteristics of ecological environment and human activities changes in the Qinghai Lake basin including vegetation,snow cover,permafrost and land use changes are analyzed,and response of lake water bodies on ecological environment and human activities of the basin are discussed.The main conclusions are as follows:(1)The climate characteristics of Qinghai Lake Basin showed that air temperature increases significantly and precipitation tends to increase.The whole basin is characterized by warmming and wetting.The mean annual air temperature of Qinghai Lake Basin has increased year by year since 1961 with a temperature increase rate of0.39 ?/10 yr.Although annual precipitation fluctuates,the overall trend showed a significant upward trend with an increased rate of 16.9 mm/10 yr.The warming temperature was mainly affected by the increase of temperature in winter,and the increasing precipitation was mainly dominated by the increase of precipitation in summer and autumn.With the influence of warming and wetting climate characteristics in the basin,river discharge in the basin increased significantly after entering the 21 st century,and evaporation showed an obvious downward trend.(2)Lake water level of Qinghai Lake showed an obvious downward trend with a rate of-34 cm/10 yr from 1961 to 2019.Lake water level had two obvious changing processes in the past 60 years.The mean annual lake level showed a significant downward trend from 1961 to 2004 with a decline rate of 80 cm/10 yr.Mean annual lake level showed a continuous upward with an increased rate of 1.8 m/10 yr since 2004,which was 2.25 times of downward rate from 1961 to 2004,and the lake water level rose continuously for 15 years to reach the level in the 1960 s.Lake area and lake water volume showed characteristics of shrinking(1961-2004)and expansion(2004-2019)around 2004.Lake surface precipitation and inflow runoff showed a significant increasing trend after 2003,while lake surface evaporation showed a decreasing trend after 2000.In the first stage,precipitation and runoff were low,while evaporation was relatively high,and lake water balance was negative.In the second stage,lake water balance was positive with an increased trend of precipitation and runoff into the lake,and a decreased trend of evaporation.The increase in lake runoff and lake surface precipitation and the decrease in lake surface evaporation together led to a significant increase in the water level,area and water volume of Qinghai Lake after 2004.(3)The NDVI of Qinghai Lake Basin had a significant positive correlation with the river runoff and the lake water level.Vegetation coverage had a high consistency with the change trend of the river runoff and lake level,showed a significant positive correlation,indicated that the stability of the vegetation ecosystem improved the water conservation capacity of the basin,and the climate characteristics of the basin's warming and wetting and the increased of water resources also had a positive feedback effect on the ecosystem.The snow water equivalent in the Qinghai Lake Basin was significantly positively correlated with the river runoff in May,and the change of underground ice content in the basin showed a significant positive correlation with the river runoff in autumn.In years with relatively dry climate conditions,the lake water level showed an obvious downward trend,while in years with relatively humid climate conditions,the lake level showed a clear upward trend.(4)In different time series,the main influencing factors of Qinghai Lake water body were different.Under the background of climate warming and wetting in the basin,the melting of snow in spring,especially in May,recharged river runoff had a positive impact on the lake water body.From June to September in the rainy season,increasing precipitation was not only fed back to the surface runoff of the river,but also impacted the lake.From October to November,active layer melting in permafrost areas led to the melting of underground ice,which supplied groundwater and finally to rivers and lakes.The impact of land use on the Qinghai Lake was small because Qinghai Lake is located in the Qinghai Tibet plateau at a high altitude with a cold climate,low population density,and low degree of agricultural development.The increase of precipitation and river runoff into the lake was the main factor leading to a lift in lake water level,expansion of lake area and increasing water volume.Under the circumstance of less interference from human activities,the intensification of climate warming and wetting,the increase of snow melt water and precipitation,the increase of underground ice melt and groundwater increase and other comprehensive factors in different periods lead to the increase of runoff and the decrease of lake surface evaporation,which led to the water level,lake area and water volume of the Qinghai Lake have rebounded since 2004.The contribution rate of inflow runoff to the lake water level change was 48.5%,followed by the contribution rate of lake surface precipitation was 35%.Global warming leads to irreversible impacts such as glacier retreat,permafrost degradation,snow melting,precipitation and inflow increasing will increase water level of the lake in the short term,but there is great uncertainty of lake water balance in the long-term evolution process.Based on the station-observed data over the past 60 years,this study systematically clarified the changing trend of the water body and water balance in Qinghai Lake.The water changes of Qinghai Lake were more comprehensively clarified from the seasonal scale,and the contribution mechanisms of climate and cryosphere components to the discharge changes in different seasons were comprehensively revealed.These research results will help to further explore the physical mechanism,internal connection and ecological-hydrological process of lake change in Qinghai Lake,and contribute to regional climate change and water resources planning and management.It has a great significance to improve the research level of hydrological processes in cold regions and their response mechanisms to climate change.