Monitoring Of Water Volume Change And Lake Ice In Qinghai Lake Based On Multi-Source Data

Lakes are the link between cryosphere,atmosphere,hydrosphere and biosphere of the earth's surface system.As an important part of the terrestrial hydrosphere,the shrinkage and expansion of lakes has a great effect on the natural environment and human life.Lake ice,one of the physical characteristics of lakes most directly affected by climate change,can not only reflect the small-scale environmental changes caused by atmosphere and hydrology,but also the large-scale climate changes such as global warming.Qinghai Lake is the largest inland saltwater lake in China,which acts as a natural regulator for the climate around the lake and is a significant water vapor source in the northeastern part of the Qinghai-Tibet Plateau.Based on NOAAAVHRR,Terra MODIS and Landsat MSS/TM/OLI remote sensing images,this paper studied the changes of Qinghai lake's area,water level,water quantity and lake ice phenology characteristics by comprehensively applying RS and GIS technologies according to the differences of the reflectance of ice and water in the visible and infrared bands.And,the meteorological data was combined to analyze the main climatic factors affecting the lake ice phenology and to clarify the changes of area and ice of large lakes on the Qinghai-Tibet Plateau how to response the change of climate.The main conclusions are as follows:Between 1973 and 2018,the area of Qinghai Lake was generally reduced first and then increased,in which,the area in 2004 was the smallest,in 2018 was the largest.The change rate of lake area in the expansion period was faster than that in the shrinkage period.Affected by the terrain,Qinghai Lake mainly expanded towards the west,east and northwest,especially towards the west.Since 1961,Qinghai lake water level mainly experienced a decline process and a recovery process.The lowest water level occurred in 2004.During the period 1961~2004,the water level decreased by3.22 m,resulting to the negative water balance with the water volume decrease of13.24 km3.While from 2004 to 2017,the water level increased by 2.07 m,the water showed a positive balance with the water volume increase of 12.90 km3.With the change of water level and area,the coastline of Qinghai Lake has also undergone significant change in the past 45 years.The Shadao Lake on the east bank,the Haixi Island and the Tiebuka Bay on the west coast and the entrance area of the Shaliu Riveron the north bank are the regions experienced the most significant changes.Qinghai Lake is a closed inland lake,which is mainly recharged by runoff and precipitation.Therefore,the total runoff of the Buha River and Shaliu River is one of the key factors determining the rise and fall of the water level in Qinghai Lake.Precipitation and evaporation also play an important role in the change of water volume.The sudden increase of precipitation in the past two years directly leads to the rapid expansion of Qinghai Lake.In addition,the influence of wind speed and temperature also cannot be ignored.Four freeze-thaw dates of Qinghai Lake between 1980 and 2018 derived from AVHRR HRPT and MODIS data showed that the Qinghai Lake usually begins to freeze from mid-December to early January,and is completely covered by ice at the end of December or early January.The completely frozen ice layer gradually melts away and disappears completely in early or mid-April until the end of March.In the past 38 years,the average freeze duration(FD)and the completely freezing duration(CFD)of Qinghai Lake were 96 d and 88 d,and the average lake ice coverage duration(ICD)and ice ablation duration(AD)were 109 d and 8 d.The changes of phenological characteristics of lake ice in Qinghai Lake showed great differences between 1980 and 2018.The freeze-up start(FUS)and freeze-up end(FUE)dates were respectively delayed at a rate of 0.16 d/a and 0.19 d/a with a total delay of 6.08 d and 7.22 d.The dates of break-up start(BUS)and break-up end(BUE)respectively showed an advanced trend of-0.36 d/a and-0.42 d/a with a total of 13.68 d and 15.96 d in advance.ICD,FD and CFD showed a significant decreasing trend,the change rates were-0.58 d/a,-0.60 d/a and-0.52 d/a respectively,and the total shortened periods were 22.04 d,22.81 d and 9.76 d respectively.According to the daily MODIS remote sensing images,we found that the first crushed ice appeared on the east,north and west of Qinghai Lake and the first fixed ice formed.Then the ice layer gradually expanded from the lake shore to the lake center.In terms of ablation,the ice first began to ablate in the northern and western parts of the lake area with the faster rate after the main ice surface cracked.The southeast and east parts of the lake area were the latest to melt.The freezing-ablation spatial mode of Qinghai Lake can be divided into two types: the similar spatialpatterns of freezing and ablation in the west and the opposite freezing and ablation spatial modes in the east of the lake.Climate factors are closely related to the phenology of lake ice,especially the negative accumulated temperature in winter half year.In the year when the accumulated temperature in the winter half year is relatively large,the later the freezing time of Qinghai Lake is,the earlier the ablation time is,and the shorter the freeze duration is.And the influence of radiation and air pressure on the lake ice are also improtant.When the average radiation or pressure is large in the winter half year,the freezing time of the lake is delayed and the ablation time is advanced.Wind speed and precipitation play different roles in the process of lake freezing and ablation,and freezing time is more sensitive to changes of wind speed and precipitation.