Firn Line Of Glacier In Headwaters Of Urumqi River Comparison Of Remote Sensing Extraction Methods

Mountain glaciers are used as indicators to study climate change.Glacier mass balance provides a direct response to climate change.Mass balance monitoring needs a lot of material and manpower guarantee and technical support,and it is difficult to represent the mass balance state of the whole mountain glacier with only a few glacier monitoring data.Therefore,alternative methods are needed to determine whether glaciers in the mountain system are growing or shrinking.In recent years,the research results of extracting snowline by remote sensing show that the snowline altitude at the end of ablation season can be used to indicate the altitude of equilibrium line,and even in some years,the snowline altitude at the end of ablation season can be used to approximately replace the altitude of equilbrium line.However,the above results are obtained by different remote sensing methods on different glaciers,and the comparability is poor.It is rare to use different remote sensing methods to extract the altitude of firn line on the same glacier,especially in High Asia including Xinjiang.Whether the snowline altitude of mid latitude continental glaciers can indicate or approximately replace the mass balance line altitude is still controversial.This paper mainly selects Landsat TM/ETM+/OLI image,HJ-CCD image,GF-1WFV image,SRTM DEM data,through remote sensing image preprocessing(image registration,radiometric calibration,FLAASH)Atmospheric correction,topographic correction,inversion of albedo by narrow-band to wide-band conversion),using visual interpretation method,snow cover index method,single band threshold method,remote sensing inversion of albedo method and random forest method,the elevation value of firn line at the end of ablation period was extracted based on DEM.Taking Urumqi No.1 glacier as the experimental area,the accuracy and characteristics of extracting firn line of No.1 glacier at the source of Urumqi River were compared,This paper analyzes the causes of the errors,and uses the best method to extract the snowline elevation of the glacier in the headwaters of the Urumqi River from 2002 to2019,analyzes its temporal variation characteristics,discusses the image of temperature and precipitation on the firn line altitude of the glacier in the headwaters of the Urumqi River,and the uncertain factors affecting the snowline altitude.Based on the above research,the main conclusions are as follows:(1)The results show that the visual interpretation method,albedo remote sensing inversion method and random forest method have strong adaptability,and the accuracy of snow line extraction is high,followed by snow cover index method,and the single band threshold method is poor.(2)The NDSI threshold value of 0.60?0.98 is suitable for extracting firn line in the source area of Urumqi river.The threshold setting of single band threshold method should be determined according to the specific research area.When the inversion method of albedo remote sensing is used to extract the glacier firn line,the glacier firn line can be obtained according to the distribution characteristics of the retrieved glacier surface albedo.The random forest method needs to determine the snow sample first,which can quickly obtain the glacier firn line.Among the five methods of snow line extraction,the degree of automation and efficiency are the highest,and the adaptability is the strongest.(3)Except for the years with extremely negative mass balance,the altitude of snowline at the end of ablation season is a good indicator of the altitude of snowline.When the remote sensing image is selected at the end of the ablation season and the mass balance is less negative,the altitude of the glacier snowline is 13m?18m higher than the altitude of the equilibrium line;when the mass balance is positive,the altitude of the glacier snowline is 13m?23m lower than the altitude of the equilibrium line.The altitude of snowline at the end of ablation season can be used to approximate the altitude of snowline in the years with small negative and positive mass balance.(4)In most years,the firn line altitude values obtained by the five methods are lower than the measured mass balance line altitude in varying degrees,and the absolute error increases when the extreme negative value of material balance appears.Affected by the available image acquisition time,the firn line obtained by remote sensing is not the theoretical highest snowline altitude in the year of Glacier Mass Balance in most cases,which is an important reason for affecting the accuracy of glacier firn line extraction.(5)From 2002 to 2019,the variation of firn line altitude in the headwaters of Urumqi River is mainly controlled by the temperature in warm season,and the firn line altitude has an upward trend.(6)The accuracy of obtaining glacier firn line altitude by remote sensing is affected by DEM Resolution and DEM accuracy,image type,image registration accuracy and image resolution,image imaging time and so on.In this study,the uncertainty of DEM Resolution and DEM accuracy on the altitude of firn line are as follows ±30m and ±20m,The uncertainty caused by different image types is about11m?16m;The uncertainty of image registration accuracy and image resolution on firn line altitude is-56m?56m,The uncertainty of firn line altitude caused by imaging date is less than 61 m.