Construction And Variation Analysis Of Lake Water Level Series In Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau is a sensitive area for global climate change,and the plateau lake group is one of the most sensitive factors to climate change.Due to the special geographical location and harsh climate environment of the Qinghai-Tibet Plateau,most lakes have not established monitoring stations.The rapid development of satellite remote sensing technology has brought new directions for the large-scale and long-term monitoring of lakes in this region.In this paper,based on ICESat,Cryosat-2,Sentienl-3A/B and ICESat-2 four kinds of spaceborne altimeter data and auxiliary data,such as Landsat series remote sensing images,the lake water level extraction and accuracy evaluation work of multi-source spaceborne altimeter were carried out.This constructed the 2003-2020 water level sequence of 161 lakes on the Qinghai-Tibet Plateau with an area greater than 50 km2,and evaluated the consistency of the water level sequence based on the measured and verified data.In this study,water level extraction and evaluation are carried out by multi-source satellite-based altimeters,and the water levels of the lakes are extracted in different ways with different observation modes of the altimeters and different observation equipment on board.For ICESat/GLAH,Cryosat-2/SARIn and ICESat-2/ATLAS spaceborne altimeter GDR data,the lake water level is directly calculated based on the principle of spaceborne altimeter extraction,while for Cryosat-2/LRM and Sentinel-3/SAR spaceborne altimeter L1 b waveform data,lake water level is extracted based on waveform re-tracking algorithm.The inversion of lake water levels on the Tibetan Plateau using satellite-based altimeter GDR data products and the accuracy evaluation of water level inversion is carried out in conjunction with lakes with actual measurement data.Based on the seven water levels（Cryosat-2（4）,ICESat,Sentinel-3 and ICESat-2（1 each））directly calculated by four satellite-based altimeter GDR data products,the data quality of the multi-source satellite-based altimeter water level inversion is evaluated in combination with the measured water level data.The results show that the accuracy of ICESat-2/ATL 13 satellite-based altimeter data in laser altimetry mode is the highest;followed by Sentinel-3/SIRAL and Cryosat-2/ SARIn satellite-based altimeter data with higher accuracy;the accuracy of ICESat/GLAH satellite-based altimeter data is poor,but the goodness of fit R2 = 0.84 with the measured data,which satisfies the requirements for constructing lake level series were met.A comparative analysis of Cryosat-2/LRM and Sentinel-3/SAR L1 b waveform retracking was carried out.Based on the Cryosat-2/LRM and Sentinel-3/SAR waveform data,seven waveform retracking calculations were performed,including the Offset of Center of Gravity（OCOG）,Threshold（TR）,ICE-1 algorithm,Narrow Primary Peak Offset of Center of Gravity（NPPR-COG）,and Narrow Primary Peak Threshold（NPPR-TR）（threshold levels of 30%,50%,80%）.Algorithms,two kinds of satellite-based altimeter to obtain a total of 14 kinds of water level results,combined with Cryosat-2 and Sentinel-3 satellite-based altimeter GDR data products themselves have 4 and 1 algorithm,respectively,a total of 19 kinds of Qinghai Lake,8 kinds of Geren Co water level results for accuracy evaluation and analysis.The results show that Sentinel-3 satellite-based altimeter with NPPR-TR 80% threshold is the optimal retracking algorithm;Cryosat-2/LRM satellite-based altimeter with NPPR-TR 30%threshold is the optimal retracking algorithm.Constructing time series of lake water levels on the Tibetan Plateau with area larger than 50 km2 and accuracy verification.Based on the evaluation results of multisource satellite-based altimeter lake water levels,the rules for the construction of lake water level sequences on the Qinghai-Tibet Plateau are established.ICESat-1/2 and Cryosat-2/SARIn satellite-based altimeter GDR data are used to extract water levels directly,and Cryosat-2/LRM and Sentinel-3A/B satellite-based altimeters are used to select the optimal waveform retracking based on the accuracy evaluation results.algorithm to extract the water level.The water levels extracted by four kinds of starborne altimeters are unified to WGS₈4 ellipsoid and EGM 2008 level,respectively,to construct the water level time series of 161 lakes on the Qinghai-Tibet Plateau with area larger than 50 km2 from 2003 to 2020,and verify the accuracy of the series,and find that except for Dogaicoring_Qangco and Mingjing Lake,the water levels of the lakes involved in the verification and other high-precision water level data products Pearson the correlation coefficient CC<0.7 and the root mean square error RMSE<0.8m indicate that the accuracy of the lake level series constructed in this paper is high in the Tibetan Plateau.Based on the constructed lake level time series of Qinghai-Tibet Plateau,the spatial and temporal changes of 161 lakes in Qinghai-Tibet Plateau were analyzed by combining the M-K trend analysis method.The results show that 118（73.3%）lakes on the Qinghai-Tibet Plateau are in an increasing trend between 2003 and 2020,of which87（54%）are significantly increasing;43（26.7%）lakes are in a decreasing trend,of which 26（16.1%）are significantly decreasing.The lakes with rising water levels on the Tibetan Plateau are mainly located in the northern part of the Tibetan Central Instream Region and the northeastern part of the Tibetan Plateau,while the lakes with decreasing water levels are mainly concentrated in the Yarlung Tsangpo River Basin,the southern part of the Tibetan Central Instream Region and the Indus River Basin.Analysis of the water level-area changes of typical lakes on the Qinghai-Tibet Plateau,the results showed that the correlation between water level-area changes of two typical lakes was high,with R2=0.94 for Qinghai Lake and R2=0.67 for Mapam_Yumco,but the changes were consistent.Combined with the data from meteorological stations,the relationship between climate change effects on different types of lakes was analyzed by precipitation difference product,correlation analysis and M-K mutation test,and it was found that the main factor affecting the changes of inland lakes was precipitation,followed by temperature;for outland lakes,both precipitation and temperature had less influence on the changes of lakes.