Method Of Monitoring Water Level Change Of Main Lakes In China With Radar Altimeter Data

Lake is important water resources.Lake level is a sensitive indicator of climate and environment change.Satellite altimetry is used as an alternative tool to measure lake levels since the 1990 s.In this study,water level of 27 major lakes in Chinese was extracted using ENVISAT/RA-2 and Cryosat-2/SIRAL radar altimeter data to verify the level extraction ability of Cryosat-2/SIRAL and monitor Chinese lake level change.The detailed content and results are as follows:(1)Comparative analysis of waveform retracking methods.This study used 6 kinds of algorithms to retrack Cryosat-2/SIRAL LRM level 1b data to extract water levels from 2010 to 2015,including the primary peak Offset Center of Gravity(OCOG),primary peak threshold,primary peak 5-? parameter,traditional OCOG,traditional threshold and traditional 5-? parameter methods.Furthermore,Together with three different retrackers of Cryosat-2/SIRAL GDRs of LRM mode,UCL,refined CFI and refined OCOG,there were 9 kinds of water level results.The accuracy of all these different algorithms extracting water level of inland and outflow lakes was analyzed compared with measured water level of hydrological station.The results showed that,the waveform retracking can effectively improve the accuracy of water extraction.For the inland lake Qinghai Lake,the water level extraction accuracy could reach 0.1 m,and the water level result based on primary peak 5-? parameter retracker is the best.While for outflow lakes,the accuracy is about 0.2 m,and the primary peak OCOG algorithm and primary peak threshold algorithm performed better.The water level extraction accuracy of retrackers based on primary peak is better than retrackers based on the whole waveform generally.Comparing the three kinds of Cryosat-2/SIRAL GDR products for LRM patterns,the data based on Refined OCOG algorithm is more suitable for the extraction of lake level.(2)Water level series extraction and validation of major lakes in China using radar altimeter data.Primary peak 5-? parameter retracker for inland lakes and primary peak OCOG algorithm for outflow lakes were selected to retrack Cryosat-2/SIRAL LRM 1b waveform data.The 2002 to 2015 water level time series of these 27 lakes greater than 500 km2 in China were obtained combined with the ENVISAT/RA-2 and Cryosat-2/SIRAL GDR data by the lakes boundary extraction,water level calculation,abnormal water level elimination and height system transformation.The accuracy of the lake level sequence is verified with the example of Qinghai Lake and Poyang Lake.The results show that,there is good correlation between the lake level of altimeter and measured water level,with correlation coefficient of 0.95 and RMSE of 0.1 m.Their level change trend is consistent.The lake level sequence extracted from altimeter could be used to analyze water level changes.(3)Lake level changes and change reasons analysisThe inter-annual variation trends were expressed by the average water level change rate of 2002-2015,while seasonal variations were analyzed with the month the highest water level appears and the amplitude of water level variation.Natural and anthropogenic factors effects on lake level change were discussed on the basis of previous research.The results showed that,lakes in the Tibetan Plateau are mainly inland lake with small anthropogenic influence,and the change of water level is mainly influenced by the change of temperature,precipitation,evapotranspiration and frozen soil.These Lakes had strong regularity of seasonal level variation.During the year of 2002-2015,most lakes water level rised and changes quickly generally,and the mean annual change rate of Selincuo reached 0.377 m/a.The area of Inner Mongolia and Xinjiang has a large span,and water level of lakes in this region rised or fell.Lakes in Northern Xinjiang and Tianshan Mountains are affected by agricultural irrigation.Xingkai Lake water level in Northeast Plain and Mountainous region had a rising trend of 0.022 m/a overall,because the flood discharge capacity of Songacha River decrease with sediment deposition.In addition to Taihu and Chaohu,Eastern Plain lakes level showed a slow downward trend during recent 14 years.These lakes had large variation range of water level and early water level peak in a year.The water level changes were complex with the effect of human activities,such as the Three Gorges project,the establishment of the dam gate,and reclaiming farmland from lakes.