| Changes in lake levels are an important indicator of water balance and climate change.As an alternative method of water level observation,radar altimetry technology has been widely used in monitoring lake water level in recent years,especially in lakes without hydrological stations or transboundary lakes.Traditional pulse-limited radar altimeters are susceptible to the land noise signals when observing small and medium-sized lakes due to the large footprints.With the application of the Delay Doppler technology and Open-Loop Tracking mode in Cryosat-2,Jason-2/3,Sentinel-3A,the estimation of the water level of small and medium-sized lake is becoming possible.In this paper,the waveform processing and applications over the water level of small and medium-sized lake is conducted by using Cryosat-2,Jason-2/3,Sentinel-3.The main research contents and conclusions are as follows:(1)According to the problems of multi-peak waveforms and invalid off-nadir points,an automatic multiscale-based peak detection retracker(AMPDR)was presented in this study.The retracker can extract a robust threshold level for each track,then the stable lake level can be obtained from the multipeak waveforms using a shortest-path algorithm.Additionally,the retracker can be used for mountain lakes and for flat lakes,and is also suitable for different kinds of altimetry data.To validate the lake levels derived by the AMPDR retracker,the in-situ gauge data of nine lakes in China were used.Moreover,seven existing retrackers were compared to evaluate the performance of the proposed AMPDR retracker.The results suggest that AMPDR can efficiently process many complex multipeak waveforms,and the AMPDR has the lowest mean of all track standard deviations over all lakes.The lake level time series obtained from AMPDR are more stable.The average RMSE between the lake levels over the nine lakes obtained by the AMPDR retracker and the in-situ data from Cryosat-2,Sentinel-3,and Jason-2/3 are0.149,0.139,and 0.181 m,respectively.Additionally,the AMPDR algorithm also has outstanding performance for the fusion level time series,and the average RMSEs of selected lake level series and the in-situ water levels are 0.124m.In summary,the novel AMPDR can further improve the retracking performance of the waveform from different altimeter data,even though dealing with the seriously contaminated waveform,which helps to obtain a more accurate lake level.(2)A construction method of lake level time series by using multi-altimetry based on noise orbit removal,state-space model and STL methods was constructed.Compared with the in-situ data,the RMSEs of the water level series obtained by this study in the Zhari Namco and Hongze Lake is 0.128m and 0.162m,respectively;compared with the Hydroweb water level products,the robustness of the 19 lakes has been significantly improved in this study,especially for small lakes.(3)In this study,through waveform retracking,lake level time series construction,multi-altimeter time series fusion and outlier detection,changes in water level of 340lakes(>10 km~2)in China between 2016 and 2019 were monitored using multi-altimeter data from Cryosat-2,Jason-2,Jason-3,and Sentinel-3A.Then the spatial and temporal characteristics and driving factors of the above lake water level changes were analyzed.Overall,the water levels of these lakes showed a mean increasing trend of 0.343m/yr.The water levels of 274 lakes have a mean increasing trend of 0.465 m/yr and those of 66 showed a mean decreasing trend of-0.164 m/yr.The annual rate of change of lake water levels increased from south to north and decreased from west to east in terms of spatial distribution.In different basins,the changes in lake level varied owing to the spatial heterogeneity:most lakes in the Songhua and Liao River Basin and Yellow River Basin showed an increasing trend;all lakes in the Huai River Basin were relatively steady;most lakes in the Yangtze River Basin showed a decreasing trend;the great majority of lakes in the Inland River Basin showed an increasing trend;and the lakes located in the southeastern Qiangtang Basin,near the Nyainqen Tanglha Mountain,showed a decreasing trend.In addition,some lakes in the western Qiangtang Basin also showed a decreasing trend.Factor analysis indicated that the driving factors in lake level varied among different areas.However,temperature changes in autumn and winter were significantly correlated with changes in lake level.(4)The water level changes of two typical lake groups in the Hoh Xil and Qaidam basins were monitored by using Cryosat-2,Sentienl-3,Jason-2/3 and SARAL altimeter data during 2010 to 2019,and the reason of water level changes were analyzed.The results show that the water level change of the Hoh Xil lake group has changed significantly after the outburst of the Zhuonai Lake in 2011.The water level of the Salt Lake showed an increasing rate of 1.489 m/yr after 2011,which may risk at the project of Qinghai-Tibet Railway;The Dabson Lake in the Qaidam Basin lake group declined at a rate of-0.231 m/yr with the affection by human interference in recent years.Research on the relationship between water level changes in lake groups can help reveal the influence of flood disasters,natural environmental,and human interference on lakes. |
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