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3.2 Lake level variations based on SARAL and Sentinel-3 altimetry data

In order to examine whether abrupt water level increase during 2016-2018 occurred similarly in other Tibetan lakes, we investigated the water level changes of a total of 12 lakes from 2013 to 2018 by combining SARAL and Sentinel-3 altimetry data. Six of them were aimed to inter-compare with the Hydroweb and DAHITI altimetry data. Fig. 4 presents the comparisons of water-level time series from Hydroweb/DAHITI data archives and SARAL/Sentinel-3 data between 2013 and 2018 for Qinghai Lake, Nam Co, Ayakkum, Migriggyangzham Co, Zhari NamCo and Tangra YumCo. The water level time series for all the lakes indicate that not only are the general trends derived from two data sources comparable but the abrupt changes and intra-annual variations of lake level are overall consistent. For example, both data sources clearly depict a sudden increase of about 1.0 m during the wet season (July-September) of 2017 for Zhari NamCo. The characteristics of water levels rises in wet seasons and declines in dry seasons are also in good agreement for all the lakes, suggesting that SARAL and Sentinel-3 data have reliable results.
Based on the above validation, the SARAL and Sentinel-3 data were used to monitor the lakes larger than 100 km² in the TP, and the water level time series are shown in Fig. 5. The six lakes display an evident trend of water level increase at varying rates during 2016-2018. For example, the alpine lakes between the Tanggula and Kunlun Mountains, including XijingUlan Lake, Mingjing Lake and UlanUla Lake, showed a significantly rise (exceeding 1 m/a). In contrast, the lake level for Gyariog Co showed an inter-annual fluctuation.