[Insert Table 1 here]
Both two data archives cover a certain number of lakes in the Tibetan Plateau (Table 2). The long-term water level change information of 16 lakes was obtained based on Hydroweb and DAHITI data archives. However, the number of lakes cannot fully meet the needs of our lake level monitoring (refer back to Fig. 1). Therefore, in this study, SARAL and Sentinel-3 altimetry data were used to supplement the missing observations from the LEGOS Hydroweb and DAHITI data archives. We interpreted and combined SARAL and Sentinel-3 altimetry data to obtain a total of 12 lake level time series during the period 2013-2018. Six of them, covering the same lakes as Hydroweb and DAHITI data archives, were used to verify the reliability of SARAL and Sentinel-3 altimetry data. Others were used as the data supplement.
2.2.2 Supplementary altimetry data

2.2.2.1 SARAL/AltiKa data

SARAL is a common mission of the Indian Space Research Organization (ISRO) and the France space agency (Project 2010). The mission, launched on 25 February 2013, is a continuation of the ERS series and Envisat missions. The orbit and revisit cycle of SARAL satellite were the same as Envisat, but after July 2016, SARAL satellite was then transformed to drifting ground track mode. Compared to the previous altimetry satellites, the improvement of SARAL/AltiKa was that it was the first altimeter to measure in the Ka band, with higher spatial resolution. In addition, AltiKa’s bandwidth (500 MHz) had been enhanced, which resulted in a higher vertical resolution and higher pulse repetition frequency (Project 2010). We downloaded the SARAL data (spanning from March 2013 to June 2016) from ftp://avisoftp.cnes.fr.

2.2.2.2 Sentinel-3/SRAL data

The Surface Topography Mission uses the Synthetic Aperture Radar Altimeter (SRAL) instrument, which transmits pulses occasionally at Ku band for altimeter range measurements, supplemented by a C-band frequency to correct range delay errors. The SRAL instrument acquires data in two measurement modes, the Low-Resolution Mode (LRM) which is useful over homogeneous open-ocean surfaces, and the Synthetic Aperture Radar (SAR) mode which achieves high along-track resolution over relatively flat surfaces. Sentinel-3 satellite has multiple new functions and modes with the dual-frequency SAR radar altimeter (SRAL) (Donlon et al. 2012). Among them, the tracking mode is the most critical characteristic(Jiang et al. 2019). We downloaded the needed data from March 2016 to December 2018 through https://sentinel.esa.int/web/sentinel/ sentinel-data-access.