Monitoring of lake level changes on the Tibetan Plateau and Tian Shan by retracking Cryosat SARIn waveforms

Summary In this study we present, for the first time, the application of Cryosat SARIn mode data to monitor lakes in mountainous areas and to find their water balance. By applying a novel retracker, tailored for lake level observations, we find at least four useable passes for 125 lakes on the Tibetan Plateau and Tian Shan areas over the period February 2012 to January 2014. From these 125 lakes, 30 are passed at least ten times, for which we compute trends and periodic variations, and 16 lakes more then twenty times, for which we additionally apply a water slope correction. This slope correction accounts for geoid inaccuracies or wind effects. We compared the results over two lakes, Langa Co and Bosten, with Jason-2 measurements. Over Langa Co we find an RMS difference of 0.55 m, while for Lake Bosten this is only 0.26 m. For Lake Bosten, the estimated trends, annual and semi-annual variations from the Cryosat and Jason-2 datasets are compared as well. The estimated amplitudes are comparable, while derived phases differ by a few days. Using the trends of all lakes passed at least ten times, a water volume balance of natural lakes is estimated. A loss of 1.51 ± 0.11 km 3  y −1 is observed in the lakes in the Tian Shan area. In Tibet, a positive mass balance is estimated of 1.76 ± 0.24 km 3  y −1 .