## 2 Study Area and Data
### Data  
In this study, according to the mapping and validation aims, three different types of images in different time periods were selected. Table 1 gives a summary of all the data used. For the development of the classification method and mapping of the high-frequency glacial lakes, all available time series of C-band Sentinel-1A/1B ground range detected (GRD) images from both the ascending and descending orbits at a spatial resolution of 10 m, were accessed from Google Earth Engine cloud computing platform ([[https://code.eartheengine.google.com/](https://code.eartheengine.google.com/)]([https://code.eartheengine.google.com/](https://code.eartheengine.google.com/))) with a consecutive time interval of six days. These data were mainly acquired during May to October 2018 in order to ensure that most of the glacial lakes were not frozen and were easily visible in the images [18], and to further ensure the reliability and representative of the median composite pixel values derived from these intensity images. This median-based compositing approach can deal effectively with data quality, abnormal values, and other issues that will affect single scene classification. Among the mapping results, some glacial lakes at a particular time were used for the accuracy assessment, which were mapped based on the time series of the C-band Sentinel-1A/1B images from the ascending and descending orbits in July to August  
Figure 1: Geographic location of study area with the topographical characteristics and distribution of mountain glaciers in the two river basins. Pink and blue rectangles represent the frames of ascending and descending Sentinel-1 images, respectively. Green rectangles indicate the coverage of Gaofen-2 panchromatic multi-spectral (GF-2 PMS) images used for accuracy evaluation of the glacial lake mapping results.  
2018 to be consistent with the time period of the validation data from Gaofen-2 (GF-2) panchromatic multi-spectral (PMS) imagery. For the validation of the lake mapping results, we used six cloud-free GF-2 images acquired in July to August 2018 from the descending orbits to evaluate the accuracy of the mapping results of glacial lakes in a similar time period. Here, the GF-2 scenes were chosen due to the fact that they had a high spatial resolution of 1 m/4 m in the panchromatic/multi-spectral bands and a revisit cycle of five days, which is quite similar to the revisit period of the dual satellite constellation of Sentinel-1A and Sentinel-1B. The basically consistent acquisition dates for the Sentinel-1A/1B and GF-2 images can reduce the influence of lake extent fluctuations on the two-date lake maps comparison.  
Furthermore, Digital Elevation Model (DEM) data were obtained to serve as the topographic reference for the geocoding of SAR and optical data, and for removing the mountain shadows that may induce the classification errors for lake mapping. This study used the ASTER Global Elevation Model Version2 (GDEM V2) gridded data with a 30-m resolution collected from [[http://gdem.ersdac.jspacesystems.or.jp/](http://gdem.ersdac.jspacesystems.or.jp/)]([http://gdem.ersdac.jspacesystems.or.jp/](http://gdem.ersdac.jspacesystems.or.jp/)).