## Materials and Methodology  
### Datasets and Preprocessing  
Datasets utilized for the present study included S1A images ([[https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)]([https://scihub.copernicus.eu/](https://scihub.copernicus.eu/))), and Landsat-8 Operational Land Imager images (OLI, [[http://earthexplorer.usgs.gov/](http://earthexplorer.usgs.gov/)]([http://earthexplorer.usgs.gov/](http://earthexplorer.usgs.gov/))).  
Sentinel-1 is a polar orbiting two-satellite constellation (S1A and -B) that was launched by the European Space Agency (ESA) [34]. S1A and -B, the next generation of C-band (center frequency 5.405 GHz) radar missions, were launched on 3 April 2014 and 25 April 2016, respectively, from Europe's Spaceport in French Guiana. The revisit time of a single satellite is 12 days [25], whereas the two-satellite constellation provides a revisit time of 6 days [35]. The S1A utilized in the present study is a SAR standard L1 product, and belongs to the Interferometric Wide-swath mode (IW) with dual-polarization (VV/VH) [26]. This data product has a wide coverage of 250 km with medium resolutions of 5 m and 20 m in range and azimuth directions, respectively, with an equivalent number of looks of five, and imagery pixel spacing of 10 m in ground geometry [25]. For detailed descriptions regarding S1A, refer to reference [25].  
In the present study, 71 S1A scenes from 41 observations by Sentinel-1A satellite were selected from 24 May 2015 to 14 November 2016. Preprocessing of S1A imagery was performed with SNAP software provided by ESA ([[https://sentinel.esa.int/web/sentinel/toolboxes/sentinel-1](https://sentinel.esa.int/web/sentinel/toolboxes/sentinel-1)]([https://sentinel.esa.int/web/sentinel/toolboxes/sentinel-1](https://sentinel.esa.int/web/sentinel/toolboxes/sentinel-1))). The workflow included five steps. First, S1A images were allocated orbit files. Second, S1A images  
Figure 1: Location of Poyang Lake: (**a**) location of Jiangxi Province, China; (**b**) location of Poyang Lake in Jiangxi Province; and (**c**) Poyang Lake and its surrounding environment.  
were radiometrically calibrated to output radar backscatter bands (\(\sigma^{\circ}\)). Third, radar backscatter bands were orthorectified using the Range Doppler Terrain Correction algorithm with SRTM DEM and spatial resolution of 1 s. Fourth, the backscattering coefficient (in dB) was acquired from the orthorectified radar backscatter band by the equation 10 \(\times\) log\({}_{10}\) (\(\sigma^{\circ}\)) [36]. Fifth, a median filter with window size of 5 \(\times\) 5 pixel was utilized to remove speckle noises [23; 37; 38].  
A Landsat-8 satellite equipped with two sensor payloads (OLI and Thermal Infrared Sensor) was launched on 11 February 2013. In the present study, one OLI scene (acquired on 27 September 2016) was selected as the training data and applied for building the regression equation, and two OLI scenes (acquired on 9 September 2015 and 2 February 2016) were selected as testing data and applied to assess the classification accuracy of S1A. Eight OLI scenes (acquired on: 9 September 2015; 11 October 2015; 30 December 2015; 16 February 2016; 3 March 2016; 23 June 2016; 25 July 2016; and 27 September 2016) were used to extract the water surface areas of Poyang Lake and were compared with the water surface areas of Poyang Lake extracted from S1A from 24 May 2015 to 14 November 2016. Preprocessing work included atmospheric correction using the Fast Line-of-sight Atmospheric Analysis of Spectral Hypercube module, geometric correction, and data clipping based on the operational boundary of Poyang Lake. At the end, all images (S1A and OLI) were re-projected to an Asia North Albers Equivalent Conical Projection with the first standard parallel 25\({}^{\circ}\)N, the second standard parallel 47\({}^{\circ}\)N, and the central meridian 105\({}^{\circ}\)E.