## 3 Methods  
The main technical route of this study includes four major parts (Figure 2): (1) preliminary extraction of water body based on Sentinel-1 archive, (2) acquisition of the auxiliary water mask,  
\begin{table}
\begin{tabular}{p{113.8 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Data/Product Name** & **Spatial Resolution** & \begin{tabular}{c} **Temporal** \\ **Resolution** \\ \end{tabular} & **Purpose in the Study** & **Download Link** \\ \hline Sentinel-1 ground range detected (GRD) products & 10 m & 6-d & \begin{tabular}{c} Generate time series of \\ water fraction maps \\ \end{tabular} & [[https://scihub.copernicus.eu/dhus/](https://scihub.copernicus.eu/dhus/)]([https://scihub.copernicus.eu/dhus/](https://scihub.copernicus.eu/dhus/))\#home \\ \hline Landsat operational land & 30 m & 16-d & \begin{tabular}{c} Eliminate the effects of \\ western terrain shadows \\ and deserts \\ \end{tabular} & [[https://doi.org/10.5066/F7854](https://doi.org/10.5066/F7854) MZJ]([https://doi.org/10.5066/F7854](https://doi.org/10.5066/F7854) MZJ) \\ \hline Digital elevation model from Shuttle Radar (SRTM) & 30 m & static & \begin{tabular}{c} Identification of sloping \\ terrain and terrain \\ shadows \\ \end{tabular} & [[https://doi.org/10.5066/F7](https://doi.org/10.5066/F7) PR7 IFT]([https://doi.org/10.5066/F7](https://doi.org/10.5066/F7) PR7 IFT) \\ \hline Global Surface Water Explorer (GSWE maximum water extent map during 1984 to 2018 & 30 m & static & \begin{tabular}{c} Eliminate the effects of \\ western mountains \\ \end{tabular} & [[https://global-surface-water.appspot.com/download](https://global-surface-water.appspot.com/download)]([https://global-surface-water.appspot.com/download](https://global-surface-water.appspot.com/download)) \\ \hline GSWE monthly water history in 2018 & 30 m & monthly & \begin{tabular}{c} To generate threshold \\ estimation points and \\ validation points, \\ comparison of products \\ \end{tabular} & [[https://global-surface-water.appspot.com/download](https://global-surface-water.appspot.com/download)]([https://global-surface-water.appspot.com/download](https://global-surface-water.appspot.com/download)) \\ \hline Water level from satellite altimetry & 10-d & Comparison of products & [[https://dahit.dgf.tum.de/en/](https://dahit.dgf.tum.de/en/)]([https://dahit.dgf.tum.de/en/](https://dahit.dgf.tum.de/en/)) \\ \hline Copernicus Global Land Cover Layers—Collection 2 & 100 m & static & \begin{tabular}{c} As a reference for the \\ selection of threshold \\ samples and landcover \\ types \\ \end{tabular} & [[https://doi.org/10.5281/zenodo.3243509](https://doi.org/10.5281/zenodo.3243509)]([https://doi.org/10.5281/zenodo.3243509](https://doi.org/10.5281/zenodo.3243509)) \\ \hline \begin{tabular}{c} Inland surface water \\ dataset in China (ISWDC) \\ \end{tabular} & 500 m & 8-d & Comparison of products & [[http://doi.org/10.5281/zenodo.1463694](http://doi.org/10.5281/zenodo.1463694)]([http://doi.org/10.5281/zenodo.1463694](http://doi.org/10.5281/zenodo.1463694)) \\ \hline Standard China map & & \begin{tabular}{c} Define a correct \\ boundary of China \\ \end{tabular} & \
\begin{tabular}{c} [[http://bzdt.ch.mnr.gov.cn/](http://bzdt.ch.mnr.gov.cn/)]([http://bzdt.ch.mnr.gov.cn/](http://bzdt.ch.mnr.gov.cn/)) \\ data.aspr/DATAID=141 \\ \end{tabular} \\ \hline Boundary of basins & &
\begin{tabular}{c} Provide computing \\ resources \\ \end{tabular} & [[https://earthengine.google.com](https://earthengine.google.com)]([https://earthengine.google.com](https://earthengine.google.com)) \\ \hline \hline \end{tabular}
\end{table}
Table 1: All datasets and tools used in this study.  
Figure 1: (**a**) Spatial distribution of Sentinel-1 observations in 2018 and (**b**) the number of images per month used for 2016, 2017, and 2018. The boundary layer of China is from National Administration of Surveying, Mapping, and Geoinformation of China: GS (2016)1569.  
(3) post-processing for water extraction, and (4) accuracy validation of the high spatial-temporal water body dataset in China (HSWDC).