## 2 Study Area and Data
### Validation  
This research uses the Landsat 8 OLI data as a reference to validate the 7 methods. The verification data, 2016 Landsat 8 OLI data, were downloaded from the geospatial data cloud  
Figure 1: Map of the Poyang Lake, Jiangxi Province, China. website ([[http://www.gscloud.cn/](http://www.gscloud.cn/)]([http://www.gscloud.cn/](http://www.gscloud.cn/))) with a resolution of 30 m. In this study, Landsat 8 OLI image data from 17 September 2016, were used due to having less cloud cover and clear images. These image data effectively reflect the distribution of land surface water. Moreover, the corresponding verification samples were checked against the Landsat 8 OLI data, and scale transformation and integration were carried out, thereby providing the reference images for the accuracy analysis of the experimental results. Verification samples corresponding to the eight stability test areas were checked using the Google Earth data, which served as reference images for the accuracy analysis of the experimental results. Since Poyang Lake is a seasonal lake [10], the difference between its dry season and the wet season is conspicuous. Therefore, we used Landsat 8 OLI data from 17 September 2016, which is in between the two different seasons, as the reference data and selected the corresponding verification samples. The confusion matrix was calculated from the results of the classification method used in this paper, and the coefficients of OA, Kappa, f1, and miou were obtained to evaluate the classification method [24; 25; 26; 27]. The stability of the surface water extraction method was mainly limited by the high spectral and spatial diversity of the water bodies and the inhomogeneity of adjacent surfaces in different regions. Therefore, eight test sites characterized by different surface water types, landscapes, and terrains were selected to evaluate the stability of the surface water extraction methods. These water bodies at these different locations consist of many small lakes, linear or irregular rivers, artificial reservoirs, and lakes composed of waterways in coastal areas. Given that the features of these heterogeneous landscapes are spatially variable, the test areas were selected from coastal areas to inland areas, which are characterized by diverse climatic conditions, such as deserts, rain forests, deltas, and tropics. Specifically, (i) sites located in harbors are characterized by a complex composition of seawater and constructed lakes. The mixture of an urban surface appearance and building shadows at these sites creates considerable obstruct for the accurate extraction of target waters; (ii) the waters of natural coasts and bays consist mainly of small reservoirs and tidal creeks, making them ideal sites for study. Suspended matters and abundant hydrophyte are covered in such areas, making it very difficult to map waters; (iii) due to the abundance of water resources (dense river networks and alpine lakes), locations in tropical rain forests and subtropical monsoon climates are also considered; (iv) as the distance between the land increases, the middle and high latitudes become prone to misclassifications related to mountain shadows or snow cover. All eight test sites are briefly summarized in Table 2.