# Application of Image Segmentation in Surface Water Extraction of Freshwater Lakes using Radar Data
## 4 Conclusions  
Through this research we mapped the spatio-temporal variation of Poyang lake in January from 2017 to 2020 and showed that the surface water area fluctuated annually and tended to shrink both in the center and boundary of the lake over the past four years. The variation was consistent with related Poyang lake research for earlier decades. Our mapping approach involved a novel implementation of the Otsu Method and processing of Sentinel-1 data in Google Earth Engine. GEE performed well as a powerful cloud computing platform to implement an exhaustive searching algorithm. We provided detailed mathematical explanation to enumerate the advantages and limitations of the Otsu Method that were not clearly indicated in previous remote sensing research. We also demonstrated that the Otsu Method  
Figure 4: Inundation Area of Poyang lake in Winter of 2017, 2018, 2019 and 2020 (from left to right).  
Figure 3: An Example of Histogram of VV Band in dB value (4 January 2020), the x-axis represents that backscatter coefficient is calculated in dB scale, the y-axis represents that how many pixels have the same dB value in a bin and the interval of bins is 0.5.  
can be an effective classifier for threshold auto-selected algorithms to extract water surface with use of Sentinel-1 data. As a result, the Otsu Method has potential to be applied to other water related studies, such as water extraction applications for other lake regions, water pollutant detection for environmental assessment and aquatic habitat mapping for ecological conservation, using the open access scripts of the threshold algorithm contributed here.  
In the future, to reduce the influence of linear nonseparability nature of the data, the 2D Otsu Method [51] can be applied. However, since the Otsu Method is an unsupervised method, we have not compared its performance with supervised learning algorithms. The supervised learning algorithm requires training data that is unavailable for the January of 2020 at this time. In addition, the Otsu Method is affected by the penetration ability of single C-band radar signal so that it is difficult to capture water beneath the vegetation. To advance our research toward mapping aquatic habitat availability, we recommend the comparison between supervised and unsupervised methods by using different series of imagery to discriminate vegetation zones. This next step will allow us to identify and project the spatial distribution of available foraging habitat under varying hydrological conditions for species of concern like Siberian crane and other aquatic organisms.  
Conceptualization, [PERSON] and [PERSON]; methodology, [PERSON] and [PERSON]; software, [PERSON]; validation, [PERSON], [PERSON] and [PERSON]; formal analysis, [PERSON] and [PERSON]; investigation, [PERSON] and [PERSON]; resources, [PERSON] and [PERSON]; data curation, [PERSON]; writing--original draft preparation, [PERSON] and [PERSON]; writing--review and editing, [PERSON] and [PERSON]; visualization, [PERSON]; supervision, [PERSON]; project administration, [PERSON] and [PERSON]; funding acquisition, [PERSON] All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Incubator program of Institute for Regional and International Studies at University of Wisconsin-Madison, grant number AAC3592.  
We appreciate the award of Incubator program of Institute for Regional and International Studies at University of Wisconsin-Madison to support our research and the collaboration with [PERSON] of SAGE (Sustainability and the Global Environment) at the University of Wisconsin-Madison. We also gratefully acknowledge Poyang lake National Nature Reserve staff who collaborated with us to collect ground truth data.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} SAR & synthetic aperture radar \\ GEE & Google Earth Engine \\ API & application programming interface \\ \end{tabular}