# Tracking Dynamic Northern Surface Water Changes with High-Frequency Planet CubeSat Imagery
## 3 Methods  
### Water Classification  
To classify open water from surrounding land and vegetation, a 10-step procedure was used (Figure 2). First, Planet imagery were manually checked for clouds. Although Planet does provide a cloud index value indicating the percent of cloud-covered pixels for each image, the index performance is not consistent in snow and ice conditions at present and a cloud mask product is still unavailable. For every image where clouds were present, cloud cover and cloud shadow masks were manually digitized and masked out. We ignored all imagery with a solar zenith angle greater than 78 degrees or with cloud cover greater than 70%. We then calculated a Normalized Difference Water Index (NDWI) for each image, defined as the difference between the green and near-infrared reflectance (Band 2-Band 5 for RapidEye, Band 2-Band 4 for PlanetScope) divided by the sum of the green and near-infrared (Band 2 + Band 4/5) [34]. This yields an _NDWI_ index value between \(-1\) and \(1\), where open-water pixels approach 1 and are more easily distinguishable from non-water pixels. For water classification, we used a water fraction approach adapted from [29], who originally applied this method to shortwave infrared (1550-1750 nm) Landsat TM and ETM+ imagery. In complex floodplain environments such as the Yukon Flats where inundated vegetation and mixed land/water pixels are common, a multi-step, non-binary water classification enables more accurate water area extraction through accounting of fractionated water pixels [29]. First, Otsu adaptive thresholding [35] was used to delineate potential water bodies in a maximum extent image to create an initial water mask (Figure 3a). All water bodies less than 2500 m\({}^{2}\) were removed from this initial mask. The water mask was then dilated using a 60 m disk object and converted to polygons, yielding an initial buffered polygon mask containing both the maximum extent of the water body and some surrounding land and vegetation [29]. These buffered lake polygons were then intersected with the NDWI for each image, generating a histogram of NDWI pixels within the buffered water mask (Figure 3b). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Image ID** & **Sensor** & **Date** & **Observation** \\ \hline
20160623_220215_669714_RapidEye-4 & RapidEye & 23 June & 1 \\
20160624_220552_669714_RapidEye-5 & RapidEye & 24 June & 2 \\
20160725_215412_669714_RapidEye-3 & RapidEye & 25 July & 3 \\
20160813_215232_669714_RapidEyeye-3 & RapidEye & 13 August & 4 \\
222535_0669714_2016-08-16_16,0 ed & PlanetScope & 16 August & 5 \\
20160817_220351_669714_RapidEye-2 & RapidEye & 17 August & 6 \\
229647_0669714_2016-08-27_0e14 & PlanetScope & 27 August & 7 \\
229821_0669714_2016-08-27_0e20 & PlanetScope & 27 August & 7 \\
20160901_215044_669714_RapidEye-3 & RapidEye & 1 September & 8 \\
232753_0669714_2016-09-02_0e19 & PlanetScope & 2 September & 9 \\
232758_069714_2016-09-02_0e30 & PlanetScope & 2 September & 9 \\
20160907_215525_669714_RapidEye-4 & RapidEye & 7 September & 10 \\
236414_0669714_2016-09-07_0e26 & PlanetScope & 7 September & 11 \\
20161001_215759_669714_RapidEye-4 & RapidEye & 1 October & 12 \\
234040_0669715_2016-09-02_0e0e & PlanetScope & 2 September & Validation \\
234040_0669716_2016-09-02_0e0e & PlanetScope & 2 September & Validation \\
238490_0669716_2016-09-02_0e26 & PlanetScope & 2 September & Validation \\
20160908_215904_669817_RapidEye-5 & RapidEye & 8 September & Validation \\
20160908_215905_669816_RapidEye-5 & RapidEye & 8 September & Validation \\ WV02_20160830213545_103001005A6E6600_16 AUG30213545-P1B5 & WorldView-2 & 30 August & Validation \\ WV03_20160908220049_1040010021B5F000_16 SEP08220049-P1B5 & WorldView-3 & 8 August & Validation \\ WV03_20160908220050_1040010021B5F000_16 SEP08220050-P1B5 & WorldView-3 & 8 August & Validation \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of images. The water fraction of each pixel was then determined by applying dynamic land and water thresholds to each image [29].