# Tracking Dynamic Northern Surface Water Changes with High-Frequency Planet CubeSat Imagery
## 4 Results
### Temporal Changes in Water Inundation Area  
Over the course of the mid to late summer season (23 June to 1 October) we identified a total of 470 lakes \(>\)2500 m\({}^{2}\) within our 625 km\({}^{2}\) study area. These water bodies received between 4 and 9 cloud-free Planet observations over this period, with an average of 6.0 cloud-free observations. The maximum observed lake area is 1.23 km\({}^{2}\), and for all lakes \(>\)2500 m\({}^{2}\), the mean maximum lake size is 0.0735 km\({}^{2}\) or 73,470 m\({}^{2}\). On average, the studied lakes lost 0.028 km\({}^{2}\) of surface water area, or 63% of their maximum observed area over the summer season. However, not all water bodies followed this average trend, with some shrinking, some expanding, and others stable. Because a lake's areal change tends to scale with its area [40], we divided lakes into two categories, i.e., 2500 m\({}^{2}\)-10,000 m\({}^{2}\) and  
Figure 5: Lake shorelines delineated from PlanetScope (blue), RapidEye (green) and WorldView (red) imagery. PlanetScope and RapidEye shorelines shown represent automated extraction of \(>\)75% water area. WorldView shorelines are manually delineated. Imagery © 2016, DigitalGlobe, Inc., Westminster, CO, USA. lakes >10,000 m\({}^{2}\). On average, lakes <10,000 m\({}^{2}\) lost 4280 m\({}^{2}\) or 79% of their maximum observed area as the summer progressed, whereas lakes >10,000 m\({}^{2}\) lost 42,900 m\({}^{2}\) or 52% of their maximum area. The normalized variability for each lake, which we defined as the mean change in lake area between each observation, decreased significantly with area. For all lakes >2500 m\({}^{2}\), the mean variability is 47%, but the mean variability is 67% for lakes between 2500 m\({}^{2}\) and 10,000 m\({}^{2}\), and 33% for lakes >10,000 m\({}^{2}\). Yukon River effective width W\({}_{\rm e}\) varied most strongly on wide reaches surrounded by sediment bars and least on narrow constrictions (Figure 6a,b). Effective widths of Reach 3 and Reach 7 changed little over the observation period, whereas other reaches increased in mid-August (observations 4-6, Figure 6b,c) before decreasing to a seasonal low in October. Effective width is correlated with discharge [5,7,36,37,41] so to assess the validity of Planet W\({}_{\rm e}\) measurements we compared them with Yukon River discharge data from the USGS station at Eagle. We find that a daily discharge time series corresponding to our dates of observation correlates well with our effective width measurements, yielding an r\({}^{2}\) of 0.75 (\(p<0.01\)) (Figure 6c,d). While most of the Yukon Flats lakes studied here decreased in surface area over the observation period, heterogeneous patterns of lake area increase and stability are also evident (Figure 7). Overall, 83% of the studied lakes decreased in area between 23 June and 1 October 2016, with 22% losing at least half of their surface area. Between 23 June and 17 August, however, 56% of lakes increased in surface area, and 89% of lakes shrunk between 17 August and 1 October. This summertime peak in mid-August and subsequent decrease in surface extent exhibited by more than half of lakes is also reflected in the time series of river effective width (Figure 6), signifying hydrologic connectivity of these lakes to the main-stem Yukon River. Figure 6: (**a**) Map of seven river reach segments and (**b**) time series of corresponding river effective width. Color and number of time series in (**a**) corresponds to color and number of reach in (**b**); (**c**) Time series of effective width for river reach #4 and corresponding Yukon River discharge at Eagle and (**d**) scatter between effective width and Yukon River discharge (Q) at Eagle The equation of the best fit curve in panel (**d**) (shown in red) is W\({}_{\rm e}\) = 10.9\(Q^{0.38}\) (R\({}^{2}\) = 0.75).