## 2 Materials and Methods
### Dove Satellite Image Pre-Processing  
Planet Dove imagery was obtained from Planet Explorer ([[https://www.planet.com/explorer/](https://www.planet.com/explorer/)]([https://www.planet.com/explorer/](https://www.planet.com/explorer/))) and processed as shown in Figure 2. The image product is delivered as a continuous, split-frame strip, with half-frames containing red-green-blue (RGB) and near-infrared (NIR) imagery, with an observation interval of approximately 0.5 s [49].  
We used Dove ortho scenes (Planet Labs image product Level 3B: 4-Band PlanetScope Scene; spatial resolution = 3 m) from 1 July 2017 to 31 December 2019, which are orthorectified and radiometrically and sensor corrected. The geometric correction was based on digital elevation models with post-spacing between 30 and 90 m, with a root mean square error specification of <10 m [41]. We removed bad pixels using the unusable data mask provided with the products. We converted radiance to top-of-atmospheric reflectance using the scaling factor provided with the products. Dove satellites have been observed by two sensor types: Conventional type (PS2) and next-generation type (PS2.SD). Since these sensors have different relative spectral response curves, we transformed the next-generation-type values to match those of the conventional type using the coefficient described in the metadata.  
Mismatches in the band-to-band registration were confirmed by visual inspection, and all images were re-registered by the zero-mean normalized cross-correlation method [50] with edge enhancing by a Laplacian filter; images with a shift of more than 10 pixels were excluded from the analysis. To maintain the continuity of pixel-based time series change, the registration between images was processed by the same method as the band-to-band re-registration, and was based on the reference image with an accurate position; images with a shift of more than 10 m were excluded from the analysis.  
Figure 2: Flowchart of image analysis.  
We performed simple dark pixel subtraction to obtain surface reflectance [51]. Assuming the presence of a pixel with zero reflectance (i.e., the radiance recorded by the sensor is solely from atmospheric scattering), the minimum pixel value was subtracted from those of all other pixels to remove the radiance derived from atmospheric scattering. Pixels from deep ocean areas were used for this correction. In addition, we normalized the value of each band according to [PERSON] et al. (2002) [52] by dividing it by the summed value of all bands. Even on the same ground surface, the spectra observed by the satellite vary considerably because of the influence of the surface slope and atmospheric effects, but the shape of the spectra can be regarded as similar [52]. This characteristic allows suppression of the topographic and atmospheric effects by normalizing with the total sum of reflectance [53].  
These pre-processed Planet Dove images were also used to evaluate the temporal variation of the pixels available for analysis. Specifically, the acquisition rates of analyzable pixels (i.e., not cloud pollution pixels) in the five days composite image were calculated.