Extent of available data.
The Normalized Difference Water Index is an indicator of open water. It was calculated at the 10m grid scale using imagery from the Landsat satellite and Google Earth Engine. The formula is: NDWI = (Green - Near InfraRed)/(Green + Near InfraRed). Because the NDWI relies on satellite imagery, it can be calculated over large extents. This makes it useful as a uniform metric of open water over large areas. The NDWI is also considered a useful input for some predictive landscape models.