Resolvable estuaries for satellite derived water quality within the continental United States

There are a wide range of satellite sensors used for water quality measures with various spatial resolutions. Here, we focus on spatial resolution because some estuaries are smaller than the satellite nadir pixel width, and multiple pixels within an estuary are required for quality assurance . We used the United States Environmental Protection Agency's (US EPA) Estuarine Data Mapper polygon set to determine the spatial resolution required to reliably image every estuary and sub-estuary in the continental US. Kilometre resolution ocean colour sensors resolve 25% of estuaries and 36% of sub-estuaries with a 3 x 3-pixel grid. The 300 m Ocean and Land Colour Instrument (OLCI) on Sentinel-3 resolves 53% of continental US estuaries with a 3 x 3-pixel grid, and 85% of sub-estuaries. Currently orbiting 30 m resolution land imaging satellites are sufficient to resolve all US sub-estuaries and all estuaries in the Atlantic, 89% in the Gulf of Mexico, and 54% in the Pacific with a 3 x 3-pixel grid. However, 45% of Pacific estuaries cannot be resolved at 30 m resolution, because they have narrow riverine morphology. Our results indicate that a resolution of <= 15 m will be necessary to resolve these narrow riverine estuaries, which is only currently available with commercial satellites.