SWORD: A Foundation for SWOT Vector Products

Abstract:

The upcoming Surface Water and Ocean Topography (SWOT) satellite mission, planned to launch in 2022, will vastly expand observations of river water surface elevation (WSE) and slope for global rivers 100 m wide and greater. For practical interpretation and application of SWOT measurements, a global prior database of river networks and reaches is required. Here, we introduce the SWOT prior River Database (SWORD). SWORD will serve as the framework for the SWOT vector products that will contain SWOT observations of WSE, slope, width, and discharge along pre-defined river reaches (~10 km) and nodes (~200 m). We generate SWORD by combining multiple global hydrography databases into one congruent product. The two primary products we use to develop SWORD are the Global River Widths from Landsat (GRWL) database and the MERIT Hydro DEM-derived hydrography dataset. GRWL contains river centerline locations, widths, and water body type information derived from Landsat imagery for rivers ≥30 m at 30 m resolution, while MERIT Hydro provides hydrologically-corrected elevations and flow accumulation at 3-arc sec resolution. We also include HydroBASINS’s Pfafstetter coding system to define the SWORD river id structure and topology. Finally, we attach dam locations from the Global River Obstruction Database (GROD), SWOT orbit track information, and global delta map extents to the GRWL centerlines to help define river reaches. When defining river reaches, we use the merged dataset information to maximize SWOT observations along a reach, while respecting hydrologic boundaries and barriers such as dam locations and tributary junctions. SWORD contains a total of 219,179 reaches and 10.7 million nodes. Globally, 87% percent of reach lengths are between 8 and 20 km with a mean reach length of 9.8 km and median of 10.0 km. Moreover, 97% of river reaches ≥10 km will have sufficient SWOT observations to provide discharge estimates at least once per orbit cycle. In addition to defining a global system of river reaches and nodes with a consistent topological structure, SWORD contains many useful hydrologic and morphological attributes. Users can use SWORD to prepare for the SWOT mission by familiarizing themselves with the database structure or by expanding on the current SWORD database to develop tools to efficiently analyze future SWOT data.