Enhancing the SWOT A Priori Global River Database

The upcoming Surface Water and Ocean Topography (SWOT) satellite mission, planned to launch in 2021, will vastly expand observations of river water surface elevation (WSE) and slope. For practical interpretation and application of SWOT measurements, a global a priori database of river networks and reaches is required. A number of groups have put together an initial version of the SWOT a priori global river database (SWAG) by combining the Global River Widths from Landsat (GRWL) and the Hydrological data and maps based on Shuttle Elevation Derivatives at multiple Scales (HydroSHEDS) databases. GRWL contains river centerline locations and widths derived from Landsat imagery for rivers ≥30 m at 30 m resolution. HydroSHEDS provides elevations, flow accumulation, and river identification numbers at 3 arcseconds resolution. Currently, SWAG is available in two vector products (point and polyline) and encompasses defined river reaches for the majority of the globe (up to 60°N) with hydrologic attributes including river width, elevation, slope, planform, river network topology (nodes only), flow accumulation, and dam and reservoir locations from the GRanD database. However, the SWAG database displays problems in complex river environments, contains poor reach length distributions, and does not cover Arctic river systems. Here, we present a plan for improving and expanding SWAG by using the most advanced remote sensing-derived products available. To produce the updated river database, we link two existing global hydrography products: GRWL and the new Global Flow Direction Map that includes flow direction, flow accumulation, and hydrologically corrected WSEs based on the MERIT DEM. The final SWAG database will contain updated river reach networks, topology, and attributes for the entire globe. We plan to build SWAG's river topology based on an adaption of the Pfafstetter coding system, while reaches will be defined based on the SWOT mission requirements and a number of hydromorphological characteristics such as tributary junctions, sinuosity, planform, and presence of dams.