Confluence: Cloud-based global river discharge estimation software for the SWOT satellite

The NASA/CNES Surface Water and Ocean Topography (SWOT) mission generates unprecedented volumes of hydrology data requiring the use of cloud computing to produce river discharge estimates for SWOT observations. We present Confluence, a cloud-based workflow that produces discharge on over 150,000 river reaches via 16 workflow stages with unique, hydrologic, and mission requirements. The stages are written in a variety of software languages and were developed over the past decade by more than 30 different hydrologists. A large challenge has been standardizing and operationalizing these diverse stages into a single unified framework. Doing so and providing publicly archived outputs ensures standardization and eliminates redundant processing. We show Confluence costs ~$6,000 to run globally, taking ~4d14hr to complete. Discharge results are consistent with hydrologic and mission expectations. Confluence is an operational software designed to provide value to both NASA and the hydrology community for as long as SWOT is in orbit.