Transforming SWAT as a tool for fast and economic flood risk mapping in large basins

There are numerous model coupling frameworks that integrate hydrodynamic and hydrologic models to produce flood inundation maps; however, most of them come with high computational complexities and costs. Recent advancements in hydrogeomorphic, topography-based inundation mapping approaches can provide parsimonious ways of addressing these computational challenges and therefore are suited for scenarios requiring rapid decision making and situational assessment. Despite such opportunities, research to coupling these topography-based approaches with a hydrologic model to instantaneously generate streamflow and the corresponding flood inundation extents across large river networks is still in infancy. In this work, we attempted to fill this gap by coupling a topography-based approach Height Above the Nearest Drainage (HAND) with a globally used hydrologic model Soil and Water Assessment Tool (SWAT). Through an automatic reproducible workflow, the prototype SWAT-HAND model allows users to define their region of interest (subbasins within an existing SWAT model), generate hydrogeomorphic properties of the floodplain from SWAT’s geodatabase and the topography data therein, and subsequently apply SWAT-simulated streamflow to generate flood inundation maps. We successfully tested this workflow in the United States Gulf Coast basins comprising around 415,000 km2 drainage area and a river network with 15,000 individual stream segments. Because SWAT is one of the few globally applicable process-based models capable of supporting both hydrology and ecology research, our work to couple SWAT with a flood mapping tool will benefit studies beyond traditional flood management and risk mapping, by making floodplain ecosystem, water quality, and habitat research across large river network scales highly efficient.