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GSSHA (Gridded Surface/Subsurface Hydrologic Analysis) is a two-dimensional, physically based watershed model developed by the Engineer Research and Development Center of the United States Army Corps of Engineers. It simulates surface water and groundwater hydrology, erosion and sediment transport. The GSSHA model is used for hydraulic engineering and research, and is on the Federal Emergency Management Agency (FEMA) list of hydrologic models accepted for use in the national flood insurance program for flood hydrograph estimation. Input is best prepared by the Watershed Modeling System interface, which effectively links the model with geographic information systems (GIS).

GSSHA uses a square-grid, constant grid-size representation of watershed topography and characteristics, similar to a digital elevation model representation. Relevant model parameters are assigned to the model grids using index maps. Index maps are often derived from soils, landuse/land cover, vegetation, or other physiographic maps.