The GSSHA model is a significant reformulation and enhancement of the CASC2D model. The CASC2D runoff model began with a two-dimensional overland flow routing algorithm developed and written in APL (A Programming Language) by Professor P.Y. Julien at Colorado State University. The overland flow routing module was converted from APL to FORTRAN by Dr. Bahram Saghafian, then at Colorado State University, with the addition of Green & Ampt infiltration and explicit diffusive-wave channel routing (Julien and Saghafian, 1991; Julien et al., 1995). The FORTRAN code was reformulated, significantly enhanced, and re-written in the C programming language by Dr. Bahram Saghafian at the U.S. Army Construction Engineering Research Laboratory (CERL). Implicit channel routing was added to CASC2D by Fred L. Ogden (Ogden, 1994), formerly at Colorado State University, then Associate Professor, Department of Civil and Environmental Engineering, University of Connecticut, Storrs, Connecticut, now Cline Distinguished Chair of Engineering, Environment and Natural Resources, Department of Civil & Architectural Engineering and Haub School of Environment and Natural Resources, University of Wyoming. This version, named r.hydro.casc2d, was part of the GRASS GIS for hydrologic simulations (Saghafian, 1993). Work began in 1995 to re-formulate CASC2D with the addition of continuous simulation capabilities, including an interface with the Watershed Modeling System (WMS) interface developed by Brigham Young University (BYU). This version, known as CASC2D for WMS, is distinguished from its predecessors by the addition of a number of new capabilities, numerous improvements and bug fixes, and a more stringent copyright. Johnson et al. (2000) added overland and channel transport to the CASC2D model.
The GSSHA model is a direct result of the dissertation work of Charles W. Downer, USACE-ERDC-CHL (Downer, 2002), and was developed from a need to simulate watersheds with runoff producing processes other than Hortonian flow. While the capability of the CASC2D model was included in GSSHA (many of the processes were taken directly from CASC2D) the continuous nature of the GSSHA model resulting in a need to develop an entirely new model. The first release of the model respresents a fully coupled surface water/groundwater simulator with sediment transport capability (Downer and Ogden, 2006). Since the original development of GSSHA, a myriad of improvements and capabilities have been added to model: coupling of the Green and Ampt with redistribution (GAR) model to the saturated groundwater (Downer et al, 2002), improved channel routing including non-orthogonal stream networks, reservoirs, detention basins and hydraulic structures (Downer et al., 2008), improved soil mositure accounting for use with GAR (Downer, 2008), constituent transport (Downer and Byrd, 2007), and coupling of constituent transport with the Nutrient Simulation Model (NSM) (Johnson and Gerald, 2007). The new features have been tested in a variety of watersheds (Downer et al., 2002, Downer 2008a, Downer 2008b, and others).