The GSSHA model is a significant reformulation and enhancement of the CASC2D model (Ogden and Julien 2002). The CASC2D runoff model originally began with a two-dimensional (2-D) overland flow routing algorithm developed and written in a programming language (APL) by Prof. P. Y. Julien, 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; Saghafian 1992; and Julien, Saghafian, and Ogden 1995). The FORTRAN code was reformulated, significantly enhanced, and rewritten in the C programming language by Dr. Saghafian at the U.S. Army Construction Engineering Research Laboratories. This version, named r.hydro.casc2d, was part of the Geographic Resources Analysis Support System/Geographic Information System (GRASS)/(GIS) for hydrologic simulations (Saghafian 1993). Work began in 1995 to reformulate CASC2D with the addition of continuous simulation capabilities and ability to interface with the Watershed Modeling System (WMS) graphical user interface developed by the Environmental Modeling Research Laboratory (EMRL) at 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 (1997) added overland sediment transport. Development of the CASC2D model for WMS by the Department of Defense (DoD) ended with version 1.18b, which has been the working distributed hydrologic model for the DoD (Downer et al. 2002a). CASC2D Ver. 1.18b is linked with WMS 5.1 as described by (BYU 1997a and 1997b).
While developed from the CASC2D model, the GSSHA model is inherently different in that it extends the capability of the model to simulate runoff mechanisms other than infiltration excess. Also, input of parameters for the GSSHA model is significantly different from the methods employed for CASC2D. The GSSHA model is backwards compatible with CASC2D Ver. 1.18b data sets. CASC2D Ver. 1.18b, and thus GSSHA, is not necessarily compatible with prior versions of CASC2D data sets. Also, WMS no longer supports the CASC2D input format, which is based on providing parameter values for every grid cell through a number of floating point GRASS ASCII format maps. When trying to utilize old CASC2D data sets, make sure they conform to the standards described in the GSSHA User’s Manual.
Version 2.0 of GSSHA has a new stream formulation and the ability to model wetlands, lakes, and detention basins.