Building a Model:Building a GSSHA Model

To this point this manual has described the theory, processes, solutions, and various inputs and outputs that can be used in the GSSHA model. This section is intended to provide the user with a step-by-step instruction on how to build a model beginning with a blinking curser. While there are many ways to construct a model with GSSHA, significant experience by the authors suggest that the following methodology is a prudent, if not the best way, to construct a GSSHA model.

GSSHA has been closely linked to the WMS v6.1 and higher, and it is strongly recommended that this software be used to develop the needed inputs for the model. As listed in Section 13, Additional Inputs, many required and optional plain vanilla tables must be created outside the WMS framework, and these can be created with a simple spreadsheet or word processing software. GSSHA is intended to be used with the Mapping Table and the related index maps described in Section 12. This is the method employed by WMS 6.1 and higher. Experience with the CASC2D model indicates that this method of assigning parameters is superior for the typical case where parameters must be assigned based on reclassification of common index maps, such as land use, soil-texture, and vegetation (Downer et al., 2002a). In addition to the WMS and a spreadsheet to build table inputs, a GIS is usually needed, or at least helpful, in developing many of the data layers that the GSSHA model requires. While the steps required to build a GSSHA model will be presented here, WMS how-to information is contained in the WMS User Manual (Nelson, 2001) and the WMS for GSSHA Primer (Downer et al., 2002b).

An essential element in successfully developing a complex GSSHA model is to start simple, get parts or processes of the model to run, and then build upon success. It is important to follow the methodology described here. WMS can be used to build a complex model all at one time. Such a model could have millions of cells, a complex stream network, unsaturated zone calculations, saturated groundwater flow, etc. Such a model would also have little chance of ever working properly because the amount of information and possible problems is overwhelming.