Model Formulation:Inputs

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GSSHA is a distributed-parameter, process-based model that requires the user to select the processes to be simulated and then provide the model with the data necessary to drive the selected options. Three types of input data are used. An ASCII text project file is used to provide the basic project information, select processes to be simulated, assign simulation parameters, and locate data files, tables and maps. Spatially distributed parameters can be assigned with maps of ASCII gridded data with a parameter value in each grid cell, with index maps and tables of parameter values that relate to the index maps, or with uniform values in every cell. Typically the data required to assign parameter values in every cell is not available. Standard practice in the application of GSSHA has been to develop index maps based on available data sources of land use, soil type, and vegetation. Typically these maps are combined to create a master land use/soil type/vegetation index map that can be used to assign all parameter values.

Parameters for each index map are then assigned using tables that reference the values in the index maps. A detailed description of using the index maps and Mapping Tables to build a GSSHA model is provided in Section 11. If available, the detailed maps containing parameter values in each cell, as described in Ogden (2000), may be input in lieu of the index maps and table.

Since distributed parameters may be assigned with a single uniform value in the project file, a table value linked to an index map, or with an ASCII map with a parameter value for every grid cell the model has been developed to prioritize parameter specification. While internally assigning parameter values the GSSHA model looks for the most detailed information first, GRASS ASCII maps, the second most detailed information second, table values linked to index maps, and finally a single uniform value from the project file. Once data from one of these sources is located, the search ends and the parameter values are assigned inside the GSSHA model. While this rule is generally applicable it is prudent not to specify multiple sources of the same parameter value in the project file. This avoids possible confusion and improper assignment of parameter values.

The Watershed Modeling System (WMS) interface, developed at the Environmental Modeling Research Laboratory (EMRL) at Brigham Young University, is recommended for developing input files and viewing output from the GSSHA model. The WMS produces GSSHA specific files from general Geographic Information System (GIS) data. WMS does not replace the functions of a GIS, though it can accept information in a variety of GIS formats. GSSHA relies on the GRASS ASCII data file format for storing spatially distributed data. The GRASS GIS is very helpful in the preparation of GSSHA data sets. Users of ARC/INFO and ARCVIEW can export data to GSSHA through the WMS interface. For more information about WMS, DoD and EPA personnel should contact:

XMS Model Support
Hydrologic Systems Branch
Coastal and Hydraulics Laboratory
Engineer Research Development Center
3909 Halls Ferry Road
Vicksburg, MS, 39180
(601) 634-4286

Other users seeking information about WMS, should contact:

3210 N Canyon Road
Provo, Utah 84604

GSSHA User's Manual

2 Model Formulation
2.1     Processes Simulated
2.2     Time Steps and Process Updates
2.3     Inputs