Overview:Modeling Process

Application of GSSHA requires the creation of a variety of input files and grid data (or maps). GSSHA has been coupled with WMS in an attempt to minimize the time required to create the needed inputs. WMS is also intended to aid in model conceptualization and analysis of results. The basic parts of the modeling process are watershed delineation and grid construction, selection of processes to model, parameter assignment, channel routing assignment, running the model, and post-processing. The GSSHA User’s Manual (Downer and Ogden in preparation) provides detailed information on the modeling process.

Grid construction

GSSHA is a finite difference based model and requires a 2-D grid representation of the watershed being modeled. Construction of this grid is the first step in the development of a GSSHA model. WMS has a variety of tools that can be used for watershed delineation and grid generation from digital elevation data as well as data exported from the GRASS or Arc/Info GISs.

Process selection

Once the grid is constructed, the processes to be simulated must be selected based on the needs of the study. The processes to be simulated are selected in the GSSHA Job Control. It is best to start with a simple model with few processes and proceed to build more complicated models with several processes. The GSSHA User’s Manual (Downer and Ogden in preparation) should be consulted for more information on which processes are appropriate for which studies.

Model parameter assignment

Once the grid has been created and processes selected, model parameters governing the execution of GSSHA must be assigned. Model parameters include global variables such as simulation time and time-step, as well as the distributed parameters needed to simulate processes such as interception, infiltration, and overland flow routing. WMS facilitates global parameter assignment with dialog boxes and distributed-parameter assignment with the use of index maps and the mapping tables.

Channel routing

Larger watersheds normally require that the 1-D channel routing option in GSSHA be used. The channel routing method is an explicit diffusive-wave approximation of the St. Venant equations. The channel network is described with a series of links and computational nodes. A link can be a channel segment, hydraulic structure, or lake. WMS feature arcs are used to create the channel links and assign cross-sectional parameters to the links. In order to couple the channel routing with surface runoff, GSSHA must know which grid cells “contain” stream nodes. When WMS creates the appropriate input files, grid cells beneath the stream arcs are identified and the portion of each stream node in the each cell is written to the grid stream file. WMS has several tools for creating, numbering, and assigning parameters to the channel links and nodes.

Running GSSHA

Once all necessary input for a GSSHA simulation has been prepared, the GSSHA model is run. GSSHA is a stand-alone program that can be executed from the command line or through the WMS interface.

Post-processing

Results from successful GSSHA simulations may be viewed in WMS. The optional output created by GSSHA can include gridded time series of maps of a number of variables including: surface water depth, infiltration rate, cumulative infiltrated depth, spatially varied rainfall, soil moistures, groundwater heads, and others. Time series output of discharge and depth can be produced at all nodes in the channel network in this new format. WMS has capabilities to contour, shade, and animate the results for the entire 2-D grid and all along the stream network. Additionally, 2-D plots of any result variable versus time can be generated for any location in the grid or on the stream network. Post-processing techniques are used to help the user determine if a solution is reasonable or if further model modification is necessary.