One of the greatest assets of distributed hydrologic models like GSSHA is the ability to spatially distribute the parameters for processes, such as overland flow and infiltration, over the watershed. Assigning values, grid cell by grid cell, is tedious and makes all but the simplest and smallest models impossible. Using WMS, GIS coverages (layers) representing land use and soil texture can be used to assign model parameter values to groups of grid cells sharing the same characteristics.
The basic process of assigning spatially distributed parameters consists of the following steps:
- Import a GIS coverage for land use, soil texture, or vegetation type (generally this should be in the ArcView® shapefile format).
- Map the land use, soil texture, or vegetation ID to the grid cells using a spatial overlay operation.
- Define parameter values (e.g., surface roughness, hydraulic conductivity, etc.) for the unique ID numbers.
A given soil texture/land use (STLU) index map can be used to assign multiple parameters. Since most of the grid cell parameters can be referenced to either land use or soil properties, a given simulation generally requires only a single index map of each. A combination land use and soil texture index map makes it possible to relate a parameter value to the combination of land use and soil texture (for example infiltration or erosion). Once the index maps are defined, parameter values are assigned to the IDs of the index maps. The combination of the index maps, with ID numbers, and the mapping tables, with the parameter values, are used by GSSHA to internally assign parameter values to each grid cell.
The principle means of modifying and creating index maps is in the Index Map dialog but the index maps are now also able to be accessed through the Project Explorer. When an index map has been created or read into WMS a folder appears in the Project Explorer, named Index Maps, that contains all of the index maps for the simulation. Index maps can then be treated like regular datasets; they can be contoured, renamed, deleted, and edited.
Some GSSHA input parameters change from cell to cell and cannot be characterized using index maps. Continuous maps, which are stored as datasets of a 2D grid in WMS, are used where index maps are not appropriate. Examples of GSSHA input parameters that use continuous maps include soil properties for the groundwater processes and ground surface, bedrock, and water table elevations. Contours for each of the continuous maps are displayed on the 2D grid according to the contour options selected for the dataset. Continuous maps are specified as GSSHA input in either the Sub-surface Parameters (Groundwater) dialog or in the Continuous Maps tab of the GSSHA Map Table Editor dialog.
The Continuous – Grid tab of the GSSHA Maps dialog has three options for generating continuous maps:
- Using GIS Data: Select an input coverage and attribute. Enter a name for the continuous map in the dataset name field. Click the GIS Data → Dataset button to create the dataset.
- Using the Data Calculator: This option is useful for mathematically manipulating existing continuous maps. Click the Data Caluclator... button to open the Data Calculator window.
- Reclassification: Reclassify index map values to create a continuous map, if appropriate. Click the Reclassify Index Map→Dataset... button to begin.
Stream Index Maps
Sometimes, a project might have a stream-based index map instead of a grid-based index map. When defining a stream index map, define an index value greater than zero to each link in the model. There is not currently an option to automatically define stream index map values from grid or map module data. Turn on the stream link IDs in the Display Options dialog and define an index for each of these IDs. Assign index values to each of the stream links in the model. Then use stream index maps to define Manning's roughness, nutrient values, and other values in the GSSHA Map Table Editor.
WMS – Watershed Modeling System
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