WMS:Using GIS Data to Map Parameters

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Part of the MODRAT interface in WMS is an automated mapping procedure to define rainfall depth, soil type, percent impervious, and DPA zones from digital data layers. The procedure requires a digital map of the parameter, such as rainfall depth, to be imported into WMS. Digital maps of rainfall depth (isohyetal maps), soil types, land use (percent impervious), and DPA zones are all available from the LACDPW. Once the map is in WMS, functions have been created which will overlay the parameter map on the watershed model and assign the appropriate value to each sub-basin. These functions can be accessed by choosing Map Attributes from the MODRAT menu; the following dialog controls the mapping procedure:

Map MODRAT Attributes dialog

Computation Type

Three parameters may be mapped from this dialog:

LA County soil numbers – Determines the predominate soil type in each sub-basin of the watershed.

To compute the soil number for each sub-basin of the MODRAT model, read in a soil type data layer. Soil type data is generally stored in a GIS, the best way to read it into WMS is though a Shapefile. To import soil type data for use with MODRAT, create a Soil Type Coverage, then import the polygon file and ensure that the Soil Number field (often called CLASS_NO by the LACDPW) is assigned to LA County Soil type field of WMS. Once the soil type data is present in WMS, compute and assign soil numbers using the Map Attributes command from the MODRAT menu.

LA County land use – Determines the predominate land use type in each sub-basin. The percent impervious is then computed from the land use type.

To compute the Percent Impervious for each sub-basin of the MODRAT model, read in a land use data layer. Land use data is generally stored in a GIS, the best way to read it into WMS is though a Shapefile. To import land use data for use with MODRAT, create a Land Use Coverage, then import the polygon file and ensure that the Percent Impervious field (often called IMPERV_ by the LACDPW) is assigned to LA County Soil land use field of WMS. Once the land use data is present in WMS, compute and assign % impervious using the Map Attributes command from the MODRAT menu.

LA County DPA zones – This option is currently not in use. The computation will determine which DPA zones exist in a sub-basin, and the area corresponding to these zones.

LA County rainfall depth – Mapped using the GIS calculator.

To compute the rainfall depth on the sub-basins of the MODRAT model, read in a rainfall depth data layer. The isohyetal maps for LA County have been digitized and stored in ARC/INFO® Grid format by the LACDPW; these files can be found in the /MODRAT/Rain/ folder installed with WMS. Import this data by choosing Open from the File menu, then choose Rainfall Depth Grid (*.*) as the file type.
Alternatively, if there is other rainfall data in vector format (ArcView® Shapefiles, DXF, etc.), import these into WMS. Make sure to create a Rainfall Coverage (see the Coverages dialog) and map the appropriate database fields when importing this data.
Once the rainfall depth data is present in WMS, compute and assign depths using the Compute GIS Attributes command from the Calculators menu.

Coverage Type

MODRAT parameters may be mapped from two types of data: gridded (raster) data or coverage (vector) data. These options will be inactivated or activated in accordance with the type of data which exists in WMS.

Computation Step

Mapping attributes in WMS is a grid-based operation; when a parameter is mapped, each basin is broken into an imaginary grid, then each grid cell is assigned the appropriate parameter value. Either an area-weighted average of the parameter value or the predominate value can then be determined.

The computation step value defines the size of each imaginary grid cell. A larger value creates a larger cell size and thus a lower resolution grid. For example, a computation step of 100 will create cells 100 X 100 whereas a step of 50 will create smaller cells (50 X 50). Computation step may need to be adjusted depending on the size of the sub-basins in the watershed. WMS sets the default value in the Computation Step box to a number that will divide the smallest sub-basin in the watershed into a grid 25 cells X 25 cells in resolution. Generally, the default is adequate. If greater accuracy is required, a lower computation step should be used. Note however, a lower computation step will require more computational time.

When gridded attribute data are used (a rainfall depth grid, for example), the computation step will be the same as the grid cell size in the attribute dataset and cannot be changed.

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