WMS:Compute GIS Attributes

Compute GIS Attributes dialog

WMS allows defining coverages, grids, or GIS layers that define boundaries for different soils, land uses, rainfall depths, and DPA zones. Typically this information is imported from standard GIS formatted files. This GIS data is overlaid with drainage basin boundaries to compute area-weighted composite model parameters for each sub-basin.

In summary the following data are used for computing composite model parameters:

• Basin boundaries from the drainage boundary polygons on a drainage coverage.
• Land use IDs are supplied from a land use coverage in the map module or as DEM (a gridded) attributes.
• Soil IDs are supplied from a soil type coverage in the map module or as DEM (a gridded) attributes.
• A user defined table relating land use IDs to the parameters being mapped (for example SCS curve numbers, Green & Ampt parameters, etc.).
• A user defined table relating soil IDs to the parameters being mapped (for example runoff coefficients, Green & Ampt parameters, etc.).

Any combination of data sources for computation can be used (i.e. drainage coverage, land use grid, soil type coverage, etc.). If a land use or soil coverage is used, then the parameters for each polygon ID can be defined using the Attributes command in the Feature Objects menu (in the Map module) with the proper coverage being active. However, if grid attributes are used for the soil or land use ID definitions, then one way to define the parameters for each ID is by creating the mapping file with a text editor and then importing in the Compute GIS Attributes dialog (see Mapping File Formats).

NRCS soils files that are available for download on the internet often contain the hydrologic soil group attribute in a separate database file than the feature polygons themselves. The tools in the GIS module that allow joining tables or specific attributes from tables to the feature polygons can be used to link the hydrologic soil attribute to the polygons. See more information in the section on joining tables.

Once the polygon coverages and/or grid files for land use or soil types are defined, and the mapping tables set up, the project is prepared to compute parameters for one of the available methods.

When computing GIS attributes the results are automatically stored with every applicable model supported in WMS. The computation method options are described in detail below.

SCS Curve Numbers (CN)

A composite curve number for a basin can be computed by taking an area-weighted average of the different curve numbers for the different regions (soil type and land use combinations) within a basin.

Required Inputs:

• Land use data
• Soil type data (hydrologic soil group A, B, C, or D, where the infiltration capacity decreases from A to D)
• Table relating land use IDs to curve numbers for each hydrologic soil group. Download one of the following tables and use them as a template for land use data.
  • If using vector landuse data available as a shapefile from the USGS, download and use this table.
  • If using NLCD 2006 land use data as a land use grid, download and use this table.
  • If using NLCD 2015 land use data as a land use grid, download and use this table.
  • If using Global Land Cover data as a shapefile, download and use this table.
  • If using Corine (European) land use data as a land use grid, download and use this table.

One good source for curve numbers is the TR-55.pdf file.

Runoff Coefficients

Composite runoff coefficients are computed using an area-weighted average of all runoff coefficients that overlay each drainage basin. Soil data can also be used to infer runoff coefficients.

Required Inputs:

• Soil data
• Table relating soil IDs to runoff coefficients, OR
• Runoff coefficient coverage

The runoff coefficient table must be defined to relate soil type IDs to runoff coefficient values. A very simplified file named soiltype.tbl is provided with the distribution of WMS. This file can be edited to supply user defined values or a new table with the same format can be created. See Soil Type Runoff Coefficient Files to view the format of the table.

Computation Step

The computation step is only used when defining composite curve numbers or runoff coefficients for a drainage coverage. If a TIN is used, then individual basins are composed of several triangles and each triangle can be assigned a land use and/or soil type. However, for a drainage coverage, each basin is typically represented by a single polygon. The computation step is used to divide each basin polygon into a number of square cells (the computation step being the length of a side) that are each assigned a land use and or soil type ID. The smaller the step length the more accurate the composite number will be, but the more time consuming the computation as well.

Green & Ampt Losses

Maricopa County, Arizona, and other regions often use the Green & Ampt infiltration options within HEC-1.

Required Inputs:

• Land use data
• Table relating land use IDs to initial abstraction and percent impervious
• Soil data
• Table relating soil IDs to hydraulic conductivity, soil moisture deficit, and wetting front suction

The parameters required to define these values must be entered for the appropriate coverage.

HSPF Segments

Required Inputs:

• Land use data
• Table identifying land use IDs as either pervious or impervious

Rainfall Depth

A composite rainfall depth is computed using a rainfall depth grid.

Required Input:

• Rainfall depths

Debris Production (Los Angeles County)

Debris production and bulking rates for burned simulations in Los Angeles County are computed by overlaying DPA zones and land use data with drainage basins.

Required Inputs:

• DPA zones are used for determining debris production and bulking rates
• Land use data is used for determining whether or not debris will be produced according to the percent impervious value

Orange County (CA) Losses

WMS can compute both the Fm and Ybar loss parameters used for hydrology in Orange County, CA.

Required Inputs:

• Land use data
• Soil type data
• Table relating land use IDs percent impervious and to curve numbers for each hydrologic soil group

Maricopa County (AZ) $m$ and $b$ Values

Required Inputs:

• Land use data
• Table relating land use IDs to m and b values

HEC-HMS SMA Losses

Required Inputs:

• Land use data
• Table relating land use IDs to canopy and surface storage
• Soil data
• Table relating soil texture to soil and groundwater storage, infiltration rates, and percolation rates

Related Topics:

• Hydrologic/Hydraulic Calculators
• Coverage Overlays
• Importing Shapefiles
• Land Use
• Soil Type
• Coverages
• Mapping Tables
• Joining Tables