Computing Composite CN and Runoff Coefficients

SCS curve numbers are typically determined by using a table relating land use to hydrologic soil type. The hydrologic soil type can be either A, B, C, or D, where the infiltration capacity decreases from A to D. The curve numbers for each soil group for a given land use are given in most books on hydrology (a very good source is the TR55 .pdf file included with the documents on the WMS distribution CD or downloaded file). 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. The same can be done to compute a composite runoff coefficient, only in this case a table relating soil ID to runoff coefficient is used rather than a table of curve numbers.

Maricopa County, Arizona, and other regions often use the Green & Ampt infiltration options within HEC-1. Green & Ampt parameters may also be mapped from soil type and land use coverages. Initial abstractions and percent impervious are determined from a land use coverage while a soil coverage is used to define hydraulic conductivity, soil moisture deficit, and wetting front suction. The parameters required to define these values must be entered for the appropriate coverage.

WMS allows you to define a hydrologic soil coverage or grid and land use coverage or grid that define boundaries for the different soil types and land uses. Typically this information is imported from standard GIS formatted files. These data are then mapped to either drainage coverage polygons or TIN triangles (depending on how you have created your watershed and sub-basin boundaries) and used in the computation of a composite curve number. In summary the following data are used for computing composite CNs, runoff coefficients, or Green & Ampt parameters:

Basin boundaries can be defined with either TINs or feature objects (remember that boundaries defined from a DEM are converted to feature objects).
Land use IDs can be supplied from a land use coverage in the map module or as DEM (a gridded) attributes.
Soil type IDs can be 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 the different data required 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 Composite Runoff Coefficients/CNs dialog (see Mapping File Formats).

Another way to define land use or soil type is to define a land use or soil type coverage. The Attributes command in the Feature Objects menu (in the Map module) can then be used to assign IDs to the individual polygons.

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 you to join tables or specific attributes from tables to your 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, you are prepared to compute parameters for one of the available methods.

The computation method determines whether composite curve numbers, composite runoff coefficients, Green & Ampt infiltration parameters will be computed. This affects the type of mapping table that will be required and also where results are stored. When computing curve numbers the values are automatically stored with HEC-1, TR-20, TR-55, and any other model requiring the use of a CN. If runoff coefficients are computed they are stored with the rational method data.