GSDA:Creating a Table of Runoff Coefficients
WMS can compute Composite Runoff Coefficients for the basin(s) in your watershed. To accomplish this, you will need to create a table that links a soil type's ID number (from a soil type grid or polygon coverage) to a runoff coefficient. Note that WMS automatically creates a soil ID number for each different soil encountered upon importing a soil type grid or soil type shapefile. Refer to the following steps for help to create a Runoff Coefficient table. Below these steps is a table containing suggested runoff coefficients based on a soil's management type and hydrologic soil group.
Steps for selecting a runoff coefficient for a soil ID number:
For each soil type ID in WMS
- Determine the land use / vegetation category for the soil ID in question, such as cultivated, pasture, urban, etc.
- Determine the hydrologic soil group of this ID,
To determine the soil group (A, B, C, or D) of a soil ID after importing the soil type file into WMS:
- Select Display | Display Options...
- Choose the MAP tab
- Switch the Active Coverage drop-down list to Soil Type
- Toggle On the box for displaying the Soil Group Legend
- Select OK
You will now see a small legend in the upper-left corner of the WMS graphics window that lists the soil IDs with their respective soil groups.
- Pick the Runoff Coefficient for the given land use / vegetation and hydrologic soil group using table like the one below these steps.
- Create a 3-column text file containing the soil ID, a description, and its coefficient, following this format:
soil ID, description (in quotes), runoff coefficient
- Save the file, preferably with a *.tbl extension.
Example 0, "Type1", 0.1 1, "Type2", 0.2 2, "Type3", 0.3 3, "Type4", 0.4 4, "Type5", 0.5 5, "Type6", 0.6 6, "Type7", 0.7 7, "Type8", 0.8 8, "Type9", 0.9
This file can now be read into WMS in order to calculate a composite Runoff Coefficient for you watershed basin(s).
For help in performing this calculation in WMS, refer to the WMS Help menu.
Table of Runoff Coefficients for the Rational Equation: Q=kCiA
Hydrologic Soil Group Land Use, Crop, and Management A B C D CULTIVATED, with crop rotations Row Crops, poor management 0.55 0.65 0.70 0.75 Row Crops, conservation mgmt 0.50 0.55 0.65 0.70 Small Grains, poor management 0.35 .40 0.45 0.50 Small Grains, conservation mgmt 0.20 .022 0.25 0.30 Meadow 0.30 0.35 0.40 0.45 PASTURE, permanent w/moderate grazing 0.10 0.20 0.25 0.30 WOODS, permanent, mature, no grazing 0.06 0.13 0.16 0.20 Urban Residential 30 percent of area impervious 0.30 0.40 0.45 0.50 70 percent of area impervious 0.50 0.60 0.70 0.80 Hydrologic Soil Group Descriptions A -- Well-drained sand and gravel; high permeability. B -- Moderate to well-drained; moderately fine to moderately coarse texture; moderate permeability. C -- Poor to moderately well-drained; moderately fine to fine texture; slow permeability. D -- Poorly drained, clay soils with high swelling potential, permanent high water table, claypan, or shallow soils over nearly impervious layer(s).
- This table was obtained from http://pasture.ecn.purdue.edu/~engelb/abe526/Runoff/C_table.html