GSDA:Creating a Land Use Table for Curve Number Calculations within WMS
The EPA BASINS download includes an ArcView GIS land use shapefile. The EPA land use data were originally created by the United States Geological Survey (USGS). The USGS assigned classification codes to each land use polygon. These code numbers are contained in the ArcView GIS table file under the Lucode field and represent certain land uses. In the Anderson system for example, a land use polygon having a code of 32 is "Shrub and Brush Rangeland" (see Table 1 below). Data included in Table 1 was obtained from the USGS at http://landcover.usgs.gov/pdf/anderson.pdf. You will need to create your own land use table by matching land use descriptions on a curve number table of your choice and the classification codes found in Table 1 on a "best fit" basis. Do this by:
- Select a land use classification code from your land use data. For example, 41.
- Referring to Table 1, it can be seen that land use '41' is "Deciduous Forest Land".
- Since the land use descriptions from the USGS may be different from what you have on your CN table, you must match "Deciduous Forest Land" with a land use on your CN table. For example, you might say that "Deciduous Forest Land" matches well with "Wooded Forests" on your CN table. From your CN table, you can now determine a CN for soil types A, B, C, and D.
- Using a text editor, type the land use classification code number, land use description, and the four CN's in the format shown in Table 2. Make sure each element is separated by a comma. The first CN listed is for soil type A, the second CN listed is for soil type B, and so on.
- Repeat the previous 4 steps for all other land use classification codes that apply to your watershed.
- Save the text file with any name, but be sure that the file extension is .tbl. For example, the filename might be 'landuse.tbl'.
- Click here to save a land use table template. This template file contains the Anderson land use codes with their descriptions. NOTE: You will need to replace the 0 values in the template with Curve Number obtained from a table, as described in steps 3 and 4 above.
- This table can now be used in WMS to compute a CCN (Composite Curve Number) for your watershed.