Curve Number (CN) is the most important parameter to determine surface runoff when SCS equation is used. Its value varies for different soil types, land use, and Antecedent Soil Moisture Condition (AMC). The CN for the average antecedent soil condition (CNII) is usually used. Lookup tables have been established for land use, soil hydrological group, and CNII (Table 7-4).

TABLE 7-4 SAMPLE LAND USE, SOIL AND CN LOOKUP TABLE

In most cases, CNII of each catchment is estimated with the land use and soil information and applicable lookup tables. If land use and soil layers are available, GIS software is usually used to conduct the calculation. For catchments covering more than one land use or soil type, an area weighted CNII are usually calculated.

The Calculate CN tool makes the process easier. To use this tool:

  1. Add soil and land use layers. Make sure the soil layer has the soil hydrologic group data and the land use layer has the land use data.
  2. Select catchments (StanHyd, NasHyd, WilHyd or ScsHyd) to be updated in Map View or Schematic View. Note that the selected catchments must be assigned to a geometry.
  3. Click the Calculate CN button in the GIS tab. The Calculate CN window will appear. Notice that the number of selected hydrologic objects available for CN calculation is shown on the top of the window. The window has the left part for soil mapping and the right part for the land use mapping.
  4. On the left part, choose the soil layer from the Soil Layer list and then choose the Soil Group Field. The soil hydrologic group lookup table will become available, allowing the user to define which hydrological group (A, B, C and D) will be mapped for each value of the selected field. Define the soil group lookup table by selecting a proper hydrological group from the drop-down list.
  5. On the right-hand side, choose the Land Use Layer from the layer list and then choose the Land Use Field. Define the land use lookup table by selecting from the dropdown list for each land use type.
  6. Click the Update CN button at the bottom to update the CN based on soil and land use data. A message window will appear indicating that CN of selected hydrologic objects have been updated. In TatukGIS, the users will now have a drop-down menu for Mapping with TRCA, CVC, and TR-55 as options.

  1. To export the soil and land use layer mapping information, click the Export button at the left bottom corner to save the mapping information to a file. The saved file can be imported using the Import button.
  2. The Clear button at the bottom can be used to clear all field mapping information and start all over again.
  3. To see the default CN lookup table, click the CN Lookup Table button. Note that the lookup table is not editable. To use a different lookup table, please contact us.

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