L-THIA Impervious Input

Calculating the Curve Number (CN) value from percent impervious cover:

 (An Area-Weighted Method: Mixes of traditional land uses)

1.      The User provides the areas or percentages of different land use types and hydrologic soil group combinations in the area of interest.  

Note: User is required to provide only this information. The rest of the values are computed by the model.

2.      The CN value is looked up in TR-55 (based on hydrologic soil type) for each land use type.

For the current 'percent impervious input' version default CN values have been used.  They are based on the assumption that the pervious surface is 'grass' in 'fair' condition

Note: If the user needs to calculate CN values for pervious surface other than grass in fair condition, the user can do so using the formula or the process described here. Additional information can be obtained from TR-55. 

3.      Multiply % value of each land use type with the corresponding CN value.

4.      Repeat for each land use type and CN value.

5.      Add all the multiplied values and divide this total by 100.

6.      The resulting number is the custom CN value for the area.