Composite Curve Number Calculator
Calculate the composite curve number for a watershed with a connected impervious area using:
\[
CN_{comp} = f_{CIA} \times 98 + (1 – f_{CIA}) \times CN_{perv}
\]
* Enter the percentage of connected impervious area (CIA%) and the curve number for the pervious area.
Step 1: Enter Area Data
Example: 30% of the watershed is connected impervious
Example: 70 (typical for moderately permeable soils)
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