Composite Curve Number Calculator with Connected Impervious Area

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)

Formula: \[ CN_{comp} = f_{CIA} \times 98 + (1 – f_{CIA}) \times CN_{perv} \] where \( f_{CIA} = \frac{\text{CIA%}}{100} \).