The IDF Curve Equation definition type calculates the storm data using the following equation:

Where:

• i = rainfall intensity
• D = rainfall duration
• a, b, n = rainfall equation coefficients

When editing a definition of the IDF Curve Equation type, the tabbed area of the dialog contains the following controls:

Storm Data Input Tab

• Equation Duration Unit: Specify the unit to be used for duration (D) value in the equation.
• Equation Intensity Unit: Specify the unit to be used for intensity (i) value in the equation.
• a: Specify the value to be used for the ‘a’ coefficient in the equation.
• b: Specify the value to be used for the ‘b’ coefficient in the equation.
• n: Specify the value to be used for the ‘n’ coefficient in the equation.

The table on the left side is where the input is entered. Click the New button and enter a Return Period when prompted. The return period is read-only, and the coefficients a, b and n are editable.

As each coefficient is entered, the calculated intensities for that return period will be shown in the table to the right. The first column is a predefined set of durations. The equation noted at the top is used to calculate the intensity at each duration using the entered coefficients.

Notes Tab

This tab contains a text field that allows you to enter descriptive notes that will be associated with the currently highlighted storm data definition.

Library Tab

This tab displays information about the storm data definition that is currently highlighted in the list pane. If the storm data definition is derived from an engineering library, the synchronization details can be found here. If the storm data definition was created manually for this model, the synchronization details will display the message Orphan (local), indicating that the storm data definition was not derived from a library entry.