The program ONLINE_CONVOLUTION calculates a composite flood hydrograph by convolution, when presented with the hyetograph of total rainfall, the ordinates of the unit hydrograph, the time interval [of definition of the hyetograph and unit hydrograph], and the runoff curve number. It is assumed that the unit hydrograph duration is equal to the [provided] time interval [of the hyetograph].

1. First, the program sums up the hyetograph to calculate the total rainfall, in cm or in, depending on the chosen units, either SI (metric) or U.S. Customary.

2. Then the program uses the curve number to calculate the effective rainfall, or runoff, in cm or in, with the NRCS runoff curve number method.

3. Next the program calculates a φ-index, and uses this value to calculate an effective rainfall hyetograph.

4. Last, the provided unit hydrograph is convoluted with the calculated effective rainfall hyetograph to calculate the composite flood hydrograph for the given rainfall hyetograph, curve number, and unit hydrograph.