The script ONLINE_RATIONAL_COMPOSITE will calculate a peak discharge for a composite catchment using the rational method, following the Example. The calculator can take either SI or U.S. Customary units. In SI units, subareas are given in km²; in U.S. Customary units, in mi².

The time of concentration (same as rainfall duration) is given in minutes (min). The return period is given in years (yr).

The Intensity-Duration-Frequency curve (IDF curve) is defined as a function of return period T and rainfall duration t_r, which is made equal to the time of concentration t_c. Defaults values for the parameters of the IDF curve are as follows: k = 1000 (mm/hr); b = 20 (min); n = 0.2; and m = 0.7. These default values are intended for testing purposes.

The time of concentration t_B should be greater than the time of concentration t_A, as shown in the figure. The output is peak discharge Q_p in m³ or cfs, depending on the choice of units, and the rainfall duration t_rd associated with the peak discharge, or design rainfall duration.