The ERM was developed to model detention basins using a method that was consistent with the rational method, since it is usually impossible to adjust an Horton (ILSAX) model to match rational method peak flows for a range of average recurrence intervals and storm durations. Flowrates calculated by Australian Rainfall and Runoff (1987) rational method procedure are routed using the same time-area procedure as the Horton (ILSAX) model.  The operation of the model is almost exactly the same as that with Horton (ILSAX) hydrology.  When Australian Rainfall and Runoff storm burst patterns are used, as shown below, this method produces higher peak flowrates than the rational method.

It can also run with 1987 rainfalls and synthetic rainfall patterns derived from intensity-frequency-duration relationships.  Note that the 'total areas' ERM model cannot run with the 2019 data, only the 'separate areas' model.

The example Gymea Rational & ERM ARR 1987 Example - Lite.drn shows the procedure with the I-F-D-based relationship. To get an exact match it is necessary to use synthetic rainfall patterns different from the 1987 or 2019 Australian Rainfall and Runoff patterns, as shown below. The  hydrological model 'Gymea ERM - Total Areas' must be selected.