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Hydrologic routing simulates the behaviour of runoff as it flows through a catchment or a treatment node, using the continuity equation and the relationship between detention volume and discharge for each storage or treatment measure in the catchment.

The continuity equation simply states that the inflow to a defined location in a given time interval, minus the outflow, equals the change in storage for that location and time interval. It is assumed to be true for all water volumes in music. In other words, the sources and sinks that are not explicitly calculated (eg. deep seepage) are assumed to be negligible during the periods of interest. Water reuse is explicitly taken into account in the routing computation.

The storage-discharge (or S-Q) relationship at a location is a critical input to the hydrologic routing routine. From a strict analytical point of view it is the only information required to proceed with the routing computation. However, from a practical point of view, the provision of physical dimensions and layout information, as well as the hydraulic characteristics of the outlet structure are more convenient for most users. The S-Q curve is calculated by music from this information.

In music, you can specify a pipe or riser outlet (treated as an orifice of an equivalent diameter) and a weir outlet for ponds and wetlands, and a filter outlet and a weir outlet for bioretention systems. The S-Q curve for swales is derived using Manning’s equation in the channel. In this version of music, hydrologic routing is not applied to vegetated buffers or the storage bypass facility.

The hydrologic routing used here is based on Puls Method for reservoir routing, as described in Australian Rainfall and Runoff (Institution of Engineers Australia, 2001).

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