MUSIC combines the principles outlined above into a single versatile model. The same algorithm is used for sedimentation basins, ponds, wetlands, swales, and temporary ponding above a bioretention system by changing the four main inputs - the storage-discharge relationship, the background concentration, the exponential decay coefficient, and the number of CSTRs. For this reason, the model is referred to as the Universal Stormwater Treatment Model, or USTM.
The validity of this unified approach to simulating the operation of stormwater treatment measures has been demonstrated by empirical analysis of observed water quality improvements in swales, wetlands, ponds and infiltration basins and also by fitting observed water quality data from these treatment systems to MUSIC (Wong et al., 2001).
Choosing a particular treatment node will set broadly appropriate default values for k, C*, and N (the number of CSTRs). The default values for k and C* are derived from the sedimentation equations of Fair & Geyer (1954), using a typical particle size distribution for Australian conditions (as described in Appendix G: Selecting Appropriate k and C* Values). Any change in the particle size distribution will affect the values of k and C*, and so local data should be used where available.
Default values for N are derived from typical layouts and observed behaviour (eg. see figure in Continuously Stirred Tank Reactors). Although N will vary over a smaller range than k or C*, there is still scope to improve the model fit to observed data by adjusting N.
You are urged to calibrate the model with local or nearby data as much as possible. The default values have not been fitted to your catchment.
Note: Refer to Appendix G: Selecting Appropriate k and C* Values for the USTM for advice on selecting appropriate parameter values for the USTM.
Additional Reading:
Wong, T.H.F, Duncan, H.P., Fletcher, T.D. and Jenkins, G.A. (2001), A Unified Approach to Modelling Urban Stormwater Treatment, proceedings of the 2nd South Pacific Stormwater Conference, Auckland, New Zealand, 27 - 29 June 2001, pp.319-327.