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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.
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