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The number of CSTRs (N) can be related to the short-circuiting parameter (n) of Fair & Geyer (1954), or the hydraulic efficiency of 
Persson et al. (1999), by the equations , and , respectively. The figure below shows the hydraulic efficiencies of stormwater detention systems of various shapes as a guide to the appropriate selection of the number of CSTRs to use in music.
Hydraulic efficiency Image Modified of stormwater detention systems (Persson et al., 1999)
The cases considered above may be categorised into the following three groups:
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