Bioretention systems (also known as biofiltration systems or raingardens) promote the filtration of stormwater through a vegetated filter media (NOTE: whilst the user has the option to specify a non-vegetated system, it is strongly recommended that bioretention systems always be vegetated). Non-vegetated systems will generally give poor water quality outcomes and are more likely to clog prematurely. The type and composition of filter medium determines the effectiveness of the pollutant removal, as does the choice of vegetation.
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The overflow weir carries a discharge when the water level in the bioretention system pond exceeds the Extended Detention Depth. The overflow weir is modelled as a sharp broad crested weir whose discharge equation is given by:
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Evapotranspiration losses
The method of calculating evapotranspiration from bioretention systems has been enhanced. In the previous version of MUSIC, evapotranspiration was a function of soil moisture and a term, Emax, which is used to describe the maximum daily evapotranspiration rate. The Emax value was derived from experiments using biofilter columns planted with Carex appressa, and was applied as a constant rate. More recently, field experiments on biofilters (see for example Hamel et al. 2011 and 2012) have enabled the MUSIC team to develop a more sophisticated and precise prediction of evapotranspiration, taking into account seasonal variation. We have done this by developing a ratio between potential evapotranspiration (PET) and the measured ET. This scaling factor has then been used to develop a seasonally-adjusted Emax figure on a monthly basis, such that: Emaxj = scaling factor * PETj, where Emaxj is the Emax value for month j, and PETj is the potential evapotranspiration for month j.
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