Water quality treatment in bioretention systems has been revised for use in music v5. More emphasis is placed on the continuity of explanatory variables which are in principle continuous, and the analysis is based on a multiple regression approach. Additional data has been brought in from the original data files of the three column studies where appropriate under the new methodology.

Total Suspended Solids

Modelled outflow TSS concentration depends on the soil moisture at the start of the event (continuous variable) and the vegetation type (categorical variable), as shown below.

In spreadsheet terminology the modelled outflow TSS concentration from effectively vegetated filters is: 1.27 - 1.96 * LN(MAX(SoilMoisture, 0.09))

where SoilMoisture is a dimensionless fraction between 0 and 1 and outflow TSS concentration is in mg/L.

Total Phosphorus

Outflow TP concentration depends on the orthophosphate content of the filter medium, the depth of a submerged zone (if present), the filter soil moisture content at the start of the event (all continuous variables), and the vegetation type (categorical variable).

In spreadsheet terminology the modelled outflow TP concentration from effectively vegetated filters is: MAX(0.00028 * SubZoneDepth + 0.0043 * FilterOrtho + (-0.027 * LN(MAX(SoilMoisture, 0.12)) - 0.008) * (174 - SubZoneDepth) / 174 - 0.12, 0.02)

where SoilMoisture is a dimensionless fraction between 0 and 1, SubZoneDepth is the depth of the submerged zone in millimetres, FilterOrtho is the orthophosphate content of the filter medium in mg/kg, and outflow TP concentration is in mg/L.

Here are some sample modelled outflow TP concentrations.

 

  

Total Nitrogen

Outflow TN concentration depends on the total nitrogen content of the filter medium, the total nitrogen concentration of the inflow, the filter soil moisture content at the start of the event (all continuous variables), and the vegetation type (categorical variable).

In spreadsheet terminology the modelled outflow TN concentration from effectively vegetated filters is: MAX(0.74 * InflowTN + 0.0016 * FilterTN - 1.77 * LN(MAX(SoilMoisture, 0.09)) - 3.94, 0.3)

where SoilMoisture is a dimensionless fraction between 0 and 1, InflowTN is the total nitrogen concentration of the inflow in mg/L, FilterTN is the total nitrogen content of the filter medium in mg/kg, and outflow TN concentration is in mg/L.

The only difference between these last two cases is the magnitude of the constant.

Here are some sample modelled outflow TN concentrations.