*Percolation theory, *Hydraulic conductivity, *Drainage, *Green infrastructure, *Hydrological research, *Vegetation management, *Plant growing media, *Urban runoff management

Bioretention systems contribute to stormwater management through the detention of inflow as it percolates downwards through the growing media. Correct modelling of this process is key to the design of suitable bioretention cells. The SWMM LID module represents the flow of water downwards through the variably saturated growing media layer via a percolation model, which treats the entire growing media depth as a single homogeneous layer in which unsaturated hydraulic conductivity is estimated from moisture content and a simplified Hydraulic Conductivity Function (HCF). Controlled inflow detention tests were conducted on four bioretention columns, each with different vegetation treatments, over a 21-month period. Differences in outflow response due to vegetation treatment and system age were minimal. HCF, outflow, and media moisture content data support the adoption of a new form of HCF parameterised from routinely-characterised growing media properties. This new HCF leads to improved percolation estimates under low-flow conditions. [ABSTRACT FROM AUTHOR]

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