In VO, Ponding and mulch layers are additional layers of storage for above the bioretention soils. The storage capacity of the ponding and mulch layer, is following by Eq. (10.37):


where is the effective porosity of the mulch layer (m³/m³); and are the ponding and mulch layer depth (m).

The volume of water stored in the mulch layer (m³), , varies in time according to Eq. (10.40):


Where is volume of water in a mulch layer (m³) during the time step t; is the inflow to the bioretention (m³/s). , which is constant (m/s) in both single even and continuous models or using Green-Ampt Mein-Larson (GAML) equation to calculate infiltration in a continuous model. is the bottom area of the bioretention (m²); is the overflow volume (m³).