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):
(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):
(10.38)
(10.39)
(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).
