When applied to urban catchments the pervious and impervious areas are considered separately. Only the pervious areas are considered when natural catchments are being modelled (unless it is known that part of the natural catchment behaves like an impervious surface).
Three processes are modelled for the pervious areas; catchment wetting, catchment drying, and Soil Moisture Redistribution. Five levels of storage are used. These are interception, depression, upper and lower soil zone and groundwater stores.
In the catchment wetting phase rain falling on the pervious areas goes simultaneously to the interception and depression storages. Once the interception store is full it also overflows into the depression storage. The infiltration process begins as soon as there is any moisture in the depression storage and continues until the depression store is empty. The infiltrating water enters the upper soil zone.
Catchment Drying is by evapotranspiration which occurs from the interception, depression and upper and lower soil zone stores. The full potential evapotranspiration based on pan readings is applied first to the interception store. If it is depleted before the evapotranspiration requirements are met then the residual evapotranspiration capacity is applied to the depression store. If the depression store is unable to meet the requirements then the remainder is applied to the upper and lower soil zones.
Soil moisture is redistributed between the upper and lower soil zones concurrently with the catchment wetting and drying phases. Moisture may flow either to or from the upper soil zone. A certain proportion of the moisture in the lower soil zone drains to groundwater storage, which gives rise to groundwater runoff.
One storage is used for the impervious areas. It is filled by rainfall and depleted by evaporation.
Runoff is generated in three ways, by overflow of the impervious area store, overflow of the depression storage due to infiltration limitations and Groundwater Runoff.
The first two sources of runoff, which are the surface runoff components, are combined preparatory to routing. The groundwater runoff is routed separately and is added to the routed surface runoff to give the total estimated runoff volume for each day.