It is usual to use the first sub-catchment to determine dry weather Hydrographs and calculate either surface stormwater flows or sewage flows from both dry weather and wet weather sources. If another subcatchment needs to be linked to a node, the parameters can be set using the Second Subcatchment button in the Subcatchment Data window. 

Stormwater Modelling

When rural stormwater flows are being considered only it is usual to only have one (1) sub-catchment entering a node. Catchment area is input in hectares.

When urban stormwater runoff is being analysed two sub-catchments should be included to a node. These should individually reflect the pervious and impervious contributions respectively. Both catchment areas are input in hectares.

Sewer Modelling

When sewage flows are being analysed the split catchment option should be utilised to separately estimate the dry weather and wet weather (infiltration) contributions to the node. In this instance the Catchment area for wet weather infiltration is input in hectares.

Dry weather flow catchment area is however, NOT INPUT IN HECTARES. To reflect an instantaneous diurnal dry weather flow pattern it is necessary to input as catchment area EP divided by 10,000 ie. Catchment area input = EP / 10,000

Appropriate flow values per EP for the sub catchment together with an appropriate dimensionless diurnal temporal pattern is provided in the global storm database under Global Data menu.