Every LID control element in the network must be associated with a "Parent" catchment. The Parent catchment contributes flow to the LID and receives LID outflow. Once the Parent has been specified in the LID Properties (see LID Control-General), the association is represented by a dotted line in the drawing.

Although a LID control can only be associated with a single Parent catchment, a single catchment may be a "Parent" for multiple LID controls acting in parallel.

Further, a Parent catchment may represent only the area occupied by LID controls, or it may include non-LID area. The catchment must have an outflow node, which, if desired, can be another catchment.

The possibilities just described for configuring LID controls and Parent catchments translate to two main options for how to approach your network configuration. The benefits and limitations of each approach should be carefully considered against the needs of a particular model.

Option 1: LID occupies full subcatchment (usually recommended)

With this approach, a new catchment is created for each LID control in the network. It is the LID control's Parent Catchment, and represents only the area occupied by the LID control.

Flow from additional catchment area(s) is directed to the LID control by designating the LID's Parent Catchment as the additional catchment's "Outflow Node."

Under this scenario, in the Low Impact Development Control properties dialog, the attribute "Occupies Full Subcatchment" will be set to True.

The advantages of this option are:

• Multiple LIDs can be placed in series, if desired (i.e., upstream LID's parent catchment and have downstream LID's parent catchment as its outflow node)
• Explicit definition of a LID's contributing area (as opposed to defining as a percentage of impervious area, which is in turn a percentage of total catchment area)
• Ability to direct upstream pervious area to the LID control. (With a parent catchment, only impervious non-LID area can be directed to the LID control.)
• Avoids possible mistakes with parent catchment impervious area percentage and characteristic width resulting from the addition of a LID control (see Option 2)
• Enables more explicit hydrograph reporting to show LID control effects

A possible disadvantage of this option is a greater number of network elements in the model.

Option 2: LID displaces an equal amount of non-LID catchment area

With this option, the Parent catchment Area includes the total area occupied by the LID Control(s), as well as adjacent non-LID area. The LID Control attribute "Occupies Full Catchment?" is set to False, and the attributes "Area of Each Unit" and "Number of "Replicate Units" define the area that a LID control displaces from the Parent catchment.

This approach allows multiple LID controls to be associated with a single catchment. If multiple LID controls are present, they act in parallel to each treat a different portion of the runoff generated from the non-LID catchment area.

The advantage of this option is that it minimizes the number of network elements that must be placed in the model, enabling a more uncluttered appearance and reducing data entry in some cases.

The disadvantages of this approach are:

• The catchment's LID controls cannot be considered in series (i.e., the outflow from one LID control cannot be inflow for another downstream).
• Only runoff from the impervious portion of the parent catchment can be directed to the LID control(s).
• Caution must be exercised with regard to parent catchment attributes for Percent Impervious and Width. These values must reflect the non-LID portion of the catchment only. For example, if the original Parent catchment is 40% impervious, and in the course of design, 75% of this impervious area is changed to porous pavement, then the percent impervious must be adjusted to (1 - 0.75)*40/(100 - 0.75*40) = 14.3%. Similarly, the catchment's "Characteristic Width" can be affected. If estimating this value as area divided by overland flow length, the area used would typically be the non-LID area only.