# Applying PDD only to a portion of a node's demand, or to specific demand patterns

 Product(s): WaterCAD, WaterGEMS Version(s): 08.XX.XX.XX, 10.00.XX.XX Area: Modeling

# Problem

How can I apply PDD (pressure dependent demand) functions only to specific demand patterns in WaterCAD or WaterGEMS? For example I would like to have the leakage portion of junction demands be pressure dependent, while the regular demands remain fixed.

# Solution

Currently there are a few options to do this:

1) If possible, use the “percent of demand that is pressure dependent” field in the Pressure Dependent Demand alternative to only apply PDD to a portion of the total calculated demand at each node. Basically assume a percentage of total demand is leakage, and adjust the base flow of the junction demands accordingly. However, there may not be a direct correlation for all cases.

2) Use Emitter coefficients on the junctions to represent the PDD portion. This is how leakage is modeled with Darwin Calibrator Leakage Detection. The emitter coefficient is always pressure dependent (no need to use the PDD functionality), and will be added to any regular demands at the node.

3) Use separate junctions to model the leakage, create a selection set of them, and use that in the PDD selection set field in the calculation options.

The ability to specify PDD on a per-pattern basis has been submitted as Enhancement #683224