Loss from System and Loss to Groundwater refer to the evaporative and leakage losses as wastewater is carried by canals and/or conduits from demand sites and wastewater treatment plants. These loss rates are specified as a percentage of the flow passing through the link. Loss from System indicates water that disappears from WEAP's accounts, whereas Loss to Groundwater does not disappear, but will flow into the groundwater node specified. You may include fractions for both, if, for example, there are both evaporative losses (Loss from System) as well as leakage losses that go to a groundwater node (Loss to Groundwater). Losses to groundwater in which there is no groundwater node defined in WEAP should be entered as Loss from System.
Gain from Groundwater deals with infiltration from groundwater into the return flow link. Infiltration, typically via cracked pipes, can lead to sewer overflows and increased wastewater treatment costs. To model infiltration, specify the groundwater node and the amount of inflow.
Entered on: Data View, Branch: Supply and Resources \ Return Flows \ <Demand Site Name>, Tabs: Loss from System, Loss to Groundwater, Gain from Groundwater