These include inflows to and outflows from river reaches due to evaporation, flooding, surface water inflow, and losses to or gains from groundwater.
In many watersheds, surface waters and groundwater are hydraulically connected. A stream can contribute to groundwater recharge (a "losing" stream) or can gain water from the aquifer (a "gaining" stream) depending on the level of groundwater in the aquifer. These flows between surface water and groundwater can be handled in WEAP in one of two ways.
you can specify directly how much flows from surface to groundwater (Groundwater Outflow) and from groundwater to surface water (Groundwater Inflow),
WEAP can model these flows based on the level of the groundwater table and the Reach Length. For more information, see Groundwater-Surface Water Interactions.
In addition to flows to and from groundwater, flow on river reaches can be reduced by Evaporation and Flooding, and increased by Surface Water Inflow.
Evaporation losses are specified as percentages of streamflow; groundwater outflow are specified as either percentages of streamflow (Groundwater Outflow Fraction) or as a volume (Groundwater Outflow Volume); surface water inflow and groundwater inflow are only entered as volumes.
A river can overflow its banks and flood the surrounding land if the streamflow exceeds the River Flooding Threshold. In this case, a fraction of the streamflow above the threshold, the River Flooding Fraction, will flow from the river reach to the connected catchment. See Catchments \ Soil Moisture Method \ Flooding for more information.
Surface water inflow represents either non-point runoff into the river, or the confluence of streams or rivers not otherwise modeled. You may specify this inflow using the Water Year Method, the Read from File Method, or with an expression. See Specifying Inflow for details.
For groundwater interactions, you must specify to which Groundwater node each reach is connected.
If a reach is connected to a groundwater node for which you've chosen to model the flows based on the level of the water table (the "wedge" method), then you will need to enter the Reach Length -- -the horizontal length of the interface between the reach and linked groundwater. Note that this might be shorter than the actual length of the reach.
Entered on: Data View, Branch: Supply and Resources \ River \ <River Name> \ Reaches, Category: Inflows, Tabs: Surface Water Inflow, Groundwater Inflow, Groundwater Outflow, Evaporation, Reach Length.