FluidTransfer BC
Units [L]
A transfer boundary condition applies a predefined reference head combined with a conductance parameter (transfer rate).
Transfer boundary conditions are applied in cases where a reference potential is linked to the aquifer via a separating medium. This can be the case for example for rivers or lakes with a limited connection to groundwater (surface water level equals groundwater level) or for partly clogged drains.
The inflow/outflow is calculated from the relevant area, the transfer rate, and the difference between reference and groundwater head:
Q = AΦ(h_{ref}h)
where
Q: inflow or outflow to/from the model
A: relevant area
Φ : transfer rate
h_{ref}: reference water level
h: current hydraulic head in groundwater
The transfer rate is a conductance term describing the properties of a clogging layer. It is defined as
Φ = K/d
where
K: hydraulic conductivity of the clogging layer
d: thickness of the clogging layer
When setting the boundary condition, the reference water level is defined as the value for the boundary condition. The transfer rate is set separately as a material property.
3D model
The boundary condition nodes have to enclose entire faces of elements to define the area in one of the following ways:
For an inflow at one slice (either on top or bottom of the model or inside) all the nodes of an element in one slice (three nodes when using prismatic elements, four for cuboidal elements) have to be assigned the condition. 

For an inflow at a vertical element face two nodes of an element in one slice and the same two nodes in an adjacent slice have to be assigned the condition. 
The area for the inflow or outflow calculation includes the area of all the element faces the boundary condition has been assigned to. If the condition is inside the model, there are always two neighboring elements for each face therefore the area of the face is taken twice. At outer boundaries or on top/bottom of the model, the area of the face is only taken once. 
2D model
The transfer boundary condition in 2D models has to be set to at least two nodes, defining a length for the inflow or outflow. 
The length for the inflow or outflow calculation includes the length of all the element edges the boundary condition has been assigned to. If the condition is inside the model, there are always two neighboring elements for each edge. Therefore the length of the edge is taken twice. At outer boundaries, the length of the line is only taken once. 