This is also known as a first-type, Dirichlet, or constant head boundary condition. It is a nodal property so you should first define the subset of nodes for which you want to apply the condition and write them to a nodal data set.

The following instructions can be used as input to the Type instruction inside the Boundary condition...End instruction group to assign various specified head boundary conditions:

Sets the input type to be a general specified head boundary condition.

• • •

For example:

```boundary condition
type

node set
inflow

time value table
0.0    100.0
end

end ! new specified head
```

would define a specified head of 100.0 from time zero for the duration of the simulation for all of the nodes contained in the node set inflow.

These heads can be interpolated or turned on and off as discussed in Section 5.7.

## Head equals elevation

Sets the input type to be a special form of the specified head boundary condition where head is set equal to the elevation of the node. This is normally the z-coordinate, unless the instruction y vertical has been issued, in which case the y-coordinate is used instead.

• • •

For example:

```boundary condition
type

node set
inflow
end ! new specified head
```

This example shows that a time-value table is not required, since heads are derived from the nodal elevation. However, if you wish to turn the boundary condition on or off, then this can be accomplished by including a time value table instruction:

```boundary condition
type

node set
inflow

time value table
0.0    1.0
10.0   -99999.
end

end ! new specified head
```

The value 1.0 at time zero is ignored, but the NODATA value -99999 at time 10.0 causes the boundary condition to be turned off and the nodes become unconstrained.

## Head equals initial

Sets the input type to be a special form of the specified head boundary condition where head is set equal to the initial head at the node.

• • •

For example:

```boundary condition
type

node set
inflow

end ! new specified head
```

This condition can be turned on and off as discussed above for Head equals elevation.

## Notes

Each time you issue a Boundary condition...End instruction, a new boundary condition is formed with a unique ID number. HydroGeoSphere processes the boundary conditions in order and later ones may take precedence over later ones. The position in the grok file determines the order.

rgm Think about whether head bc override flux bc’s because we call the routine to process all heads first, then do fluxes later. If the node was assigned a specified head or fluid flux value by a previous instruction then it might be overwritten by later head boundary conditions, depending on whether or not both nodes are active at a given time.

Note that the definition of a seepage face (see Section 5.7.2.3) may lead to the formation of a specified head boundary condition at the seepage nodes.