Flow Boundary Conditions

You may select from the flow boundary conditions listed below.

No flow across boundary. The component of flux in the direction normal to the boundary is zero.

Specified pressure head. The pressure head at the boundary is set at the specified value.

Specified total head. The total head at the boundary is set at the specified value. Care should be exercised in determining the sign of the value. The datum for the total head is the zero tick mark of the vertical ruler. The total head at a point can be thought of as the water level in a tensiometer or piezometer opened to that point. If this water level is below the datum, the total head is negative. If this water level is above the datum, the total head is positive.

Specified flux into domain. The component of flux (units of [L]/[T]) into the domain is set at the specified value. The value is either for flux normal to the boundary or flux in the vertical direction. The former approach is the more common method for specifying boundary fluxes. The latter approach should be employed when precipitation rates are used as input. The implementation of this boundary condition is primarily aimed at simulating infiltration on land surface with possible ponding. For any cell along the specified flux boundary, if the pressure head exceeds the maximum height of ponding, the boundary condition is converted from specified flux to specified pressure head and the pressure head is set equal to the maximum height of ponding. Once this conversion has occurred, the flux at the cell is monitored by the program. If the computed flux exceeds the specified flux by 1 percent or more, the boundary condition is converted back from specified pressure head to specified flux. See discussion in Lappala and others (1983, p. 38). Note that this conversion applies to any specified flux boundary, regardless of whether it represents land surface or a vertical side of the domain. To prevent the conversion, set the maximum height of ponding to a large positive value.

Specified flux out of domain. The component of flux (units of [L]/[T]) normal to the boundary and out of the domain is set at the specified value. Users should enter a positive number into the edit box. Flux out of domain is specified separately from flux into domain because they require different transport boundary conditions.

Specified volumetric flow into domain. This boundary condition is available only when using radial coordinates. The specified value is the total volumetric flow rate (in units of [L]³/[T]) into the domain across a surface swept out by revolving the boundary segment 360 degrees about the z axis. This boundary condition is controlled by maximum height of ponding in the same manner as described for the specified flux boundary condition (See above).

Specified volumetric flow out of domain. Same meaning as above except flow is out of the domain. Users should enter a positive value into the text box. By convention flow out of the domain is negative, so the given flow rate will automatically be converted to a negative number. Volumetric flow out of domain is specified separately from volumetric flow into domain because they require different transport boundary conditions.

Evaporation/Transpiration. Evaporation and/or transpiration occurs along the boundary. This boundary condition is available only when the options to "simulate evaporation" and/or "simulate transpiration" are selected in the "Basic" pane of the Model Options dialog box. Note that for evaporation and/or transpiration to occur during a recharge period, the corresponding boxes for "Evap" and/or "Plant" should be checked in the Recharge Period Window.

Possible seepage face. Indicates that a seepage face might develop on the boundary. Whether or not a seepage face will develop on the boundary usually cannot be predicted in advance of the simulation. If a seepage face does not develop on the boundary, the boundary acts as a boundary of "no flow." If a seepage face occurs, the pressure head along the seepage face is set to zero and water is allowed to flow out of the domain along the seepage face.