The GHB: General-Head Boundary package pane is on the MODFLOW Features tab of the Object Properties dialog box.
The data that can be specified for the Drain package are the Starting time, Ending time, Boundary Head and Conductance. The Boundary Head is the head of the external water body (or other feature) that is connected to the GHB cell. Water enters and leaves the cell containing the GHB cell at a rate of flow proportional to the difference between the Boundary Head and the head in the cell. The Conductance is the factor that relates the difference in head to the rate of flow.
Starting time and Ending time are explained in the help for the MODFLOW Features tab.
GHB Multiplier is only used if it has been activated in the GHB: General-Head Boundary Package Pane . If GHB Multiplier is specified, the GHB conductance is multiplied by the Conductance GHB value.
If parameters are used with an object, Boundary Head must be defined for each parameter as well as a multiplier for the Conductance. MODFLOW will multiply the multiplier by the parameter value (specified in the MODFLOW Packages and Programs dialog box) to determine the conductance to apply.
In MODFLOW, the Conductance is described as having units of L2/T and is equal to KLW/M where
K = the hydraulic conductivity of the sediment in the boundary condition such as a river or drain,
L = the length of the boundary condition in the cell,
W = the width of the boundary condition, and
M = the thickness of the sediment in the boundary condition perpendicular to flow between the boundary and the cell. Usually this will be vertical thickness of the sediment.
However, the conductance is also affected by the grid cell size independent of the above formula (Mehl and Hill, 2010).
For Polyline Objects, the length of the object in the cell can be determined with the function ObjectSectionIntersectLength.
One way to to set the conductance would be with a formula that includes ObjectSectionIntersectLength and user defined Global Variables for the width of the drain and the thickness of the sediment in the drain and perhaps the hydraulic conductivity of the sediment in the drain. For example, the user could define Global Variables named "DrainWidth" and "DrainSedimentThickness" and then set the Conductance of the drain using the following formula.
((Kz * ObjectSectionIntersectLength) * DrainWidth) / DrainSedimentThickness
In the formula, it is assumed that the hydraulic conductivity of the sediment in the drain is the same of the vertical hydraulic conductivity of the cell that contains the drain. This may or may not be a valid assumption in any particular case.
A similar formula appropriate for Polygon Objects would be
(Kz * ObjectIntersectArea) / DrainSedimentThickness
However, if Conductance Interpretation is set to Calculated, as it is by default, ObjectSectionIntersectLength and ObjectIntersectArea should not be included in the formulas because they will be incorporated automatically for polyline and polygon objects. Thus, if Conductance Interpretation is set to Calculated for a Polyline Object, Conductance should be treated as if it were KW/M instead of KLW/M, its units would be L/T, and an appropriate formula would be
(Kz * DrainWidth) / DrainSedimentThickness
Conductance Interpretation has no effect for point objects. For point objects, the formulas will be applied without any modification regardless of Conductance Interpretation.
if Conductance Interpretation is set to Calculated for a Polygon Object, Conductance should be treated as if it were K/M instead of KLW/M, its units would be 1/t, and an appropriate formula would be
Kz / DrainSedimentThickness
If Conductance Interpretation is set to Direct, the formulas will be applied without any modification.
If Conductance Interpretation is set to Total per layer the formulas will be modified by multiplying the conductance by ObjectSectionIntersectLength / ObjectLength or ObjectIntersectArea / ObjectArea.
Setting the Conductance Interpretation to Calculated or Total per layer allows ModelMuse to calculate an appropriate conductance for a cell based on the length or area of intersection between the object and cell. However, if you calibrate your model and then change the grid size, the conductances assigned by ModelMuse may no longer be appropriate and you may have to recalibrate your model. The conductance is a coefficient of proportionality between the head difference in the cell and the flow to the boundary. When the grid size changes, the head in the cell will be averaged over a different area than before. This conductances will need to be adjusted because of this change in head.
Auxiliary Variables If the Groundwater Transport process in MODFLOW 6 is active, auxiliary variables will be included. They represent the concentration of flow into the groundwater from the boundary. Flow out of the groundwater will be at the concentration in the cell containing the boundary.
Density: Density is the density of fluid entering groundwater from the boundary. Density is only present if the Buoyancy package is active and Specify Density is selected.
Viscosity: Viscosity is the viscosity of fluid entering groundwater from the boundary. Viscosity is only present if the Viscosity package is active and Specify Viscosity is selected.
Density and Viscosity are only used with the General Head Boundary, River, Lake, Stream, and Multiaquifer Well packages. However, they may also appear in other packages where they will be treated as auxiliary variables but not used by MODFLOW.
Of all the head-dependant flux boundaries, only the General-Head Boundary package has a linear relationship between head at the boundary and flux through the boundary. This linear characteristic is advantageous for automated calibration of models.
