The DRN: Drain 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, Elevation and Conductance. The Elevation is the elevation of the drain. Water can leave the groundwater through the drain boundary when the head is higher than the elevation and the rate of flow will be proportional to the difference between the head and the elevation. 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.
If parameters are used with an object, Elevation 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.
Discharge Scaling Length (DDRN) is an auxiliary variable used in MODFLOW 6 for discharge scaling. The idea behind discharge scaling is that the area represented by a cell may have a variety of elevations and as the head in the cell increases, a larger area of the cell become saturated and can discharge water. This can be represented by changing the conductance of the cell up to the elevation at which the entire cell surface is saturated.
The effect of DDRN is explained in detail in Chapter 3 of the MODFLOW 6 – Supplemental Technical Information (mf6suptechinfo.pdf). If it is specified as 0, it has no effect. If DDRN is non-zero, the drain conductance is scaled using either linear or cubic scaling. Linear scaling is applied if the standard flow formulation is used. Cubic scaling is applied if the Newton-Raphson formulation is used.
With a positive value of DDRN, the conductance is zero when the head in the node is equal to the drain elevation and increases up to its maximum value when the head is equal to the drain elevation plus DDRN. If DDRN is negative, the effective drain elevation is the specified drain elevation plus the (negative) value of DDRN. With discharge scaling, the drain package can be used as an alternative to the groundwater seepage option in the Unsaturated Zone Flow (UZF) Package.
Conductance Multiplier is only used if it has been activated in the DRN: Drain Package. If Conductance Multiplier is specified, the drain conductance is multiplied by the Conductance Multiplier value.
