Basics Tab

The Basics tab is on the MODFLOW Layer Groups dialog box.

Layer Group (Aquifer) Name

This is the name of the layer group. It is for the convenience of the user.

Layer type

In the Layer Property Flow package and the Hydrogeologic Unit Flow package, layers may be either Confined or Convertible.

In the Block-Centered Flow package, layers may be Confined or Unconfined, Limited Convertible, or Fully Convertible. Only the top layer can be Unconfined. If the top layer group is set as Unconfined and it is discretized into more than one layer, the top layer will be considered unconfined and the other layers in the layer group will be treated as Fully Convertible. In a Limited Convertible layer, transmissivity of the layer is constant. In a Fully Convertible layer, transmissivity of the layer varies. It is calculated from the saturated thickness and hydraulic conductivity. In MODFLOW 6, individual cells rather than layers are specified as confined or unconfined using the IDOMAIN data set.

Layer groups can also represented nonsimulated material in a quasi-3-D model. (Nonsimulated layers are allowed when using the BCF6 or LPF flow packages but not when using the HUF2 package. Nonsimulated layers are not allowed in MODFLOW 6.)

In MODFLOW 6, modelers specify individual cells as being confined or unconfined using the Cell_Type data set. Thus, in MODFLOW 6, the Layer type is deactivated.

Compute saturated thickness using starting head

This option can only be used if the THICKSTRT option in the LPF package is selected in the MODFLOW Packages and Programs dialog box. If the option is selected, the cell thickness for conductance calculations will be computed as initial head minus the bottom elevation of the cell rather than the top of the cell minus the bottom of the cell. This is normally done because the layer is really an unconfined layer being treated as confined layer to promote model stability. When this check box is checked, LAYTYP for unconfined units will be exported as -1 instead of 1.

Method of calculating interblock transmissivity

In the Layer Property Flow package, there are three methods of calculating the transmissivity between blocks in the horizontal direction. These are:

•0—Harmonic mean (This is most appropriate for confined and unconfined aquifers with abrupt boundaries in transmissivity at the cell boundaries or for confined aquifers with uniform hydraulic conductivity.)

•1—Logarithmic mean (This is most appropriate for confined aquifers with gradually varying transmissivities.)

•2—Arithmetic mean of saturated thickness and logarithmic-mean hydraulic conductivity. (This is most appropriate for unconfined aquifers with gradually varying transmissivities.)

In the Block-Centered Flow package, there is another option

•3—arithmetic mean of saturated thickness and logarithmic-mean hydraulic conductivity. (This is most appropriate for unconfined aquifers with gradually varying transmissivities.)

There is no horizontal flow in nonsimulated layers so the user does not specify this method for nonsimulated layers.

In MODFLOW 6, the method for calculating interblock transmissivity is specified for the whole model in the NPF: Node Property Flow package.

Method of specifying vertical hydraulic conductivity

In the Layer Property Flow package, the user has the option of specifying the vertical hydraulic conductivity of a layer in one of two ways:

•0—vertical hydraulic conductivity

•1—the ratio of horizontal to vertical hydraulic conductivity

In nonsimulated layers, horizontal hydraulic conductivity is not specified so vertical hydraulic conductivity is always specified directly.

In MODFLOW-6, vertical hydraulic conductivity is always specified directly.

Horizontal anisotropy (TRPY)

In the Block-Centered Flow (BCF) package, the user specifies the anisotropy of the entire layer with a single number. Unlike the Layer Property Flow package and Hydrogeologic Unit Flow package, there is no option to specify a an anisotropy that varies within a layer so the Ky data set is not used with the BCF package.

In MODFLOW-6, horizontal anisotropy is not specified. Instead the Kx and Ky data sets are specified directly.