The Hydrogeologic Unit Flow (HUF2) package is one of several packages in MODFLOW that can be used to simulate flow in the saturated zone. The pane for this package is on the MODFLOW Packages and Programs dialog box under Flow Packages.
In the Hydrogeologic Unit Flow package, aquifer properties are assigned by hydrogeologic units (defined in the MODFLOW Hydrogeologic Units dialog box) rather than by layer. MODFLOW then uses the top elevations and thickness of the hydrogeologic units to determine how much each hydrogeologic unit should contribute to the hydraulic properties of individual cells. In convertible layers, the contribution of individual hydrogeologic units can change through the course of the simulation as the water table changes.
The ability to define aquifer properties by hydrogeologic unit rather than by layer makes the Hydrogeologic Unit Flow package particularly well suited for modeling complex geology.
The hydraulic properties of the hydrogeologic units must be defined through parameters; the Hydrogeologic Unit Flow package does not provide a method of defining hydraulic properties just as an array of numbers.
The Hydrogeologic Unit Flow package provides options for saving the heads and flows for each hydrogeologic unit. These values are calculated by combining the heads and flows in individual cells intersected by each hydrogeologic unit. To activate these options, the Save heads for hydrogeologic units (IOHUFHEADS) or Save flows for hydrogeologic units (IOHUFFLOWS) checkboxes can be checked.
If any KDEP parameters are defined (see below), the user can choose whether the reference surface used to calculate depth is the top of the model or a separate array. The radio buttons in the group labeled Reference surface for calculating depth. If a separate array is chosen, the data set used to define the array will be named HUF_Reference_Surface.
Parameters used in the Hydrogeologic Unit Flow package are defined in this pane. The types of parameters that can be defined are:
•HK – horizontal hydraulic conductivity.
•HANI – HANI, horizontal anisotropy (ratio of hydraulic conductivity along columns to hydraulic conductivity along rows). When using the LVDA capability, HANI values need to be less than or equal to 1.
•VK – vertical hydraulic conductivity, for units for which HGUVANI is set to zero.
•VANI – vertical anisotropy, for units for which HGUVANI is set greater than zero. If the Hydraulic-Conductivity Depth-Dependence Capability is used, the vertical hydraulic conductivity changes according to the changes in the horizontal hydraulic conductivity.
•SS – the specific storage.
•SY – the specific yield.
•SYTP – when all model layers are confined, defines the storage coefficient for the top active cell at each row, column location. The value specified is not multiplied by model layer thickness.
•KDEP – defines the depth-dependence coefficient used to modify horizontal hydraulic conductivity with depth for the hydrogeologic unit(s) specified in Item 4. NOTE: horizontal hydraulic conductivity is only modified for cells in units defined by depth-dependence parameters and not for other cells, so depth-dependence parameters do not need to be specified for the entire model.
•LVDA - defines the angle between the grid axis and the principal direction of horizontal hydraulic conductivity. Angle is positive in a clockwise direction with the positive x direction being zero. The angle can vary between –90 and 90 degrees. Hydraulic conductivity along the principal axis (Kmax of figure 2) is defined in the HUF Package input file by an HK parameter; the hydraulic conductivity along the minor axis (Kmin of figure 2 in the LVDA documentation) axis is defined by a HANI parameter in conjunction with the HK parameter. Using the LVDA capability, HANI values need to be less than or equal to 1.
Each parameter must have a unique Name. Only ASCII characters are allowed in parameter names.
The Value of the parameter is used to determine the input to MODFLOW.
The Use Zone and Use Multiplier options are only available here for the SYTP and LVDA parameters. For the others, the use of zone or multiplier arrays are specified for each hydrogeologic unit in the MODFLOW Hydrogeologic Units dialog box
If Use Zone is checked in the MODFLOW Packages and Programs or Manage Parameters dialog boxes, a boolean data set will be created that can be used to determine where the parameter applies. The parameter will apply everywhere where the data set is true. The name of the data set will be the name of the parameter followed by "_Zone.." If Use Zone is not checked, the parameter will apply everywhere.
If Use Multiplier is checked in the MODFLOW Packages and Programs or Manage Parameters dialog boxes, a real-number data set will be created that can be used to determine how the parameter Value is related to the input. The name of the data set will be the name of the parameter followed by "_Multiplier." The parameter value will be multiplied by the values in the multiplier data set for each cell to determine the input value for MODFLOW at that cell.
If more than one parameter of the same type applies to the same cell, the input value for MODFLOW will be the sum of the input values determined for that cell for all the parameters that apply to the cell. For example, suppose that there are two HK parameters defined: HK1 and HK2. They have Values of 10 and 20 respectively. If they both apply to the cell in column 1, row 1, layer 1 and neither has multiplier data sets, the hydraulic conductivity for that cell would be 10 + 20 = 30. Now suppose that the multiplier for HK1 was 5 at that cell. The hydraulic conductivity for that cell would be 10*5 + 20 = 70.
Parameters can also be edited in the Manage Parameters dialog box.
The input to the HUF2 package can be split into up to three different input files. If any KDEP parameters are defined, a separate input file will be created with data related to depth-dependent hydraulic conductivity. If any LVDA parameters are defined, a separate input file will be created with data related to variable-direction horizontal anisotropy. The remainder of the input for the HUF2 package is in the main input file for the HUF2 package.
If the HUF2 package is selected but hydrogeologic units have not yet been defined, the MODFLOW Hydrogeologic Units dialog box will be displayed after the OK button is pressed.
ModelMuse has a number of functions for displaying data related to ModelMuse.
The following functions are used to display the values assigned to hydrogeologic units.
The following functions are used to display the values assigned to model cells.
