Three Packages for MODFLOW

Copyright, 1997 by Richard B. Winston (rbwinston@mindspring.com)

Download The example input data sets are the data sets used to validate the new packages.

Package 1, Modified River Package "RIV6W"

This is a modified version of RIV5 in MODFLOW96 (Harbaugh and McDonald, 1996a, 1996b). (I have added my last initial to the end of the name of the package to distinguish it from a future possible release of a modified version of the River Package by the USGS.)

RIV6W addresses a problem with maintaining correct boundary conditions when a River cell goes dry. When the head in an aquifer is lower than the river level, the river can recharge the aquifer. However, in MODFLOW, when the head gets sufficiently low, the cell containing the river cell may convert to an inactive cell thus removing this source of recharge. Such a loss of recharge may well be unrealistic if a single aquifer is modeled with two or more model layers. To address this problem, RIV6W allows the modeler to specify a layer beneath each river cell. If the River Cell becomes dry, the River Stress will be applied to the highest active layer down to and including the layer specified by the modeler. For example, suppose that the River cell that becomes dry is in layer 2 and the layer beneath it designated by the modeler is layer 4. When the river cell in layer 2 becomes dry, the stress will be applied to layer 3. If that also becomes dry, the stress will be applied to layer 4. If layer 4 becomes dry, the stress will not be applied to layer 5. Until one of the cells to which the stress can apply becomes wet, the stress will no longer be applied.

RIV6W makes use of an auxiliary parameter named "LOWLAY" to specify the lowest layer to which the river stress can apply. If "LOWLAY" is not included in the list of auxiliary parameters on line 1 of the RIV6W input file, RIV6W will act exactly the same as RIV5. Thus RIV6W is compatible with existing River package input files because those files will not include "LOWLAY" as an auxiliary parameter.

Package 2, Modified Streamflow Routing Package "STR3W"

STR3W is derived from version 2 of the Streamflow-Routing package (Prudic, 1989). It makes changes in the Streamflow-Routing package analogous to those used to modify the RIV5 package and for similar reasons.

Like RIV6W, it uses an auxiliary parameter "LOWLAY" to designate the lowest layer to which the stream stress can apply. Thus, part of the changes to the Streamflow-Routing package, was to add the ability to read auxiliary parameters to the Streamflow-Routing package. In addition, the first three data items can now be read in free format. The input formats for the remaining three data items remain unchanged.

If "LOWLAY" is not included as an auxiliary layer, STR3W acts exactly like version 2 of the Streamflow-Routing Package. Thus STR3W is compatible with existing Streamflow-Routing package input files because those files will not include "LOWLAY" as an auxiliary parameter.

Package 3, Time Variant Conductance Package "TVC1"

The Time Variant Conductance package "TVC1" is based in part on the Block-Centered Flow Package (BCF5) (Harbaugh and McDonald, 1996a, 1996b) and in part on the Horizontal-Flow Barrier package (HFB1) but it departs significantly from both packages and provides additional functionality not provided by either of those packages. TVC1 should be especially useful in for three situations: 1. multiaquifer wells, 2. horizontal wells, and 3. flow along active faults for which the hydraulic conductivity changes as a function of time.

TVC1 allows the user to change the conductance between any pair of adjacent cells at the beginning of any stress period. These changes will then remain in effect for the remainder of the model run or until explicitly changed in a later stress period. If the conductance between TVC1 cells is sufficiently high, all the linked TVC1 cells will maintain the same head. If any one of the linked TVC1 cells is also a well cell, the well flux will, in effect, be applied to all the cells. In addition, the model and not the modeler will determine the appropriate flux for each cell. This could be especially advantageous for horizontal wells where the appropriate flux to maintain a uniform head in a uniform aquifer will depend not only on the cell size but also the position of an individual cell along the horizontal well. (The cells at the ends of a horizontal well will have a higher flux than those at the middle.)

Code Validation

The new packages have been validated as described in the users guides.

Distribution

Those interested in obtaining copies of the revised packages or a compiled version of MODFLOW containing them should contact me. The three new packages may be purchased for $350. This includes

(On request, the compiled version can be for Windows 3.1 and DOS.)

References

Harbaugh, A.W., and McDonald, M.G. 1996a. User's Documentation for MODFLOW-96, An Update to the U.S. Geological Survey Modular Finite-Difference Ground-Water Flow Model.: U.S. Geological Survey Open-File Report 96-485, 56 p.

Harbaugh, A.W., and McDonald, M.G. 1996b. Programmer's Documentation for MODFLOW-96, An Update to the U.S. Geological Survey Modular Finite-Difference Ground-Water Flow Model.: U.S. Geological Survey Open-File Report 96-486, 220 p.

Hsieh, P.A. and Freckleton, J.R., 1993, Documentation of a computer program to simulate horizontal-flow barriers using the U.S. Geological Survey modular three-dimensional finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 92-477, 32 p.

Prudic, D.E., 1989, Documentation of a computer program to simulate stream-aquifer relations using a modular, finite-difference, ground-water flow model: U.S. Geological Survey Open-File Report 88-729, 113 p.