RIV6W and STR3W: Documentation of Modified Versions of the River and Streamflow-Routing Package that Retains Correct Boundary Conditions When Cells Go Dry

By

Richard B. Winston

2163 Tulip St.

Baton Rouge, La, 70806

504-344-0646

rbwinston@mindspring.com

Copyright, 1997

This is an abbreviated copy of the users guide for RIV6W and STR3W. The programmer's documentation has not been included here.

Introduction *

Code Verification *

References *

Programmerís documentation RIV6W package *

Input instructions RIV6W package *

EXPLANATION OF PARAMETERS READ BY THE RIV PACKAGE *

Example Input Files *

Name File *

BASIC Package *

RIV6TEST.I0 (IUNIT Array, Layers 1 and 2) *

RIV6TEST.H0 (Initial head, Layers 1 and 2) *

RIV6TEST.BCF (Block-Centered Flow) *

RIV6TEST.SO (Primary Storage Parameter, Layer 1) *

RIV6TEST.S1 (Primary Storage Parameter, Layer 2) *

RIV6TEST.K0 (Hydraulic conductivity, Layers 1 and 2) *

RIV6TEST.WEL (Well Package) *

RIV6TEST.RIV (River Package) *

RIV6TEST.PCG (Preconditioned Conjugate Gradient Package 2) *

RIV6TEST.OC (Output Control Option) *

Changes in the Main Program *

Module Documentation for the Revised River Package *

Module RIV6WAL *

Module RIV6WRP *

Module RIV6WFM *

Module RIV6WBD *

Programmerís documentation STR3W package *

Input instructions STR3W package *

Explanation of Fields used in Input Instructions *

Example Input files *

MODFLOW.BF *

Name File, (STR3) *

Basic Package (STR3.bas) *

Block-Centered Flow Package (STR3.bcf) *

Modified Stream Package (STR3.STR) *

Well Package (STR3.WEL) *

Output Control Option (STR3.OC) *

Preconditioned Conjugate Gradient Package 2 (STR3.PCG) *

Changes in the Main Program *

Module Documentation for the Revised Streamflow-Routing Package *

Module STR3WAL *

Module STR3WRP *

Module STR3WFM *

Module STR3WBD *

Module UGETLAY *

 

Introduction

MODFLOW (McDonald and Harbaugh, 1988) is one of the most commonly used groundwater modeling programs. However, it does have limitations. One of those limitations is the way that it deals with river cells when those cells become dry. Under normal conditions, water can flow into or out of the river depending on whether the head is higher in the river or in the cell containing it. Thus when, the head in the aquifer is low, a river can act as a source of water to the aquifer. However, if the river cell goes dry, it is designated as an inactive cell and the river will no longer supply water to the aquifer. In cases where a single aquifer is modeled using two or more model layers, such a cessation of recharge may be inappropriate. The Streamflow Routing package has similar limitations (Prudic, 1989). This paper describes a modification of MODFLOW that allows river stresses to be applied to lower layers when a river cell goes dry. Because the user may wish to limit the depth to which recharge from a River or Streamflow Routing cell will apply, a new parameter must be specified for each River or Stream cell designating the lowest layer to which the river stress can apply. Generally, this will be the layer just above the highest significant aquitard beneath the River or Stream cell.

Code Verification

The revised versions of the River and Streamflow Routing packages were checked by comparing the results when they were employed against comparable models that did not employ them. Three models were prepared for each revised package. One model, the "test" model, employed the revised package. It had two layers one of which contained river or stream cells that would go dry and then rewet during model execution. The second model, the "two-layer control" model was identical to the first model except that it did not use the revised package. Because cells went dry during execution, it was anticipated that heads in the two-layer control model would be different from those of the test model. The third model, the "one-layer control" model, was identical to the other two models except that it only had one layer. Because cells did not go dry in the one-layer control, it was anticipated that its heads would be similar to those of the test model but different from those of the two-layer control model. In both the test model and the two-layer control model, the vertical hydraulic conductivity was set to 10 times the horizontal hydraulic conductivity so that heads in both layers would be nearly the same. This made these models more comparable to the one layer model than would otherwise have been the case.

Figure 1. Grid used in test of revised River and Streamflow-Routing Packages.

 

In tests of both the revised River and Streamflow Routing packages, the models had 10 rows and 11 columns. The first and last columns were designated as constant head cells. A river or stream was in the top layer in column 6 and wells were in the bottom layer in column 6 (fig. 1). The pumping rate in the wells was adjusted so that it was sufficient to make the top layer go dry in the cells containing the wells. The conductances of the River or Stream cells were adjusted so that leakage from the River or Stream could supply a significant part of the water budget.

Both the revised packages resulted in a closer match to the one-layer control models than did the two-layer control models. When the river cells went dry, head in the underlying well cells dropped rapidly in the two-layer control but not in the test model with the revised package (fig. 2). Eventually, the Well cells also went dry in the two-layer control but not the test model. Once the Well cells went dry, head in the remaining cells rose above the levels in the one-layer control. In the test with the revised Streamflow Routing package, the results were similar except that the well cells did not go dry (fig. 3).

Figure 2. Comparison of results of modified and unmodified River packages.

 

 

 

Figure 1. Comparison of results of modified and unmodified Streamflow-Routing packages.

References

Harbaugh, A.W., and McDonald, M.G. 1996. Userís Documentation of 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.

McDonald, M.G. and Harbaugh, A.W., 1988, A modular three-dimensional finite-difference ground-water flow model. U.S. Geological Survey Techniques of Water-Resources Investigations Book 6, Chapter A1, 586 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.