Kansas Geological Survey, Open-file Report 99-24

Documentation of a combined watershed and stream-aquifer modeling program based on Swat and Modflow

Vol. 1, Users' manual

by Samuel P. Perkins and Marios Sophocleous

KGS Open File Report 99-24
June 1999, revised Jan. 7, 2000
[Note: Source code available as Open-file Rept. 99-25.]

Preface

The purpose of this report is to document a watershed modeling program, referred to here as SWATMOD, which incorporates both Swat (Arnold et al., 1994), a watershed modeling program, and Modflow (McDonald et al., 1988), a stream-aquifer modeling program. This report is intended to serve as a user's manual for the program, to document its application to the Lower Republican River Basin as described by Perkins (1999), and to provide a starting point for an update of the Swat-Modflow linkage scheduled for completion July, 2000 that is to incorporate the latest versions of Swat and Modflow.

The program is based on code development efforts from three separate projects. In each of these we have linked Modflow to a watershed model code and then applied the combined code to develop calibrated computer models for use in examining watershed management questions. Sophocleous and Perkins (1999) present an overview of these studies. This report describes the latest version of this program, which benefits from the experience gained in the course of these projects. The code incorporates the capabilities developed for these studies, yet is simpler and easier to apply than earlier versions.

Initial tests of the combined SWAT-MODFLOW computer code were based on a model of a small watershed with an area of 1.23 km² at Riesel, TX that is gaged and operated by USDA-ARS and identified as Y7. A SWAT -based model of Y7 was used by ARS for SWAT program verification; results from this model are summarized in the SWAT manual (Arnold et al., 1994). Results from our tests of Y7 are described in progress reports to KWO for a study of the Lower Republican River Basin (Sophocleous et al., 1995a and b).

Swat and Modflow were combined for application to Rattlesnake Creek watershed in south central Kansas and to the Lower Republican River Basin in north central Kansas. Similar but separate versions of the combined code were developed on parallel schedules, and are documented separately. The version applied to the Rattlesnake Creek watershed is documented in Perkins and Sophocleous (1998: KGS Open-File Report 98-59), which is an update of Appendix I to the Combined KSU/KGS Final Report (Sophocleous et al., 1997a). In this study, KSU provided the expertise to develop a watershed model based on Swat, while KGS developed the Swat-Modflow linkage and the calibrated stream-aquifer model. The combined Swat-Modflow code was applied to the Lower Republican River Basin in a study for the Kansas Water Office, and is documented in Perkins and Sophocleous (1997: KGS Open-File Report 97-9) as a companion volume to the study's final report (Sophocleous et al., 1997: KGS Open-File Report 97-8). The development and application of these computer codes to Rattlesnake Creek and Lower Republican River basins, respectively, are also reported in Sophocleous et al. (1998) and in Perkins and Sophocleous (1999).

We drew on our experience and computer codes from these studies to develop a combined code for a study of Wet Walnut Creek watershed. In this case Swat was replaced by the program POTYLDR ("Potential Yield Revised", Koelliker, 1994). This was again a collaborative effort in which KSU applied Potyldr to develop a calibrated watershed model, while KGS applied Modflow to develop a calibrated stream-aquifer model, and the two groups coordinated closely to produce a combined calibrated model.

This report is supplemented by appendices as follows. Appendix A was written as an extension to Chapter 3 of Swat's manual (version 2), added as Section 3.4, to describe the format for input data files read by Swat, including additional input required for coordinating Swat with Modflow. Appendix B provides operational details of running the combined programs of Swat and Modflow, and coordinating file device numbers. Appendix C describes input changes made to MODFLOW for its application to the combined watershed-stream-aquifer models. Volume 2 describes compiling and linking the programs, and lists the source code that was written or modified to implement the linkage.

Introduction

This document describes a revised Swat-Modflow linkage with application to the Lower Republican River Basin. The main objectives of the revisions are to simplify and generalize the linkage, both in terms of the code's maintenance and its application to watershed modeling. The version of MODFLOW used in the POTYLDR-MODFLOW linkage for application to Wet Walnut Creek (Sophocleous and Perkins, 1998) was revised by substituting SWAT v.'94 for POTYLDR for application to the Lower Republican River Basin. The calibrated model previously developed for this basin was updated to run under this simplified version of SWATMOD. The base case of our study of the Lower Republican River Basin is used as an example for input data file instructions and in procedures for running the linked program and examining simulation results.

This version is an improvement in several aspects over the original linkage used for the Lower Republican River basin study (Sophocleous et al., 1997) and documented in Perkins and Sophocleous (P & S, 1997):

Techniques for passing data between SWAT and MODFLOW are simpler but more flexible, whether data are passed by file or by reference as subroutine arguments.
Conceptual models for hydrologic connections between the two model codes satisfy continuity more accurately. This is shown by improvements in calibration residuals and in overall watershed water balances (Perkins, 1999).
Two alternatives to the original conceptual model for spatial heterogeneity (HRU scheme) have been installed as options to account for areal fractions of subbasins underlain by bedrock with no aquifer; and to further distinguish between deep and shallow areas of ground water. The two-way coupling between unsaturated and saturated zones arising from shallow ground water is represented by one of the alternative schemes. This coupling of SWAT and MODFLOW solutions was implemented with separate execution of SWAT and MODFLOW and required relatively minor changes to the two programs. Its implementation utilizes the techniques of (a) time-varying HRU weights to reflect ground water response to weather cycles, and (b) successive approximation to pass MODFLOW's results concerning shallow ground water to SWAT. Section 4 of this report discusses options for representing this two-way coupling
The operating procedure for applying the SWAT-MODFLOW linkage to simulations has been simplified by using a single instance of SWAT's Control Codes (~.cod) input file to show in a table how the component HRUs of a scheme are organized to represent spatial heterogeneity. This ~.cod file is used as input to all of the HRU simulations by SWAT and by program SWBAVG, which takes an average over SWAT's results.
The procedure for representing spatial heterogeneity with SWAT as originally developed for application to the Lower Republican River basin has been retained. In this procedure, SWAT, SWBAVG, and MODFLOW are executed separately. This is favored over the procedure outlined in Fig. 1 of the proposal (S & P, 1999), in which SWAT calls MODFLOW as a subroutine. However, the technique employed here of simulating HRUs individually with SWAT and averaging them externally is considered to be an optional mode of operation that should not interfere with alternative approaches that make use of SWAT's capabilities to simulate and average HRUs internally.
The preliminary version of the updated SWAT-MODFLOW linkage incorporates the latest versions of MODFLOW packages developed for the linkage of POTYLDR (Koelliker, 1994) to MODFLOW for application to the Wet Walnut Creek watershed (Sophocleous et al., 1998).

Input data requirements for the combined Swat-Modflow program (SwatMod)

Most of the input data for the combined Swat-Modflow program are described by the respective manuals for Swat (Arnold et at, 1994) and Modflow (McDonald et at, 1988; Prudic, 1989). The manual for Swat describes input for the program but not the format in which it is read, under the assumption that input files are written by a preprocessor available with Swat that runs under GRASS, a public domain, raster-based graphical interface system (GIS). However, Arc-Info is the GIS used here in Kansas by the parties interested in the combined Swat-Modflow program (KGS, KU, KSU, DWR, and KWO). Because a preprocessor for Swat that runs under Arc-Info was not available, a preliminary version of one was developed (L. Bian, personal communication), although it does not take into account revisions to input data format for Swat (v. 2).

Revisions to Swat's original input data format are restricted primarily to its input control file (~.cod), where options regarding variations on original Swat procedures and coordination of Swat and Modflow are specified. Some format revisions were made to input files for soils and weather data to be consistent with formats for version 1 of Swat (1993).

For the combined Swat-Modflow program, an extension to the Swat manual has been written to describe Swat's input format, including revisions. This extension, reproduced here as Appendix A (also inserted after Section 3.3 of the Swat manual as Section 3.4), provides some rudimentary instructions for input files that are to be constructed without the aid of a GIS-based preprocessor. For the purpose of data entry, some preprocessing assistance can be provided by spreadsheet templates for exporting text files that conform to Swat input file formats described by Appendix A.

Changes to Modflow input format have been made that allow the program to operate as expected, given standard input as described by Modflow's manual. The Stream package contains some nonstandard input options, although only standard input is used for the Rattlesnake Creek watershed model. The Well package was modified to allow pumping wells to be distinguished from model wells used to represent nonzero flow boundary conditions.

The Modswb package reads an input file primarily to associate (a) subbasin outflow locations with reaches of a stream network represented by Modflow's Stream package; and (b) the domain of each subbasin with grid cells of the aquifer through an integer array, IBSHED, similar to the IBOUND array read from the input file for Modflow's Basic package.

If Swat and Modflow are executed separately, Modswb also reads a hydrologic summary of results from Swat for each aquifer time step from a balance input file. Alternatively, Swat can make subroutine calls to Modflow, and Swat's results are passed to Modflow by reference; see overviews for linked Swat-Modflow execution control, Preswb, and Modswb.

The complete text of this report is available as an Adobe Acrobat PDF file.

OFR 99-24 (Adobe Acrobat, 1.4 MB)

Kansas Geological Survey, Geohydrology
Placed online Jan. 30, 2019
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