EPA/600/2-91/065December 1991

The RETC Code for Quantifying the Hydraulic Functions

of Unsaturated Soils

by

M. Th. van Genuchten, F. J. Leij and S. R. Yates

U.S. Salinity Laboratory

U.S. Department of Agriculture, Agricultural Research Service

Riverside, California 9250 1

IAG-DW12933934

Project Officer

Joseph R. Williams

Extramural Activities and Assistance Division

Robert S. Kerr Environmental Research Laboratory

Ada, Oklahoma 74820

ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY

OFFICE OF RESEARCH AND DEVELOPMENT

U. S. ENVIRONMENTAL PROTECTION AGENCY

ADA, OKLAHOMA 74820

DISCLAIMERS The information in this document has been funded in part by the United States Environmental Protection Agency under IAG-DW12933934 to the Agricultural Research Service, U. S. Department of Agriculture. It has been subjected to the Agency’s peer and administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. This report documents the RETC computer program for analyzing or predicting the unsaturated soil hydraulic properties. RETC is a public domain code and may be used and copied freely. The code has been tested against a large number of soil hydraulic data sets, and was found to work correctly. However, no warranty is given that the program is completely error-free. If you do encounter problems with the code, find errors, or have suggestions for improvement, please contact M. Th. van Genuchten or F. J. Leij U. S. Salinity Laboratory 4500 Glenwood Drive Riverside, CA 92501 Tel. 714-369-4846 Fax. 714-369-4818

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ABSTRACT

input preparation and sample input and outputfiles. A listing of the source code is also provided.

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ACKNOWLEDGMENTS The authors wish to thank the many individuals who have contributed in small or large parts to improve the RETC program over the past several years. In particular, we thank Walter Russell and Renduo Zhang of the U. S. Salinity Laboratory, and Fereidoun Kaveh of Chamran University (Ahwaz, Iran), for their help in running the code for a large number of data sets, and for providing various plotting routines to graphically evaluate the computer output. We also thank Joseph Williams of the Robert S. Kerr Environmental Research Laboratory (U. S. Environmental Protection Agency, Ada, Oklahoma) for his many helpful suggestions, and his critical review of this report.

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FOREWORD EPA is charged by Congress to protect the Nation’s land, air and water systems. Under a mandate of national environmental laws focused on air and water quality, solid waste management and the control of toxic substances, pesticides, noise and radiation, the Agency strives to formulate and implement actions which lead to a compatible balance between human activities and the ability of natural systems to support and nurture life. The Robert S. Kerr Environmental Research Laboratory is the Agency’s center of expertise for investigation of the soil and subsurface environment. Personnel at the Laboratory are responsible for management of research programs to: (a) determine the fate, transport, and transformation rates of pollutants in the soil, and the unsaturated and saturated zones of the subsurface environment; (b) define the processes to be used in characterizing the soil and subsurface environment as a receptor of pollutants; (c) develop techniques for predicting the effect of pollutants on ground water, soil and indigenous organisms; and (d) define and demonstrate the applicability and limitations of using natural processes, indigenous to the soil and the subsurface environment, for the protection of this resource. The EPA uses numerous mathematical models to predict and analyze the movement of water and dissolved contaminants in the saturated and unsaturated zones of the subsurface environment. The usefulness of these models, and the accuracy with which model predictions can be made, depends greatly on the ability to reliably characterize the hydraulic properties of the unsaturated zone. This report discusses several theoretical models which may be used to quantify the unsaturated soil hydraulic properties involving the soil water retention and hydraulic conductivity fuctions. The report includes a computer program which predicts, among other things, the unsaturated hydraulic conductivity from independently measured soil water retention data. Several examples illustrate the applicability of the model to

different types of hydraulic data. The information in this report should be of interest to all those concerned with the development of improved methods for predicting or managing water and contaminant transport in partly saturated soils.

Clinton W. Hall Director Robert S. Kerr Environmental Research Laboratory

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CONTENTS

Page

DISCLAIMERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

. . .ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivFOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

. . .FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x

1. INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

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