retc_win(土壤水分特征曲线)

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

ii

ABSTRACT

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

iii

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

2. PARAMETRIC MODELS FOR THE SOIL HYDRAULIC FUNCTIONS...........4

2.1. Soil Water Retention Models...........................................4

2.2. Mualem's Hydraulic Conductivity Model..................................13

2.3. Burdine's Hydraulic Conductivity Model..................................30

2.4 Parameter Estimation...............................................323. THE RETC USER GUIDE..............................................42

3.1. Program Options...................................................42

3.2. Code Structure and Program Preparation.................................444. SUMMARY AND CONCLUSIONS 51REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52APPENDICES

A. Listings of the Control, Input and Output Files for Five Examples

FIGURES

Page

1.

Soil water retention curves based on (7) for various values of n

7

2. Semi-logarithmic (a) and regular (b) plots of soil water retention

curves based on (7) with

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4. Observed and fitted retention curves for Touchet silt loam

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...18

8. Dimensionless semilogarithmic plot of the relative hydraulicconductivity,

Mualem’s model with for various values of m. The curves were predicted from (7) using 0.5). 21

10. Predicted relative hydraulic conductivity curves for Touchet silt

loam (Mualem’s model with 21

viii .

. 22

Predicted relative hydraulic conductivity curves versus pressure

head (a) and volumetric water content (b) for Sarpy loam (Mualem’s

in Mualem’s model (Eq. 8 with

25

Observed and fitted unsaturated soil hydraulic functions for crushed

Bandelier tuff. The calculated retention (a) and hydraulic conductivity

(b) curves were based on (7) and Calculated curves for the relatively hydraulic conductivity versus

reduced pressure head (a) and reduced water content (b) as predicted

from the retention curves in Figure 2 using Burdine’s model (Eq. 46)

.. . 33

versus reduced water content, S,, for variousvalues of

= 2 and assuming .

TABLES

1. Fitted soil hydraulic parameters for the retention curves plotted in

Figures 3 through 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102. Fitted soil hydraulic parameters for crushed Bandelier Tuff 293. Average values for selected soil water retention and hydraulic

conductivity parameters for 11 major soil textural groups according

to Rawls et al. [1982]

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7. Outline of the control file RETC.CTL 468. Schematic of the output generated with RETC

1. INTRODUCTION

Interest in the unsaturated (vadose) zone has dramatically increased in recent years becauseof growing evidence and public concern that the quality of the subsurface environment is beingadversely affected by industrial, municipal and agricultural activities. Computer models are nowroutinely used in research and management to predict the movement of water and chemicals intoand through the unsaturated zone of soils.Such models can be used successfully only if reliableestimates of the flow and transport properties of the medium are available.Current technology ofdeveloping sophisticated numerical models for water and solute movement in the subsurface seemsto be well ahead of our ability to accurately estimate the increasing number of parameters whichappear in those models. This is especially true for the unsaturated soil hydraulic properties whichby far are the most important parameters affecting the rate at which water and dissolved chemicalsmove through the vadose zone. While a large number of laboratory and field methods have beendeveloped over the years to measure the soil hydraulic functions [Mute, 1986], most methods arerelatively costly and difficult to implement.Accurate in situ measurement of the unsaturatedhydraulic conductivity has remained especially cumbersome and time-consuming. Thus, cheaper andmore expedient methods for estimating the hydraulic properties are needed if we are to implementimproved practices for managing water and chemicals in the unsaturated zone.

One alternative to direct measurement of the unsaturated hydraulic conductivity is to usetheoretical methods which predict the conductivity from more easily measured soil water retentiondata. Such theoretical methods are generally based on statistical pore-size distribution modelswhich assume water flow through cylindrical pores and incorporate the equations of Darcy andPoiseuille. A large number of models of this type have appeared in the soil science and petroleumengineering literature during the past several decades.These include the models by Gates and Lietz

[1950], Childs and Collis-George [1950], Burdine [1953], Millington and Quirk [1961], and Mualem

[1976a], among others. An excellent review of previously published pore-size distribution modelswas given recently by Mualem [1986]. Implementation of these predictive conductivity models stillrequires independently measured soil water retention data.Measured input retention data may begiven either in tabular form, or by means of closed-form analytical expressions which containparameters that are fitted to the observed data.While a large number of analytical soil waterretention functions have been proposed, only a few functions can be easily incorporated into the

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predictive pore-size distribution models to yield relatively simple analytical expressions for theunsaturated hydraulic conductivity function.

The use of analytical functions in soil water flow studies has several advantages. For example,they allow for a more efficient representation and comparison of the hydraulic properties ofdifferent soils and soil horizons. They are also more easily used in scaling procedures forcharacterizing the spatial variability of soil hydraulic properties across the landscape. And, if shownto be physically realistic over a wide range of water contents, analytical expressions provide amethod for interpolating or extrapolating to parts of the retention or hydraulic conductivity curvesfor which little or no data are available. Analytical functions also permit more efficient datahandling in unsaturated flow models, particularly for multidimensional simulations involving layeredsoil profiles.

Because of their simplicity and ease of use, predictive models for the hydraulic conductivityhave become very popular in numerical studies of unsaturated flow. Results thus far suggest thatpredictive models work reasonably well for many coarse-textured soils and other porous mediahaving relatively narrow pore-size distributions, but that predictions for many fine-textured andstructured field soils remain inaccurate. Because of the time-consuming nature of direct fieldmeasurement of the hydraulic conductivity, and in view of the field-scale spatial variability problem,it nevertheless seems likely that predictive models (including those that predict the hydraulicproperties from soil texture and related data) provide the only viable means of characterizing thehydraulic properties of large areas of land, whereas direct measurement may prove to be cost-

effective only for site-specific problems

(2) a more flexible choice of hydraulic parameters to be included in the parameter optimizationprocess, and (3) the possibility of evaluating the model parameters from observed conductivity datarather than only from retention data, or simultaneously from measured retention and hydraulicconductivity data. Although the models used in RETC are intended to describe the unsaturatedsoil hydraulic properties for monotonic drying or wetting in homogeneous soils, the code can beeasily modified to account for more complicated flow processes such as hysteretic two-phase flow

[Lenhard et al., 1991] or preferential flow [Germann, 1990].

3

is

approximated by the slope

the hydraulic conductivity

0):

where

(2)(3)

Parametric models of these functions are reviewed in detail below.

2.1. Soil Water Retention Models

Several functions have been proposed to empirically describe the soil water retention curve.One of the most popular functions has been the equation of Brook and Corey [1964], further

referred to as the BC-equation:

(ah

1)(4)

4

where

are the residual and saturated water contents, respectively;

whose inverse is often referred to as the air entry value or bubbling pressure, and

in (4) specifies the maximum amount of water in a soil that will

not contribute to liquid flow because of blockage from the flow paths or strong adsorption onto thesolid phase [Luckner et al., 1989]. Formally,

and

The saturated water content,

of field soils is generally about 5 to

10% smaller than the porosity because of entrapped or dissolved air. Following van Genuchten andNielsen [1985] and Luckner et al. [1989],

in this study are viewed as being essentiallyempirical constants in soil water retention functions of the type given by (4), and hence withoutmuch physical meaning.

Equation (4) may be written in a dimensionless form as follows

=

1

(ah

is the effective degree of saturation, also called the reduced water content (0

l/a.Because of their simple form, (4) and (5) have been used in numerous unsaturated flowstudies. The BC-equation has been shown to produce relatively accurate results for many coarse-

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textured soils characterized by relatively narrow pore- or particle-size distributions (large R-values).Results have generally been less accurate for many fine-textured and undisturbed field soils becauseof the absence of a well-defined air-entry value for these soils.

Several continuously differentiable (smooth) equations have been proposed to improve thedescription of soil water retention near saturation.These include functions introduced by King

A related smooth function with attractive properties is the equationof van Genuchten [1980],

further referred to as the VG-equation:

[l

=mn. The same limiting curve also appears when n in (7) is allowed to go to

infinity, while simultaneously decreasing m such that the product, mn, remains the same at 0.4. Asshown in Figure 2, the limiting BC equation exhibits a sharp break in the curve at the air entryvalue