ASTM D3404-91(1998)
(Guide)Standard Guide for Measuring Matric Potential in the Vadose Zone Using Tensiometers
Standard Guide for Measuring Matric Potential in the Vadose Zone Using Tensiometers
SCOPE
1.1 This guide covers the measurement of matric potential in the vadose zone using tensiometers. The theoretical and practical considerations pertaining to successful onsite use of commercial and fabricated tensiometers are described. Measurement theory and onsite objectives are used to develop guidelines for tensiometer selection, installation, and operation.
1.2 The values stated in SI units are to be regarded as the standard. The inch-pound units given in parentheses are for information only.
1.2 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D 3404 – 91 (Reapproved 1998)
Standard Guide for
Measuring Matric Potential in the Vadose Zone Using
Tensiometers
This standard is issued under the fixed designation D 3404; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 2.1.3 precision (repeatability)—the variability among nu-
merous measurements of the same quantity.
1.1 This guide covers the measurement of matric potential
2.1.4 resolution—the smallest division of the scale used for
in the vadose zone using tensiometers. The theoretical and
a measurement and it is a factor in determining precision and
practical considerations pertaining to successful onsite use of
accuracy.
commercial and fabricated tensiometers are described. Mea-
surement theory and onsite objectives are used to develop
3. Summary of Guide
guidelines for tensiometer selection, installation, and opera-
3.1 The measurement of matric potential in the vadose zone
tion.
can be accomplished using tensiometers that create a saturated
1.2 The values stated in SI units are to be regarded as the
hydraulic link between the soil water and a pressure sensor. A
standard. The inch-pound units given in parentheses are for
variety of commercial and fabricated tensiometers are com-
information only.
monly used. A saturated porous ceramic material that forms an
1.3 This standard does not purport to address all of the
interface between the soil water and bulk water inside the
safety problems, if any, associated with its use. It is the
instrument is available in many shapes, sizes, and pore
responsibility of the user of this standard to establish appro-
diameters. A gage, manometer, or electronic pressure trans-
priate safety and health practices and determine the applica-
ducer is connected to the porous material with small- or
bility of regulatory limitations prior to use.
large-diameter tubing. Selection of these components allows
1.4 This guide offers an organized collection of information
the user to optimize one or more characteristics, such as
or a series of options and does not recommend a specific
accuracy, versatility, response time, durability, maintenance,
course of action. This document cannot replace education or
extent of data collection, and cost.
experience and should be used in conjunction with professional
judgment. Not all aspects of this guide may be applicable in all
4. Significance and Use
circumstances. This ASTM standard is not intended to repre-
4.1 Movement of water in the unsaturated zone is of
sent or replace the standard of care by which the adequacy of
considerable interest in studies of hazardous-waste sites (1, 2,
a given professional service must be judged, nor should this
3, 4) ; recharge studies (5, 6); irrigation management (7, 8, 9);
document be applied without consideration of a project’s many
and civil-engineering projects (10, 11). Matric-potential data
unique aspects. The word“ Standard” in the title of this
alone can be used to determine direction of flow (11) and, in
document means only that the document has been approved
some cases, quantity of water flux can be determined using
through the ASTM consensus process.
multiple tensiometer installations. In theory, this technique can
2. Terminology be applied to almost any unsaturated-flow situation whether it
is recharge, discharge, lateral flow, or combinations of these
2.1 Definitions of Terms Specific to This Standard:
situations.
2.1.1 accuracy of measurement—the difference between the
4.2 If the moisture-characteristic curve is known for a soil,
value of the measurement and the true value.
matric-potential data can be used to determine the approximate
2.1.2 hysteresis—that part of inaccuracy attributable to the
water content of the soil (10). The standard tensiometer is used
tendency of a measurement device to lag in its response to
to measure matric potential between the values of 0 and −867
environmental changes. Parameters affecting pressure-sensor
cm of water; this range includes most values of saturation for
hysteresis are temperature and measured pressure.
many soils (12).
This guide is under the jurisdiction of ASTM Committee D-18 on Soil and
Rock and is the direct responsibility of Subcommittee D18.21 on Ground Water and
Vadose Zone Investigations. The boldface numbers in parentheses refer to a list of references at the end of
Current edition approved May 15, 1991. Published October 1991. the text.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 3404
4.3 Tensiometers directly and effectively measure soil-water combined with matric-potential data to estimate flux. In either
tension, but they require care and attention to detail. In case, the accuracy of the flux estimate needs to be assessed
particular, installation needs to establish a continuous hydraulic dK dK
carefully. For many porous media, and are large, within
connection between the porous material and soil, and minimal dc du
disturbance of the natural infiltration pattern are necessary for certain ranges of c or u, making estimates of K particularly
successful installation. Avoidance of errors caused by air sensitive to onsite-measurement errors of c or u. (Onsite-
invasion, nonequilibrium of the instrument, or pressure-sensor
measurement errors of c also have direct effect on „(c + Z)in
inaccuracy will produce reliable values of matric potential.
Darcy’s Law). Other sources of error in flux estimates can
4.4 Special tensiometer designs have extended the normal
result from: inaccurate data used to establish the K (c)or K (u)
capabilities of tensiometers, allowing measurement in cold or
functions (accurate measurement of very small permeability
remote areas, measurement of matric potential as low as −153
values is particularly difficult) (16); use of an analytical
m of water (−15 bars), measurement at depths as deep as 6 m
expression for K (c)or K (u) that facilitates computer
(recorded at land surface), and automatic measurement using
simulation, but only approximates the measured data; an
as many as 22 tensiometers connected to a single pressure
insufficient density of onsite measurements to define ad-
transducer, but these require a substantial investment of effort
equately the u or c profile, which can be markedly nonlinear;
and money.
onsite soil parameters that are different from those used to
4.5 Pressure sensors commonly used in tensiometers in-
establish K (c)or K (u); and invalid assumptions about the
clude vacuum-gages, mercury manometers, and pressure trans-
state of onsite hysteresis. Despite the possibility of large errors,
ducers. Only tensiometers equipped with pressure transducers
certain flow situations occur where these errors are minimized
allow for the automated collection of large quantities of data.
and fairly accurate estimates of flux can be obtained (6, 17).
However, the user needs to be aware of the pressure-transducer
The method has a sound theoretical basis and refinement of the
specifications, particularly temperature sensitivity and long-
theory to match measured data markedly would improve
term drift. Onsite measurement of known zero and “full-scale”
reliability of the estimates.
readings probably is the best calibration procedure; however,
5.3 The concept of fluid tension refers to the difference
onsite temperature measurement or periodic recalibration in the
laboratory may be sufficient. between standard atmospheric pressure and the absolute fluid
pressure. Values of tension and pressure are related as follows:
5. Measurement Theory
T 5 P 2 P (2)
F AT F
5.1 In the absence of osmotic effects, unsaturated flow
obeys the same laws that govern saturated flow: Darcy’s Law where:
M
and the Equation of Continuity, that were combined as the T 5
F
the tension of an elemental volume of fluid, ,
F G
Richards’ Equation (13). Baver et al. (14) presents Darcy’s
LT
Law for unsaturated flow as follows:
P 5 the absolute pressure of the standard atmosphere,
AT
q52K„~c1 Z! (1)
M
, and
F 2G
where:
LT
L
q 5
the specific flow, ,
F G
T
P 5 the absolute pressure of the same elemental volume
F
L
K 5
M
the unsaturated hydraulic conductivity, ,
F G
T
or fluid .
F G
LT
c5 the matric potential of the soil water at a point, [L],
Z 5 the elevation at the same point, relative to some
Soil-water tension (or soil-moisture tension) similarly is
datum, [L], and
equal to the difference between soil-gas pressure and soil-water
−1
„5 the gradient operator, [L ].
pressure. Thus:
The sum of c + Z commonly is referred to as the hydraulic
T 1 P 5 P (3)
W G W
head.
5.2 Unsaturated hydraulic conductivity, K, can be expressed
where:
as a function of either matric potential, c, or water content,
T 5 the tension of an elemental volume of soil water,
W
3 3
u[L of water/L of soil], although both functions are affected M
,
F 2G
by hysteresis (5). If the wetting and drying limbs of the K (c)
LT
function are known for a soil, time series of onsite matric-
potential profiles can be used to determine: which limb is more
P 5 the absolute pressure of the surrounding soil gas,
G
appropriate to describe the onsite K (c); the corresponding M
, and
F G
values of the hydraulic-head gradient; and an estimate of flux
LT
using Darcy’s Law. If, instead, K is known as a function of u,
onsite moisture-content profiles (obtained, for example, from
neutron-scattering methods) can be used to estimated K, and
D 3404
P 5 the absolute pressure of the same elemental volume
W
M
of soil water, .
F 2G
LT
In this guide, for simplicity, soil-gas pressure is assumed to
be equal to 1 atmosphere, except as noted. Various units are
used to express tension or pressure of soil water, and are related
to each other by the equation:
1.000 bar 5 100.0 kPa 5 0.9869 atm 5
1020 cm of water at 4°C 5
1020 g per cm in a standard
gravitational field.
(4)
A standard gravitational field is assumed in this guide; thus,
centimetres of water at 4°C are used interchangeably with
grams per square centimetre.
5.4 The negative of soil-water tension is known formally as
matric potential. The matric potential of water in an unsatur-
ated soil arises from the attraction of the soil-particle surfaces FIG. 1 Enlarged Cross Section of Porous Cup-Porous Medium
Interface
for water molecules (adhesion), the attraction of water mol-
ecules for each other (cohesion), and the unbalanced forces
across the air-water interface. The unbalanced forces result in
where:
the concave water films typically found in the interstices T 5 the soil-water tension relative to atmospheric pres-
W
between soil particles. Baver et al. (14) present a thorough sure, in centimetres of water at 4°C,
P 5 the atmospheric pressure, in centimetres of water
discussion of matric potential and the forces involved.
A
at 4°C,
5.5 The tensiometer, formally named by Richards and
P 5 the average pressure in the porous cup and soil, in
W
Gardner (18), has undergone many modifications for use in
centimetres of water at 4°C,
specific problems (1, 11, 19-31). However, the basic compo-
r 5 the average density of the mercury column, in
Hg
nents have remained unchanged. A tensiometer comprises a
grams per cubic centimetre,
porous surface (usually a ceramic cup) connected to a pressure
r 5 the average density of the water column, in grams
H O
sensor by a water-filled conduit. The porous cup, buried in a
per cubic centimetre,
soil, transmits the soil-water pressure to a manometer, a
r 5 the reading, or height of mercury column above
vacuum gage, or an electronic-pressure transducer (referred to
the mercury-reservoir surface, in centimetres,
in this guide as a pressure transducer). During normal opera-
h 5 the height of the mercury-reservoir surface above
tion, the saturated pores of the cup prevent bulk movement of
land surface, in centimetres, and
soil gas into the cup.
d 5 the depth of the center of the cup below land
5.6 An expanded cross-sectional view of the interface be-
surface, in centimetres.
tween a porous cup and soil is shown in Fig. 1. Water held by
5.7 Although the density of mercury and water both vary
the soil particles is under tension; absolute pressure of the soil
about 1 % between 0 and 45°C, Eq 5 commonly is used with
water, P , is less than atmospheric. This pressure is transmitted
W
r and r constant.
Hg H O
through the saturated pores of the cup to the water inside the
5.7.1 Using r 5 13.54 and r 5 0.995 (the median
Hg H O
cup. Conventional fluid statics relates the pressure in the cup to
values for this temperature range) yields about a 0.25 % error
the reading obtained at the manometer, vacuum gage, or
(1.5 cm H O) at 45°C, for Tw ’ 520 cm H O. This small, but
2 2
pressure transducer.
needless, error can be removed by using the following density
5.6.1 In the case of a mercury manometer (see Fig. 2(a)):
functions:
T 5 P 2 P 5 ~r 2r !r2r ~h 1 d! (5)
W A W Hg H O H O r 5 13.595 2 2.458 3 10 ~T! (6)
2 2 Hg
D 3404
FIG. 3 Porous-Cup and Tube Designs
FIG. 2 Three Common Types of Tensiometers: (a) Manometer; (b)
water and then setting the gage to zero while immersing the
Vacuum Gage; and (c) Pressure Transducer
porous cup to its midpoint in a container of water. This setting
is done at the altitude at which the tensiometer will be used and
it needs to be repeated periodically after installation either by
and
removing the tensiometer from the soil or by unscrewing the
25 26 2
r 5 0.9997 1 4.879 3 10 ~T! 2 5.909 3 10 ~T! (7)
H O
gage and measuring a tension equal to that used in the original
where: r and r are as defined above, and calibration. The gage then reads directly the tension in the
Hg H O
T 5 average temperature of the column, in° C. porous cup. Use of a vacuum gage without an adjustable zero
5.7.2 Average temperature of the buried segment of water reading could result in inaccurate measurements because the
column can be estimated with a thermocouple or thermistor in zero-reading could become negative and, therefore, would be
contact with the tubing, buried at about 45 % of the depth of indeterminate.
the porous cup. Air temperature is an adequate estimate for 5.9 Pressure transducers convert pressure, or pressure dif-
exposed segments. ference, into a voltage (or current) signal. The pressure
5.8 Most vacuum gages used with tensiometers are gradu- transducer can be connected remotely to the porous cup with
ated in bars (and centibars) and have an adjustable zero- tubing (22, 24) att
...
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