Environmental Engineering Reference
In-Depth Information
by these phenomena than other EM methods
(Ferré
et al
.,
2007
).
Ground-penetrating radar (GPR) is another
approach based on measurement of the dielec-
tric permittivity of soils and water. GPR meas-
urements are obtained over a longer distance
than TDR measurements (
Table 5.1
). GPR meas-
urements can be made on land surface; meas-
urements can also be obtained between two
or more boreholes in the subsurface (Huisman
et al
.,
2003
). Surface measurements are useful
for identifying different soil textures, but quan-
tification of soil-water content has not met with
much success (Ferré
et al
.,
2007
). Borehole GPR
can provide accurate estimates of water con-
tent (Binley
et al
.,
2001
), but instrumentation is
expensive, and the requirement for two bore-
holes limits its use.
The electromagnetic induction (EMI) method
measures the electrical conductivity of a bulk
soil/water mixture (Ferré
et al
.,
2007
). EMI is
widely used to determine electric and hydraulic
properties of geologic material and to map areas
of similar properties. Land-surface-based and
airborne EMI surveys are run along transects
that can stretch for hundreds of meters. EMI is
also an important borehole logging technique.
EMI can provide qualitative information on soil
texture, but attempts to obtain quantitative
estimates of soil-water content with EMI have
met with only limited success (Cook and Kilty,
1992
; Scanlon
et al
.,
1999
).
The neutron moderation method for deter-
mining soil-water content has been in use since
the early 1950s. Instrumentation has improved
over the years, but the basic theory remains
the same (Hignett and Evett,
2002
). A radio-
active source within a probe emits fast-moving
neutrons. Hydrogen atoms are very effective at
slowing the movement of the neutrons because
the atoms are about the same size as the neu-
trons. A sensor within the same probe detects
the number of slowed or thermalized neutrons.
Water molecules account for most of the hydro-
gen atoms in the subsurface, so neutron meters
can be calibrated to determine soil-water con-
tent, usually by comparing instrument read-
ings with gravimetric analysis of soil samples
(Hignett and Evett,
2002
). Measurements are
obtained by logging a cased borehole or spe-
cial access tube. Generally, readings must be
taken manually because of the radioactive
source. Special licensing is usually required to
use a neutron probe. In contrast to emplaced
sensors (such as TDR probes), which offer high
frequency water-content measurements at a
limited number of depths, borehole logging
tools, such as the neutron probe, allow many
depths to be sampled, but generally at a much
lower sampling frequency.
The temporal gravity technique described
in
Section 2.3.3
is included in
Table 5.1
. As
discussed in
Section 2.3.3
, gravity measure-
ments can be used to estimate total change
in subsurface water storage. However, sur-
face measurements cannot distinguish stor-
age changes within specific depth intervals of
the unsaturated zone (i.e. intervals above or
beneath the zero-flux plane). Therefore, gravity
measurements can be used with the zero-flux
plane method only if an independent approach
is available for estimating storage changes
between land surface and the zero-flux plane.
5.2.2 Pressure head
Several devices are available for
in-situ
measure-
ment of pressure head (sometimes referred to as
soil-water tension or matric potential), includ-
ing tensiometers, heat-dissipation probes, and
thermocouple psychrometers (Dane and Topp,
2002
). Unfortunately, there is no universal
method for measuring pressure head that can
be applied at any site. Each technique is applic-
able only within a specific range of pressure
heads. Tensiometers are generally regarded as
the most accurate instrument for measuring
pressure heads in the range of 0 to -10 m of
water, and for this reason they are commonly
used in applications of the ZFP and Darcy meth-
ods (Cooper
et al
.,
1990
). Tensiometers consist of
a sealed water-filled tube with a porous ceramic
cup that is placed in contact with soil. Soil-water
pressures are transmitted across the ceramic
cup and measured with an attached pressure
transducer, manometer, or other pressure
gauge. Pressure transducers permit electronic
recording of soil-water pressures. Tensiometers
are delicate in nature; field installation and
