Geoscience Reference
In-Depth Information
The TDR technique is based on the time for a voltage pulse to travel down a soil probe and back,
which is a function of the dielectric constant (ε) of the porous media being measured. By measuring
ε, θ can be determined through calibration (Dalton, 1992). The
ε is calculated with Equation (2.13)
from Topp et al. (1980):
2
2
ct
l
l
lv
=
a
ε=
(2.13)
2
p
pp
where
c
is the propagation velocity of an electromagnetic wave in free space (2.997 × 10
8
m s
−1
),
t
is
the travel time (s),
l
p
is the real length of the soil probe (m),
l
a
is the apparent length (m) as measured
by a cable tester, and
v
p
is the relative velocity setting of the instrument. The relationship between
θ and ε is approximately linear and is influenced by soil type, ρ
b
, clay content, and OM (Jacobsen
and Schjønning, 1993).
By measuring the resistive load impedance across the probe (
Z
L
), EC
a
can be calculated with
Equation (2.14) from Giese and Tiemann (1975):
=
ε
0
c
Z
Z
EC
0
(2.14)
a
l
L
where ε
0
is the permittivity of free space (8.854 × 10
−12
F m
−1
),
Z
0
is the probe impedance (Ω), and
Z
L
=
Z
u
[2
V
0
/
V
f
− 1]
−1
, where
Z
u
is the characteristic impedance of the cable tester,
V
0
is the voltage
of the pulse generator or zero-reference voltage, and
V
f
is the final reflected voltage at a very long
time. To reference EC
a
to 25°C, Equation (2.15) is used:
−1
EC
=
KfZ
(2.15)
a
c
t
L
where
K
c
is the TDR probe cell constant (
K
c
[m
−1
] = ε
0
cZ
0
/
l
), which is determined empirically.
The advantages of TDR for measuring EC
a
include (1) a relatively noninvasive nature, (2) an
ability to measure both θ and EC
a
, (3) an ability to detect small changes in EC
a
under represen-
tative soil conditions, (4) the capability of obtaining continuous unattended measurements, and
(5) a lack of a calibration requirement for θ measurements in many cases (Wraith, 2002). However,
because TDR is a stationary instrument with which measurements are taken from point-to-point,
thereby preventing it from mapping at the spatial resolution of ER and EM approaches, it is cur-
rently impractical for developing detailed geo-referenced EC
a
maps for large areas.
Although TDR has been demonstrated to compare closely with other accepted methods of EC
a
measurement (Heimovaara et al., 1995; Mallants et al., 1996; Reece, 1998; Spaans and Baker, 1993),
it is still not sufficiently simple, robust, and fast enough for the general needs of field-scale soil
salinity assessment (Rhoades et al., 1999a, 1999b). Currently, the use of TDR for field-scale spatial
characterization of θ and EC
a
distributions is largely limited. Even though TDR has been adapted to
fit on mobile platforms such as ATVs, tractors, and spray rigs (Inoue et al., 2001; Long et al., 2002;
Western et al., 1998), vehicle-based TDR monitoring is in its infancy, and only ER and EMI have
been widely adapted for detailed spatial surveys consisting of intensive geo-referenced measure-
ments of EC
a
at field extents and larger (Rhoades et al., 1999a, 1999b).
2.2.3 f
R o M
o
b s e R v e d
a
s s o c i a t i o n s
t o
ec
a
-d
i R e c t e d
s
of i l
s
a M P l i n g
Much of the early observational work with EC
a
correlated EC
a
to soil properties measured from soil
samples taken on a grid, which required considerable time and effort. This early work noted the
spatial correlation between EC
a
and soil properties and subsequently between EC
a
and crop yield.
However, some of these observational studies were not solidly based on an understanding of the
