than the TDR measurements used by Topp and
the dielectric constant to volumetric moisture con-
his co-workers.
tent using time domain reflectometry (TDR) tech-
Since there are obvious problems associated
niques to measure the dielectric constant. Among
with this method for determining soil moisture, it
the first researchers to do this were Topp et al.
seems appropriate to justify the effort being ex-
(1980), whose curve is a third-order polynomial
that matches the calibration curve quite closely
pended to develop it:
and has been used by other researchers as a "uni-
1. The ideal soil moisture measurement system
versal" calibration equation for volumetric mois-
would have the following characteristics:
ture contents. Another group of researchers, Roth
Accuracy
et al. (1990), generated a much more elaborate
Good long-term stability
equation that is based on dielectric mixing theory
Reliability
and that requires a knowledge of the dielectric
Easily read using untrained observers
constant of the dry soil as well as its porosity. It
Low cost
should be noted that the use of these equations
Easily adapted to dataloggers
requires independent calibrations to establish the
Easily calibrated
error limits for any particular soil.
Easily installed
Topp et al.'s equation (based on an empirical fit
Ruggedness
of available calibration data):
Resistance to environmental parameters
such as temperature and humidity.
θ = (5.3 102) + (2.93 102) ε
2. A detailed, serious review of the soil mois-
(5.5 104) ε2 + (4.3 104) ε3 (4)
ture literature will reveal that there is no pres-
ently available method that can be considered to
where θ = volumetric water content.
meet the ideal characteristics (Campbell 1988). In
Roth et al.'s equation (based on dielectric mixing
fact, most methods have serious failings in sev-
theory):
eral of these areas.
3. It is believed that the capacitance measure-
[
]
1 /α
εc = θ ε α + (1 - η) ε α + (η - θ) ε α
ment system described here (or one very similar
w
a
s
to it) can and will ultimately meet the require-
ments for an ideal system.
where εc
=
composite dielectric constant
θ
=
volumetric water content
εw
=
dielectric constant of water
SYSTEM RELIABILITY AND
εs
=
dielectric constant of the soil
REPEATABILITY CONSIDERATIONS
εa
=
dielectric constant of air
η
=
the soil's porosity
α
As part of the initial testing program for this
=
0.46.
soil moisture measurement system, the perfor-
The alpha term (α) is required by dielectric mix-
mance limits of the electronics were established.
ing theory and accounts for the molecular orien-
The objective of these initial tests was to establish
tation of the three materials in this system, namely
the accuracy, reliability, and repeatability of the
air, soil, and water. For a soilairwater mixture,
electronic circuits as a separate portion of the
the authors determined that the best fit was ob-
total soil moisture measurement procedure.
tained between gravimetrically determined soil
To determine how well the electronic cir-
moisture data and this equation when α = 0.46.
cuits performed, three test circuits consisting of
It is also possible to calibrate this soil moisture
resistancecapacitance (RC) networks were fab-
measurement technique directly for volumetric
ricated. A series of tests were performed at differ-
moisture content. For example, the probe might
ent times: once in the spring of 1991 and again in
be placed directly in a test apparatus with a known
the fall of 1991. The circuits for each test sequence
volume of soil and then known volumes of water
are shown in Figure 3. The tests were conducted
added to provide calibration data. Such a proce-
at two different times, with the data taken by two
dure could then be used to determine how well
different observers in order to provide some ran-
equations such as Topp and his colleagues' would
domness to the measurement process. Note that
apply when the dielectric constant is measured at
these tests were conducted with the following
10 MHz using capacitance measurements rather
electronic measurement equipment:
4
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