Table 14. Soil moisture calculations.
Moisture by volume
Moisture by volume
using calibration
using equations of
Dielectric
equation
Topp et al.
Roth et al.
Date
constant
Temp
(%)
(%)
(%)
19 Nov 90
13.7
2.4
17.9
25.5
28.5
20 Nov
13.4
2.1
17.0
25.0
28.0
27 Nov
13.8
1.9
18.2
25.6
28.7
30 Nov
13.9
3.8
18.5
25.8
28.9
3 Dec
13.8
1.6
18.2
25.6
28.7
4 Dec
16.3
1.0
26.2
29.5
32.8
5 Dec
15.3
1.3
22.8
28.0
31.2
6 Dec
14.9
1.0
21.6
27.4
30.6
7 Dec
14.1
0.5
19.1
26.1
29.2
10 Dec
14.4
0.7
20.1
26.6
29.7
11 Dec
13.2
0.1
16.3
24.6
27.7
12 Dec
12.6
0.0
14.5
23.6
26.6
13 Dec
12.7
0.1
14.8
23.8
26.8
14 Dec
12.6
0.1
14.5
23.6
26.6
17 Dec
12.4
0.2
12.2
23.3
26.3
18 Dec
12.5
0.2
12.3
23.4
26.4
19 Dec
12.5
0.1
12.3
23.4
26.4
20 Dec
12.8
0.1
15.1
24.0
27.0
21 Dec
12.8
0.1
15.1
24.0
27.0
26 Dec
13.5
0.1
17.3
25.1
28.2
31 Dec
9.8
0.1
9.4
18.4
21.3
2 Jan 91
7.6
0.8
7.0
13.9
16.4
3 Jan
8.0
0.7
7.4
14.7
17.3
4 Jan
7.5
1.7
6.9
13.7
16.2
7 Jan
7.5
1.8
6.9
13.7
16.2
8 Jan
7.2
2.5
6.6
13.0
15.5
9 Jan
6.9
3.3
6.3
12.4
14.7
10 Jan
7.0
2.3
6.4
12.6
15.0
11 Jan
7.0
2.3
6.4
12.6
15.0
14 Jan
7.1
2.0
6.5
12.8
15.2
16 Jan
7.2
0.6
6.6
13.0
15.5
17 Jan
7.9
0.4
7.3
14.5
17.1
21 Jan
7.2
1.8
6.6
13.0
15.5
23 Jan
6.6
5.9
5.9
11.7
14.0
24 Jan
6.7
4.2
6.0
11.9
14.2
28 Jan
6.8
3.1
6.2
12.1
14.5
needed to actually determine how well the equa-
is the expected accuracy of a given soil moisture
tions of Topp and Roth and their colleagues could
determination using this particular method for
be used to determine soil moisture when using
field measurements?" The term "accuracy" is used
the dielectric constants measured by the method
here to mean the application of a "universal" equa-
described here rather than those found by the
tion such as those that Topp and Roth and their
TDR method used by Topp and Roth. However,
colleagues developed. This implies that the cir-
the data presented in Table 14 and Figure 8 sug-
cuits developed for this measurement method
gest that it is very likely that either Topp et al.'s or
would be used for the laboratory tests and that
Roth et al.'s equation could be modified to work
the measurement frequency would be 10 MHz. It
very well with the dielectric constants found us-
should be noted that the circuits used for the tests
ing this method of capacitance measurement.
described here are unique. They are not the stan-
Other researchers have found that at low frequen-
dard capacitance bridge measurement systems
cies there is a "break" in the curve and have of-
that have been used to measure and report soil
fered various explanations (see Campbell [1988],
capacitances and dielectric constants in most of
for instance).
the research literature. Instead, these circuits were
Another question that might be better answered
developed through Dartmouth College specifi-
with a controlled set of laboratory tests is, "What
cally to produce DC output voltages that could be
20
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