SM150T User Manual 1.0
Operation
25
Soil calibration
Damp soil is essentially a mixture of soil particles, air and water,
and together these components determine its dielectric properties,
including the refractive index
. The refractive index of the mixture
is approximated simply by adding the contributions from the
individual components [ref 4.].
For a particular soil, the contribution from the soil particles can be
assumed to be constant, so the refractive index measured by the
SM150T is only affected by changes to the water content,
. This
relationship simplifies to:
1
0
a
a
where the coefficients
a
0
and
a
1
conveniently parameterise the
dielectric properties of soils.
Note that:
soil
dry
a
_
0
is usually between 1.3 to 2.3
a
1
corresponds approximately to
1
water
and usually takes a
value about 8.0. Real soil values for
a
0
and
a
1
can vary significantly
from these guidelines when they are affected by other factors
– in
particular, bound water in clay may result in higher values of
a
1
.
0.0
2.0
4.0
6.0
8.0
0.0
0.2
0.4
0.6
0.8
So
il
r
e
fr
ac
ti
ve
in
d
e
x
(√
ε
)
Soil moisture (m
3
.m-
3
)
Soil calibrations
Offset (a
0
)
Slope (a
1
)
