SM150T User Manual 1.0
Technical Reference
35
Temperature response of soil moisture readings
The effect of temperature on the SM150T soil moisture readings in any particular soil
will depend on a combination of effects:
The SM150T soil moisture electronics has very low temperature sensitivity, and
makes a negligible contribution to the overall sensitivity.
The refractive index of water (
, see
Calibration
section) reduces as the
temperature increases. This produces a negative temperature response
particularly in soils or substrates with high water content.
Water that is bound to the surface of soil particles has a much lower refractive
index than free water. The percentage of bound water decreases with
temperature and this produces a positive temperature response particularly in clay
soils at lower water contents.
The last two effects partially offset each other, but in soil conditions where one or the
other effect dominates, the SM150T will appear to have a significant temperature
response. This illustration is based on the model in reference 7, see page 41.
Note: ice has a quite different refractive index from water, so SM150T soil moisture
readings cannot be interpreted reliably when inserted into soil below 0°C.
0
0.1
0.2
0.3
0.4
0.5
0
10
20
30
40
50
A
p
p
are
n
t
so
il
m
o
is
tu
re
c
o
n
te
n
t
(m
3
.m
-3
)
Soil temperature (
°
C)
Illustrating temperature dependence of SM150T readings in a clay soil
θ ~ 0.4
θ ~ 0.3
θ ~ 0.2
θ ~ 0.1
bound water
