SM150 User Manual 1
Technical Reference
28
Temperature response of soil moisture readings
The effect of temperature on the SM150 soil moisture readings in
any particular soil will depend on a combination of effects:
The SM150 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 SM150 will appear to
have a significant temperature response. This illustration is based
on the model in reference 7, see page 34.
Note: ice has a quite different refractive index from water, so SM150 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
ar
en
t s
o
il
m
o
is
tu
re
c
o
n
te
n
t
(m
3
.m
-3
)
Soil temperature
(
°
C)
Illustrating
temperature dependence
of SM150 readings in a clay
soil
θ ~ 0.4
θ ~ 0.3
θ ~ 0.2
θ ~ 0.1
bound water
