Geoscience Reference
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is the mass of dry soil (g). The density of water is
close to 1 g/cm 3 so the weight of water can be
assumed to equal the volume of water; hence the
symbol in the equation above.
Gravimetric analysis is simple and accurate but
does have several drawbacks. Most notable of these
is that it is a destructive sampling method and
therefore it cannot be repeated on the same soil
sample. This may be a problem where there is a
requirement for long-term monitoring of soil
moisture. In this case a non-destructive moisture-
sampling method is required. There are three
methods that fit this bill, but they are indirect
estimates of soil moisture rather than direct
measurements as they rely on measuring other
properties of soil in water. The three methods are:
neutron probes , electrical resistance blocks and time
domain reflectometry . All of these can give good
results for monitoring soil moisture content, but are
indirect and require calibration against the
gravimetric technique.
(a)
(b)
Figure 4.9 The interactions between a river and the
groundwater. In (a) the groundwater is contributing
to the stream, while in (b) the opposite is occurring.
Neutron probe: A neutron probe has a radioactive
source that is lowered into an augured hole; nor-
mally the hole is kept in place as a permanent access
tube using aluminium tubing. The radioactive
source emits fast (or high energy) neutrons that
collide with soil and water particles. The fast
neutrons are very similar in size to a hydrogen ion
(H + formed in the disassociation of the water
molecule) so that when they collide the fast neutron
slows down and the hydrogen ion speeds up. In
contrast, when a fast neutron collides with a much
larger soil particle it bounces off with very little
loss of momentum. The analogy can be drawn to a
pool table. When the cue ball (i.e. a fast neutron)
collides with a coloured pool ball (i.e. a water
particle) they both move off at similar speeds, the
cue ball has slowed down and the coloured ball
speeded up. In contrast, if the cue ball hits the
cushion on the edge of a pool table (i.e. a soil
particle) it bounces off with very little loss of speed.
Consequently the more water there is in a soil the
more fast neutrons would slow down to become
'slow neutrons'. A neutron probe counts the number
Measuring water beneath the surface
Measurement of soil water
Gravimetric method: The simplest and most accurate
means for the measurement of soil water is using the
gravimetric method. This involves taking a soil
sample, weighing it wet, drying in an oven and then
weighing it dry. Standard practice for the drying
of soils is 24 hours at 105°C (Gardner, 1986). The
difference between the wet and dry weights tells
you how wet the soil was. If it is volumetric soil
moisture content that is required then you must
take a sample of known volume. This is commonly
done using an undisturbed soil sampler and
utilising equation 4.6.
θ =≈
V
V
MM
V
(4.6)
w
t
w
d
t
Where is the volumetric water content (cm 3 /
cm 3 ); V w is the volume of water (cm 3 ); V t is the total
volume of soil (cm 3 ); M w is the mass of water (g); M d
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