Geoscience Reference
In-Depth Information
of water through both vertical infiltration and
horizontal flow (in reality this is a combined vector
effect). It is important to realise that this occurs in
both the unsaturated and saturated zones, although
at a slower rate in the unsaturated.
(4.2)
MM
M
−
=
θ
ρρ
w
d
G
=
d
b
w
where
G
is the gravimetric water content (g/g);
ρ
b
is the bulk density of soil (g/cm
3
; see equation
4.3); and
ρ
w
is the density of water (g/cm
3
). As the
density of water is close to 1 g/cm
3
it can be ignored.
Soil bulk density (
Water in the unsaturated zone
ρ
b
) is the ratio of the mass of
dry soil to the total volume of the soil (equation
4.3).
The majority of water in the unsaturated zone is
held in soil. Soil is essentially a continuum of solid
particles (minerals, organic matter), water and air.
Consideration of water in the soil starts with the
control over how much water enters a soil during
a certain time interval: the
infiltration rate
. The
rate at which water enters a soil is dependent on the
current water content of the soil and the ability of
a soil to transmit the water.
M
V
(4.3)
d
t
ρ
b
=
As described above, the density of water is very
close to 1 g/cm
3
(but temperature dependent; see
Figure 1.3), therefore the weight of water is often
assumed to be the same as the volume of water. The
same cannot be said for soil: the bulk density
depends on the mineralogy and packing of particles
so that the volume does not equal the weight. Soil
bulk density gives an indication of soil compaction
with a cultivated topsoil having a value of around
1 g/cm
3
and a compacted subsoil being as high
as 1.6 g/cm
3
(McLaren and Cameron, 1996). It is
important to note that because of this, gravimetric
soil moisture content is not the same as volumetric
soil moisture content, and care must be taken in
distinguishing between them as they are not inter-
changeable terms.
A third way of expressing soil water content is as
a percentage of saturated.
Saturated water content
is the maximum amount of water that the soil can
hold. Soil water content as percentage of saturated
is a useful method of telling how wet the soil
actually is.
Porosity
(
; cm
3
/cm
3
) is another important soil
water property. It is the fraction of pore space in the
total volume of soil (equation 4.4).
Soil water content
Soil water content is normally expressed as a
volu-
metric soil moisture content
or soil moisture
fraction and given the Greek symbol theta (
) -
equation 4.1.
V
V
(4.1)
w
t
θ =
where
V
w
is the volume of water in a soil sample and
V
t
is the total volume of soil sample.
This is normally kept as unitary percentage (i.e.
1 = 100 per cent). As
is a volume divided by a
volume it has no units, although if is sometimes
denoted as m
3
/m
3
. Volumetric water content of a
soil is effectively a depth ratio that is easily related
to other equivalent depths such as rainfall and
evaporation (when expressed in mm depth).
Soil water content may also be described by
gravimetric soil moisture content
(
G
). Gravi-
metric soil moisture content is the ratio of the
weight of water in a soil to the overall weight of the
soil. Gravimetric and volumetric soil water content
can be related to each other by the soil bulk density:
the density of soil in situ (equation 4.2).
V
V
ρ
ρ
(4.4)
p
b
p
ε
==−
1
t
where
V
p
is the volume of pores (cm
3
); and
ρ
p
is the
density of soil particles (g/cm
3
).
